Nanosurf easyPLL Manual

1

easyPLL plusVersion 3.0

Operating Instructions

V <200

< 50

>200

RangeSet Point Phase Shift

0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz

easyPLL plus CONTROLLER

I-Gain

off

on

0˚

+60˚

Power

Hz

Monitor Input Ref dF Offset OutputLocked

Hi

Lo

Range

10mV - 10V10kHz - 2MHz

10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset

easyPLL plus DETECTOR

2

NANOSURF AND THE NANOSURF LOGO ARE TRADEMARKS OF NANOSURF AG, REGISTERED AND/OR OTHERWISE

PROTECTED IN VARIOUS COUNTRIES.© NOV. 2004 BY NANOSURF AG, SWITZERLAND, PROD.:BT01334, V3.0 R0

3

Table of Contents

The easyPLL plus 5Introduction ...........................................................................................5Technical Data ......................................................................................6

Controller 6Detector 7

Connectors and Indicators ...................................................................9Controller 9Detector 10

Content of delivery ............................................................................ 11

Installation 12Installing the easyPLL Software ........................................................ 12Installing the Hardware ..................................................................... 14

Connecting to a generic system 15Connecting to a JEOL SPM 4500 system 16Connecting to an Omicron system 18Connecting a PSI UHV system 21Connecting to an RHK SPM1000 system 23

Operation 26Self Exciting Oscillation mode ........................................................... 26

Starting the Oscillation 27Optimising the Phase shift 28Locking the Detector to the resonance frequency 29

Constant Amplitude and Constant Excitation mode ......................... 29Starting the Oscillation 30

Optimising the amplitude control ...................................................... 31Preparing the system for approach / scan ........................................ 32Lock-In mode ..................................................................................... 33

Software Reference 35The easyPLL main window ............................................................... 35Center Frequency search dialog ....................................................... 40Configuration dialog .......................................................................... 43

Troubleshooting 45

Theory of Operation 46Components of an easyPLL plus system.......................................... 46

4

About this manual

This easyPLL plus Operating Instructions manual should be read byanyone who wishes to set up and operate the easyPLL plus system. Itapplies to version 3.0 of the easyPLL plus software. Users who want tocontrol the Detector using external software should refer to the easyPLLplus programmers manual, which is delivered in PDF-format on theeasyPLL plus software installation CD.

The sensor 46The easyPLL Controller 47The easyPLL Detector 47

Operation of the easyPLL plus modes .............................................. 50Self Exciting Oscillation mode 50Constant Amplitude mode 52Constant Excitation mode 53Lock-In mode 54

Choosing an Operating mode. .......................................................... 55Lock-In mode or FM-mode ? 55Which FM mode should be used? 59

Literature ........................................................................................... 59

5

The easyPLL plus

IntroductionThe Nanosurf easyPLL plus system is an electronic device for controllingand measuring the resonance of vibrating sensors that detect changes intheir environment by changing their resonance frequency. The systemconsists of a Controller and a Detector. The Controller regulates theresonant amplitude and phase of the of the sensors vibration, the Detec-tor outputs an analog voltage that is proportional to the change in thesensors vibration frequency. Although the easyPLL plus Detector canalso be used separately with a third party controller, the Detector andController are usually used together, therefore this manual describes theoperation of the two together.

The easyPLL plus is generally used to control the resonance of an AtomicForce Microscope’s (AFM) sensor (e.g. tuning fork or cantilever), and tomeasure its resonance frequency. The output voltage of the Detector’sanalog frequency output can then be used to control the tip-sampledistance feedback loop of an AFM system produced by third parties.Several operating modes are possible, these differ in the way the systemregulates the sensors vibration frequency and amplitude. The easyPLLplus Detector alone can measure in the Self Exciting Oscillation mode.Together with the easyPLL plus Controller, also measurements in Con-stant Amplitude, Constant Excitation and Lock-In mode can be made.In addition to changes in resonance frequency, the Controller/Detectorset can also measure changes in vibration amplitude, phase shift or dissi-pation, depending on the operating mode.

The patented design of the easyPLL plus, that combines analog signalprocessing with a digital Phase Locked Loop (PLL) allows it to measurethis frequency with higher speed, resolution and stability than conven-tional measurement devices. Moreover, all functionality of the Detectoris controlled through the easyPLL plus software, which is installed on aPC with an MS Windows operating system.

THE EASYPLL PLUSINTRODUCTION

6

Several other Nanosurf easyPLL products can be used in conjunctionwith the easyPLL plus system:

• The easyPLL UHV preamplifier amplifies the signal from the oscillat-ing sensor, when mounted inside an Ultra High Vacuum (UHV) cham-ber.

• The easyPLL Manual Controller allows you to change the referencefrequency without using a PC.

Technical Data

Controller

Detector In

frequency range: 10 Hz – 1 MHz

voltage range: max. 1 Vpeak

Input filter 40 kHz, on/off, 80 dB/Dec

Drive out

Gain Signal range noise

x1 ±10 V <2.5 mVeffx0.1 ±1 V <0.3 mVeffx0.01 ±0.1 V <0.2 mVeff

Signal output

noise <1mVeff

Dissipation output

Proportional gain noise

10 <0.3 mVeff 100 <3 mVeff1000 <10 mVeff

THE EASYPLL PLUS TECHNICAL DATA

7

Amplitude control

Setpoint 0 - 1 V

proportional gain × 10 - × 11000

integral gain on/off

bandwidth 1 kHz

Analog phase shifter

0 - 360°

Other

Power supply 110/230 VAC (50/60 Hz)

Fuse 0.40 AT (220 - 240 VAC)0.80 AT (100 - 130 VAC)

Detector

Input

frequency: 10 kHz – 2 MHz

amplitude: 10 mV – 10 V peak (5 gain steps, with autorange)

Reference output

frequency: 10 kHz - 2 MHz

voltage: ±10 V (14 Bit)

Digital phase shifter: 0 - 360° (19 Bit)

Frequency shift measurement

Operating modes: Self Exciting Oscillation, Constant Amplitude,Constant Excitation, Lock-In

Demodulation bandwidth (BW): 120/400/1200 Hz

Spectrum analysis: amplitude, phase

Temperature coefficient 3.5 mHz/°C

THE EASYPLL PLUSTECHNICAL DATA

8

Measurement range Measurement resolution

±183 Hz 5 mHz ±366 Hz 11 mHz ±732 Hz 22 mHz±1464 Hz 44 mHz

System output noise [Hzeff]*Measurement Bandwithrange 120Hz 400Hz 1200Hz

± 183 Hz < 4 mHz <13 mHz < 60 mHz± 366 Hz < 8 mHz <13 mHz < 50 mHz± 732 Hz <13 mHz <25 mHz < 75 mHz±1464 Hz <25 mHz <50 mHz <100 mHz

*Output gain = 1, Signal input frequency = 200 kHz

Manual Controller Offset input

bandwith 27 kHz

input voltage ±10 V input, differential

Connection Signal

Pin1 +10 V reference voltage outputPin2 + Offset inputPin3 - Offset inputPin4 -10 V reference voltage outputShield ground

Output

Output gain Output signal range

0.1 ±1 V 1 ±10 V 10 ±10 V100 ±10 V

Output polarity selection yes

THE EASYPLL PLUS TECHNICAL DATA

9

Software

Operating system Windows 95/98/ME/NT/2000/XPRemote control COM Automation (LabView/Visual Basic/Del

phi/C++/...)

Other

PC connection Standard PC parallel port

Power supply 90 – 230 VAC (50/60 Hz)

Specifications are subject to change without notice

Connectors and Indicators

Controller

Monitor switch

Mode switchdisplay of the

monitored signal

Amplitude Set Point potentiometer

Reference signal from Detector

Amplitude signal

Amplitude controller switches and potentiometer

Phase Shift switches and potentiometer

Range switch for phase shift

Dissipation signal

Sensor signal input Signal out to Detector

Drive out to excitation piezo

V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

Monitor selector

Amplitude: Displays the RMS vibration amplitude of the 10xamplified sensor signal.

SetPoint: Displays the Setpoint for the amplitude input is dis-played in Self Osc. and Const. Ampl. mode.

THE EASYPLL PLUSCONNECTORS AND INDICATORS

10

Dissipation: Displays the peak amplitude of the Drive out sig-nal.

Mains power voltage selector switch

Fuse

mains power connector

AC PowerSerial Nr.

NANOSURF AG - 4410 LIESTAL - SWITZERLAND

230

DetectorThe Detector has several BNC connectors for input and output as wellas LEDs for quick status overview.

signal monitor switch

display of the monitored signal

sensor signal input

out of range indicators of input amplitude

reference signal output

input frequency out of range indicator

output voltage proportionalto frequency shift

output for z-feedback loop (error signal)

Manual Controller offset input

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


Monitor selector

The switch selects the signal to be displayed on the display.

THE EASYPLL PLUS CONNECTORS AND INDICATORS

11

TipGuard response potentiometer

Parallel Port connector

AC PowerParallel InterfaceSerial Nr.

NANOSURF AG - 4410 LIESTAL - SWITZERLAND

TipGuard

mains power connector

TipGuard response potentiometer

With the TipGuard response potentiometer, you can adjust the voltageto which the tip is retracted when the PLL unlocks (see chapter the easyPLLsoftware, section TipGuard).

Content of deliveryAfter unpacking the instrument, check for the following items:

Controller

1 - easyPLL plus Controller

2 - Mains cable

3 - Two 25 cm long BNC cables

Detector

1 - easyPLL plus Detector

2 - Software installation CD

3 - Parallel port cable

4 - Mains cable

5 - This manual

THE EASYPLL PLUSCONTENT OF DELIVERY

12

Installation

This section gives instructions for installing the easyPLL plus system

Installing the easyPLL SoftwareYou need a PC with MS Windows 95 or newer operating system inorder to install the easyPLL software:

- Turn on your computer and start Windows if necessary.

Important!

- Log in with Administrator rights when your are using Windows NT,2000, XP or newer.

- Insert the (backup copy of your) installation CD into the drive.

An installation menu should now appear. If this does not happen:- Open the CD and start the programme StartCDMenu.exe.

- Select Install easyPLL software.

INSTALLATION INSTALLING THE EASYPLL SOFTWARE

13

The installation screen now appears.

- Click , to install the easyPLL software.

The setup will ask for the directory in which the program files are to becopied:

- We recommend putting them in the suggested directory.

Afterwards, the setup will ask for the program group in which easyPLL isto be placed:

- Click Ok or type another name.

The setup will now ask for the parallel (LPT) port:

- Select the parallel port to which you have connected the Detector. (Seesection Assembly)

INSTALLING THE EASYPLL SOFTWARE INSTALLATION

14

Make sure that the parallel port of your PC is configured to either SPP-Mode or normal Mode. This configuration can be set in the BIOS ofyour PC. Please refer to your PC documentation for help.

- Click OK.

Restart your PC.

Installing the HardwareThis section gives instructions on how to set up your easyPLL plus sys-tem. It starts with general instructions, and then continues with in-structions for connecting the Detector to the following systems: JEOL,Omicron, PSI, RHK. Software settings are also given for each instru-ment. If you have another instrument, or have found better settings,please contact Nanosurf or your local distributor.

Important!

• Before connecting the electronics, check that the voltage indicated onthe Mains power voltage selector switch on the rear panel of the Con-troller corresponds to your mains voltage. Using the wrong settingwill blow the fuse or may even damage the electronics.

INSTALLATION INSTALLING THE HARDWARE

15

- Turn off your computer.

- Connect the easyPLL plus Detector to a free parallel port (LPT-Port)on your computer with the parallel port cable.

- Connect the Detector to the mains with the mains cable.

- Connect the Controller to the mains using the mains cable.

The other connections depend on both the operating mode (see chapterOperating the Controller) and the AFM you are using. Use 50Ω BNCcables to make all BNC connections.

Connecting to a generic system

Cables for Detector only

- Connect the (amplified) sensor signal to the Input of the Detectorusing a 50Ω BNC cable.

- Connect the Output of the Detector to the z-feedback error signalinput of your scan control electronics using a 50Ω BNC cable.

Cables for Controller/Detector set

- Use the two BNC cables to connect the Controller to the Detector, asis shown in the figure Connecting the Controller to the Detector.

V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


Connecting the Controller to the Detector

INSTALLING THE HARDWARE INSTALLATION

16

- Connect the (amplified) sensor signal to the ‘Detector in’ input of theController.

- Connect the ‘Drive out’ output of the Controller to the excitationinput of the sensor.

- If you have an Oscilloscope available:

- Connect the sensor signal to Channel 1 on the Oscilloscope aswell.

- Connect Drive out to Channel 2 on the Oscilloscope as well.

- Connect the Output of the Detector to the Z-feedback error signalinput of the scan electronics.

- Connect the Dissipation output of the Controller to an auxiliary ADCinput of the scan electronics.

- Connect the Amplitude output of the Controller to an auxiliary ADCinput of the scan electronics.

- Connect the dF output of the Detector to an auxiliary ADC input ofthe scan electronics.

easyPLL plus software settings

For a description of the parameters that should be set, refer to ChapterSoftware Reference.

Connecting to a JEOL SPM 4500 system


- Connect the A-B output of the AFM AMP to the Input of the Detec-tor.

- Connect the Output of the Detector to the AFM input of the SPMCONTROL.


17

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


AFM AMP (Rear)

CNTIA-B

SPM CONTROL (Rear)

AFM


- Connect the A-B output of the AFM AMP to the Detector In input ofthe Controller.

- Connect the CNTI input of the AFM AMP to the Drive Out outputof the Controller


- Connect the A-B output to Channel 1 on the Oscilloscope as well.

- Connect the Drive Out to Channel 2 on the Oscilloscope as well.

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

AFM AMP (Rear)

CNTIA-B

SPM CONTROL (Rear)

AFM


18

- Connect the AFM input of the SPM CONTROL to the Output out-put of the Detector


Input Setting

Lock range +-732 Hz

Input:Auto range on

Output:Gain x1Positive polarity off

TipGuard:Retract tip if unlocked onPositive direction off

JEOL WinSPM software settings

- In the Advanced dialog, set the STM/AFM box to FM.

Connecting to an Omicron system


- Insert a BNC-T connector between the FN cable from the UHV sys-tem and the FN input of the AFM CU.

- Connect the FN signal of the BNC-T connector to the Input of theDetector.

- Connect the Output of the Detector to the F IN input on the SPMCU.

- Optionally, you can connect the dF output of the Detector to theEXT 1 Input of the SPM CU.


19

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


AFM CU

FN IN

FL IN

Σ IN

SPM CU

F IN

AFM CU

FN OUT

SPM CU

EXT 1

UHV AFM-Chamber

Standard Omicronmulticore cable

BNC-Splitter


- Insert a BNC-T between the FN cable from the UHV system to theFN input of the AFM CU.

- Connect the FN signal of the AFM CU to the Detector In of theController.

- Connect the Drive out output of the Controller to the Excite cablefrom the UHV system.


- Connect the FN signal to Channel 1 on the Oscilloscope as well.


- Connect the Output of the Detector to the F IN input on the SPMCU.

- Depending on the operation mode, connect the Amplitude or Dissi-pation output of the Controller to the EXT 1 Input of the SPM CU.


20

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

AFM CU

FN IN

FL IN

Σ IN

SPM CU

F IN

AFM CU

FN OUT

SPM CU

EXT 1

UHV AFM-Chamber

Standard Omicronmulticore cable

BNC-SplitterFNExcite


Input Setting

Lock range +-732 Hz

Input:Auto range on



Omicron ScanControl settings

- Select the mode AFM non contact in the menu Topography Preset.


21

The Button Feedback set in the Panel Measurement Control is now use-less. The corresponding value is now set either digitally with the easyPLLplus software with the value OffsetFrq or manually with the optionallyavailable easyPLL Manual Controller.

When measuring the Amplitude or Dissipation signal using EXT 1:

- Open the calibration settings via the menu Setup/Edit Hardware/Mis-cellaneous Calibration.

- Enter the following values in the row for EXT 1:Name: Amplitude or DissipationMin: -10Max: +10Unit: VMin: -10.0Max: +10.0

Connecting a PSI UHV system


Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


PSI NC Module

PIEZO-LEVERSIGNAL IN

FM ERROROUT

PSI Interface Module

FM SIGNALMONITOR

Z-MOD HF NC AFM / AUX

- Connect the FM SIGNAL MONITOR output of the PSI NC Moduleto the Input of the Detector.

- Connect the Output of the Detector to the NC AFM / AUX input ofthe PSI Interface Module.


22


- Connect the FM SIGNAL MONITOR output of the PSI NC Moduleto the Detector in input of the Controller.

- Connect the Drive out of output the Controller to the Z-MOD HFinput of the PSI Interface Module.


- Connect the FM SIGNAL MONITOR output to Channel 1 onthe Oscilloscope as well.


- Connect the Output of the Detector to the NC AFM / AUX input ofthe PSI Interface Module.

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

PSI NC Module

PIEZO-LEVERSIGNAL IN

FM ERROROUT

PSI Interface Module

FM SIGNALMONITOR

Z-MOD HF NC AFM / AUX


Input Setting

Lock range +-723 Hz


23

Input:Auto range on



Connecting to an RHK SPM1000 systemThe settings and connections given here are optimised for using theRHK SPM1000 system in combination with the RHK UHV350 sys-tem. For other systems, those settings marked as UHV350 may be dif-ferent.

Cables

- Connect the NORMAL FORCE output on the AFM100 to the De-tector in Input on the Controller.

- Connect the Drive out output of the Controller to the cable markedModulation (UHV350), or to Z POSITION #2 on the SPM100 elec-tronics.


- Connect NORMAL FORCE to Channel 1 on the Oscilloscope aswell.

- Connect Drive out to Channel 2 on the Oscilloscope as well.- Con-nect the Output on the Detector to the Preamp Input (external)input on the SPM100.

- Connect the Amplitude output of the Controller to the FROM LOCK-IN input of the SPM100.


24

V <200

< 50

>200


0˚ +120˚

0˚

+180˚

Amplitude Control

x11x1

Detector in

< 1V

Drive out

< 10V

Signal out

< 10V

Ref in

< 10V

Amplitude

RMS out

Dissipation

Const.Ampl.

SetPoint

Const. Excit.

Self Osc.

Mode

off

Low Pass

Amplitude

Dissipation

Monitor

outnanoSurf

max0Amplitude

x100

x1000

x10

P-Gain Phase kHz

x 0.1

x 1

x 0.01

Power

40kHz


I-Gain

off

on

0˚

+60˚

Power

Hz


Hi

Lo

Range


10V 0˚ - 360˚

±10V ±10V ±10V Gain=0.1-100

Output

dF

Offset


AUX INPUTS

PREAMP INPUT

MODULATIONINPUTS

AFM100

SPM100

FROMLOCK-IN

Z-POSITION #2

NORMALFORCE

AUX 1

Modulation

Depending on whether you are using one of the PLL modes or the Lock-In mode, make the following connections:

- For PLL mode, connect the Dissipation output on the Controller tothe AUX 1 input on the SPM100.

- For Lock-In mode, connect the dF output on the Sensor controller tothe AUX 1 input on the SPM100.

Switches on the SPM100 electronics

SPM 100:

Feedback Linear

Input Polarity SPM


25

Polarity Normal


Input Setting

Lock range +-732 Hz

Input:Auto range on



RHK software settings

- Go to the menu Settings / I/O

- Choose User Defined for the operation mode,

- Enter ‘dF’ as name and ‘Hz’ as unit.

- Copy the calibration from the DeltaF monitor in the easyPLL plussoftware, and add a - sign (UHV350).


26

Operation

In the following, it is assumed that you are using the easyPLL plus Con-troller in combination with the easyPLL plus Detector. If you are usingthe Detector alone, only the Self Exciting Oscillation mode is available.Otherwise, the easyPLL plus set supports four modes for working with afrequency modulated sensor:

1. Self Exciting Oscillation: The Detector measures the sensor’s vibra-tion frequency. The Controller excites the sensor resonance by feedingthe measured sensor vibration back to the sensor excitation. The Con-troller also regulates the vibration amplitude to a constant value, andmeasures the necessary excitation amplitude.

2. Constant Amplitude: The Detector excites the sensor and tracks itsresonance frequency. The Controller regulates the vibration ampli-tude to a constant value, and measures the necessary excitation ampli-tude.

3. Constant Excitation: The Detector excites the sensor and tracks itsresonance frequency. The Controller sets the excitation amplitude andmeasures the vibration amplitude.

4. Lock-In (Controller: Const. Excit.): The Detector excites the sensor ata constant frequency, and measures the phase shift of the vibration.The Controller sets the excitation amplitude and measures the vibra-tion amplitude.

For more information on how to select the operating mode, refer to chapterTheory of Operation, especially section Choosing an Operating mode.

Self Exciting Oscillation modeIn the Self Exciting Oscillation mode, the Controller excites the sensorresonance frequency by feeding the sensor deflection signal back to theexcitation device (e.g. piezo). With the appropriate amplification andphase shift settings in the feedback loop, the sensor will start to self-oscillate. An amplitude controller keeps the oscillation amplitude con-

OPERATION SELF EXCITING OSCILLATION MODE

27

stant by varying the amplification of the feedback loop. The frequency ofthe oscillation is converted into a DC signal by the Detector, and canthen be used for Z-feedback.

Starting the OscillationFollow this procedure to find the optimum settings for a new or un-known sensor:

- Set the Controller controls as follows:

Control Setting

Mode switch Self Osc.Monitor switch DissipationSetPoint Amplitude 1 V

Amplitude Control:I-Gain switch offP-Gain pot. x1P-Gain switch x100

Phase Shift:switches 0°potentiometer 0°

Range switch expected resonance frequency range in kHzLow pass switch see belowDrive out switch x0.01

Turn on the Low Pass 40 kHz switch on the Detector when using asensor with a resonance frequency lower than 30 kHz, this will lead toless noise in the amplitude measurement.

- Check that the sensor is oscillating: The oscillation should be visibleon the oscilloscope and the Monitor display: The Dissipation voltageshould not be larger than about 13 V.

When the sensor is oscillating, continue with Optimising the phase shift.Otherwise, monitor the dissipation to check the result of each of thefollowing steps and continue with Optimising the phase shift when thesensor starts oscillating.

OPERATIONSELF EXCITING OSCILLATION MODE

28

Sweep the phase shift from 0° to 360°:

- Turn the Phase Shift potentiometer to search for the resonance fre-quency of the sensor.

- If adjusting the Phase Shift potentiometer up to its maximum settingdoes not result in oscillation, switch the +180° Phase Shift switch to+180° and turn the Phase Shift potentiometer back to zero again.

When oscillation does not start, increase the Amplitude Control gain:

- Turn the Amplitude Control potentiometer to x5 - x8 and increase theAmplitude SetPoint to 5 V.

- Sweep the phase from 0° to 360°.

When oscillation does not start, increase the excitation:

- Set the Drive out switch to x1.

- Sweep the phase from 0° to 360°.

When the oscillation does not start, increase the Amplitude ControlGain and the Set Point Amplitude even further.

If all these steps fail to start oscillation, measure the amplitude and phasediagram of the sensor to check whether there really is a resonance usingthe Center Frequency search dialog of the easyPLL plus software.

Optimising the Phase shiftWhen the resonant oscillation has been successfully established, theController should be adjusted to operate at the precise resonance fre-quency of the sensor. This is the case when the sensor dissipation isminimal.- Set the Monitor switch on the Controller to Dissipation.

- Turn the Phase Shift potentiometer until the monitor displays mini-mal dissipation.

- When the dissipation is still not minimal at the maximum position ofthe Phase Shift potentiometer, switch the 60° Phase Shift switch, andadjust the Phase Shift potentiometer again.

OPERATION SELF EXCITING OSCILLATION MODE

29

Important!

The 40 kHz filter causes a slight additional phase shift, therefore opti-mise Phase Shift of the Controller again after turning on the filter.

Locking the Detector to the resonance frequencyWhen the sensor has been set up to oscillate at its resonance frequencyusing the Controller, the Detector should be locked to this frequencyusing the following procedure:

- Start the easyPLL plus software if it was not already running.

- Select the Self Exciting Oscillation mode using the Operating modedrop-down menu.

- Open the Center Frequency search dialog by clicking .

- Set the Frequency Range from approximately 60% to 140% of theexpected resonance frequency.

- Click to find the CenterFrq.

Now, continue with section Optimising the amplitude control.

Constant Amplitude and Constant Excitation modeThe Constant Amplitude and Excitation mode use the phase control-ling properties of the phase locked loop to track the resonance frequencyof the cantilever. The reference signal from the Detector is only ampli-fied/weakened by the Controller, and then applied to the excitation de-vice. The resulting sensor deflection signal is passed through theController to the input of the Detector. The phase locked loop in theDetector adjusts the reference frequency until the phase shift betweenthe sensor signal and the reference signal is equal to the phase shift givenin the ‘Phase shift’ input in the easyPLL plus software. When this phaseshift is set properly, the frequency of the reference signal will follow theresonance frequency of the sensor. The frequency of the oscillation isconverted into a DC signal by the Detector, and can then be used for Z-feedback.

OPERATIONCONSTANT AMPLITUDE AND CONSTANT

EXCITATION MODE

30

In the Constant Amplitude mode, the amplitude controller in the De-tector keeps the oscillation amplitude constant by varying the excitationof the sensor. This is done by controlling the amplification of the refer-ence signal. In Constant Excitation mode, the excitation is constant.

Starting the OscillationFollow this procedure to set up the resonant oscillation with a new orunknown sensor:

- Set the Controller:

Control Setting

Mode switch Const. Excit. or Const. Ampl.Monitor switch DissipationSetPoint Amplitude 1 V

Amplitude Control Gain:I-Gain switch onP-Gain potentiometer x1P-Gain switch x10

Phase Shift:switches 0°potentiometer 0°

Low pass switch see belowDrive out switch x0.01

Turn on the Low Pass 40 kHz switch on the Detector when using asensor with a resonance frequency lower than 30 kHz, this will lead toless noise in the amplitude measurement.

Important!

The 40 kHz filter causes a slight additional phase shift, therefore adjustthe Phase shift setting in the software after turning on the filter.

- Start the easyPLL plus software if it is not yet running.

- Select the desired operating mode using the Operating mode drop-down menu.

OPERATION CONSTANT AMPLITUDE AND CONSTANT

EXCITATION MODE

31

- Open the Center frequency search dialog by clicking .

Depending on your personal preferences, you can now use either theautomatic search or the manual selection method to find the resonancefrequency.

- Set the Frequency Range from approximately 60% to 140% of theexpected resonance frequency.

- Set Frequency step to approximately:

fresonance / (3 Q) .

- Set Time/step to at least:

Q / fresonance .

- Perform automatic adjustment by clicking .

The easyPLL plus software will now search for the resonance frequencyby searching for the steepest slope of the phase/frequency. The searchresult is shown in the Marker section of the dialog.

Alternatively, you can manually select the operating frequency from theresonance curve:

- Select a smaller Frequency range and Frequency step than for the auto-matic method.

- Click .

- Select the desired operating point in the resonance curve, and click.

When the sensor is oscillating at its resonance frequency, continue withsection Optimising the Amplitude Control.

Optimising the amplitude controlWhen the resonant oscillation has been successfully established, theAmplitude Controller should be adjusted for optimal signal to noise (S/N) ratio or speed performance. This procedure should also be performedafter changing the Amplitude Set Point, in order to achieve best per-formance.

OPERATIONOPTIMISING THE AMPLITUDE CONTROL

32

- Set the desired operating amplitude with the Set Point potentiometer.(Note that the amplitude signal is equal to the RMS input amplitude atthe Detector in input, times ten.)

To achieve a high (S/N) ratio, the PLL should internally work with ashigh a voltage as possible:

- Set the Monitor switch on the Controller to Dissipation.

- Increase the setting of the Drive out gain switch until the dissipationsignal becomes larger than 1 V.

- Always decrease the setting of the Amplitude Control P-Gain switchwhen the setting of the Drive out gain switch is increased and vice versa(if possible).

Now optimise the speed of the amplitude control.

- Increase the Amplitude Control P-gain until the oscillation becomesunstable. (You generally need an Oscilloscope in order to observe this.)

- Reduce the P-gain by about a quarter of a turn.

Preparing the system for approach / scanYou can generally use a shorter setup procedure before tip-sample ap-proach if you continue using the same type of cantilever:

- Start the easyPLL plus software if it is not already running.

- Click to find the CenterFrq.

- In the Self Oscillation mode, minimize the sensor dissipation byoptimising the Phase shift.

- When using the frequency shift for Z-feedback, set the frequency shiftset point for your experiment. You can determine the set point of theZ-feedback either by entering it by OffsetFrq in the easyPLL plus soft-ware, or by adjusting the controls on the optional easyPLL manualcontroller, and reading the Monitor Offset on the Detector.

Typical offset values are -30 Hz to -1 kHz for cantilevers, and -1 Hz to-50 Hz for tuning forks.

Now the system is ready to approach and scan.

OPERATION PREPARING THE SYSTEM FOR APPROACH / SCAN

33

Lock-In modeIn the Lock-In mode, the sensor is excited by a signal with constantfrequency and amplitude. The sensor vibration amplitude and phase aremeasured, and can be used for Z-feedback. This mode is commonlycalled tapping or intermittent contact mode. Using the Lock-In is rec-ommended for sensors with low Q factors. With high-Q cantilevers, theamplitude changes very slowly and the scan speed has to be greatly re-duced.

When you want to perform Z-feedback on the vibration amplitude:- Connect the Amplitude output of the Controller to the error input of

the Z-feedback instead of the Output of the Detector (see ChapterInstalling the easyPLL plus system).

- If it is not already connected, connect the dF output of the Detector toan Auxiliary input of your scan electronics.

Control Setting

Mode switch Const. DriveMonitor switch AmplitudeSetPoint Amplitude 1 V

Amplitude Control Gain:switch x100potentiometer x1

Phase Shift:switch 0°potentiometer 0°

Low pass offDrive out Gain x0.1

- Start the easyPLL plus software if it i s not already running.

- Select the Lock-In mode using the Operating mode drop-down menu.

The signal at the dF and Output BNC connector of the easyPLL plusDetector should now be interpreted as phase information and no longeras frequency deviation.

OPERATIONLOCK-IN MODE

34

- Open the Center frequency search dialog by clicking .

- Set an appropriate Search range and Frequency step.

- Click .

- Select the desired operating point in the resonance curve, and click.

To perform measurements with feedback on the vibration amplitude,you can use an operating frequency that is not equal to the resonancefrequency:- Enter the desired offset from the resonance frequency in OffsetFrq.

Usually a positive frequency shift is used. The offset is chosen so, thatthe measured sensor vibration amplitude falls to around 90% of itsvalue at resonance.

- Define a set point for the amplitude in your scan software and start theapproach.

The Z-feedback loop now uses the amplitude as the control input. Ad-ditionally the phase can be measured using an ADC channel in yourdata acquisition hardware.

OPERATION LOCK-IN MODE

35

Software Reference

The easyPLL plus Detector is controlled by the easyPLL plus software.The functions of this software are described in this chapter.

It is also possible to control the Detector from other windows softwarethat support COM Classes (in particular the control of COM automa-tion server). This makes it easy to access the full PLL functionality froma customers home made software, LabView, Visual Basic, Windows script-ing, Delphi etc.. The installation includes application examples forLabView and Visual Basic. For more information on how to access theeasyPLL functions please refer to PDF file easyPLL Programmers Manualthat was installed with the easyPLL software.

The easyPLL main windowThe following window appears when the easyPLL plus software is started:

easyPLL main dialog

Status panel

The status panel shows the current values of the most important inputand output parameters, and important system messages.

SOFTWARE REFERENCETHE EASYPLL MAIN WINDOW

36

Input: current input frequency and input range. In Lock-In mode, theRoot-Mean-Square (RMS) input amplitude is displayed instead of theinput frequency.

DeltaF: current output value of the dF BNC connector and its outputsensitivity.

Output: current output value of the Output BNC connector and itsoutput sensitivity.

Status: displays the actual system status

• Locked - PLL is locked to the input frequency - otherwise unlocked).

• Auto - if Input Auto range is active

• Neg - if the output polarity is negative

• Guard - if the output TipGuard feature is on

System Messages:

• System ready - regular operation

• Searching... - appears during frequency sweep.

• Input amplitude too high or ... too lowThe input amplitude is too high or too low for reliable operation.The input amplitude is too high when it is larger than the Ampli-tude value in the Input section.

• Input frequency is out of lock range!The input frequency is outside the selected Lock range around theCenter frequency.

• Hardware not detectedThe Detector has no mains power, the parallel port cable is not cor-rectly connected or the wrong parallel port has been selected in theConfiguration dialog.

• Communication downPC has lost communication with the electronics during operation.Either the Detector has no main power or the parallel port cable isnot correctly connected.

SOFTWARE REFERENCE THE EASYPLL MAIN WINDOW

37

Operating Point

Center freq.: The centre frequency of the input signal (usuallythe sensors resonance frequency).

Offset freq.: An offset frequency to be added to the Output sig-nal (usually the frequency set-point of the Z-con-troller).

Phase shift: The phase shift between the Reference output andthe Internal reference signal used by the phase com-parator in the PLL. In the Constant Amplitude andExcitation mode, the phase shift indirectly deter-mines the resonance frequency. In the Lock-In mode,the Phase shift sets the Phase shift between the Ref-erence output and the signal input that correspondsto 0V output signal at the dF output.

Opens the Center Frequency search dialog that al-lows you to set the operating point search param-eters and to manually select the operating point fromthe frequency characteristic of the sensor (see sec-tion Center Frequency search dialog).

: Starts a search for the operating point with the searchsettings defined in the Center Frequency search dia-log.

In the Self Exciting Oscillation mode, the search al-gorithm increases the center frequency until the PLLlocks, then the Center frequency is set to the vibra-tion frequency of the cantilever.

In the Constant Excitation and the Constant Am-plitude mode, the search algorithm searches for thesteepest change in the Phase versus Frequency shiftplot. The search results are displayed graphicallywhen Display results of resonance search is activatedin the Configuration dialog.


38

Operating Mode

Operating Mode: The operating mode of the system.

The Mode switch on the Controller must be set ac-cordingly when changing this setting. A reminderwill appear when the corresponding configurationsetting is active.

For more information on the various operating modes,see chapters Operating the easyPLL plus system andTheory of Operation.

Lock range: The frequency range within which the deviation ofthe centre frequency is tracked.

The Lock range also determines the resolution andsensitivity of the frequency measurement, see chap-ter the easyPLL plus, section Technical Data.

Bandwidth: The bandwidth of the output signals dF and Out-put.

For low noise feedback, bandwidth of the outputsignals can be decreased using a 2nd order low passfilter.

SOFTWARE REFERENCE THE EASYPLL MAIN WINDOW

39

Input

Amplitude: The voltage range of the Detectors input amplifiercircuit.

When the input voltage is too large or too small, theIn Hi or In Lo LED will light up, and a system mes-sage is displayed. Note that all other amplitude in-dicators display RMS amplitude, which is a factorsquare root of two (0.7) smaller for sinusoidal sig-nals.

Auto range: Enables automatic selection of a suitable voltagerange for the input amplifier circuit.

Output

Gain: The amplitude gain of the output amplifier circuit.

With gain 0.1, the maximum output signal rangeis ±1 V, with the other gains, the range is ±10 V.

Positive polarity: The output voltage increases when the Frequencyshift increases. Activate this box when the Z-con-troller error should increase when the resonance fre-quency increases. Changing the state of this boxbasically changes between ‘attractive’ and ‘repulsive’imaging.


40

TipGuard

Select the behaviour of the system if the PLL circuit can no longer lockinto the input frequency in order to prevent the tip from crashing. Whenthe PLL unlocks, the output is set to the maximum to force the z-feed-back controller to retract the tip from the surface.

Retract tip if unlocked: Activates the TipGuard feature.

Positive direction: Increases the output voltage when the PLL unlocks.This box must be active when the sensor moves awayfrom the surface with increasing Output voltage.

Control buttons

: Values entered using the keyboard must be confirmedby clicking this button, or by pressing the Enter orReturn key on the keyboard.

Values changed using the arrow buttons or thecursor keys on the keyboard are automatically con-firmed.

: Opens the configuration dialog (see section Configu-ration Dialog).

: Displays the about dialog box. Here you find theversion number of your easyPLL plus software.

Center Frequency search dialogThe Center Frequency search dialog gives you control over the search forthe resonance frequency of your sensor. You can use this dialog to manu-ally select the Center Frequency, or to optimise the settings for the Auto-matic search algorithm for Constant Amplitude and Constant Excitationoperating modes.

SOFTWARE REFERENCE CENTER FREQUENCY SEARCH DIALOG

41

Frequency sweep plots

The Frequency sweep graphs display the measured Root-Mean-Squarevibration amplitude and Phase shift as a function of frequency. Whenthe mouse cursor moves over a measured point, the measured values aredisplayed numerically.

Frequency Range

You can define the frequency range for the sweep by either entering thevalues of Start and End, or by the entering the values of Center andSpan. The other value pair is adjusted automatically according to thelatest entry. Sometimes the both pairs will be adjusted because the entryis physically impossible, for example setting Span larger than half thevalue of Center.

SOFTWARE REFERENCECENTER FREQUENCY SEARCH DIALOG

42

Sampling

Frequency step: The difference in frequency between one measure-ment point and the next. The maximum number ofsteps is limited to 1023.

Time/step: The time that the easyPLL plus waits between chang-ing the excitation frequency and measuring the phase.Its value should not be smaller than roughly the Qfactor divided by the resonance frequency of the sen-sor.

Control Buttons

: Has the same effect as the Auto Search button in themain window.

: Starts measuring the Frequency sweep.

: Saves the measured amplitude and phase as a func-tion of frequency to a text file.

: Has the same effect as the Apply button in the mainwindow.

: Closes the dialog.

Marker

The Marker allows you to change the CenterFrq in the easyPLL plusmain window:

- Move the mouse cursor over one of the Frequency sweep plots to theposition of the desired CenterFrq.

- Double-click on the desired measured point. The measured values arenow displayed in the Freq., Ampl. and Phase displays.

- Click to use the displayed point as CenterFrq.

SOFTWARE REFERENCE CENTER FREQUENCY SEARCH DIALOG

43

Configuration dialog

User Interface

Display results of auto search: The Center Frequency search dialog isopened each time is clicked.

Display reminder on mode change: A reminder is displayed to changethe setting of the Mode switch on the Controllereach time the Operating Mode is changed.

Communication Settings

Status update rate: Rate at which the PLL’s status panel is updated. Thisrate lies in the range from 10ms to 1000ms.

Parallel port: The parallel port used for the communication be-tween PC and electronics.

Calibration

Quartz frequency: The internal reference clock speed. This value is setautomatically during installation and rarely needsadjusting. For fine adjustment of the PLL only.

SOFTWARE REFERENCECONFIGURATION DIALOG

44

Phase calibration: The internal phase measurement peak value. Thisvalue is set automatically during installation andrarely needs adjusting. For fine adjustment of thePLL only.

Amplitude calibration: The maximum measurable RMS amplitude. Thisvalue is set automatically during installation andrarely needs adjusting. For fine adjustment of thePLL only.

SOFTWARE REFERENCE CONFIGURATION DIALOG

45

Troubleshooting

The easyPLL digital FM-Detector hardware is not detected by theeasyPLL plus software

The easyPLL plus software only supports the easyPLL plus detector. Atthe time of writing (April 16, 2004), the newest software that supportsthe easyPLL plus is easyPLL software version 2.5. However, it is recom-mended to obtain a PLL plus detector, in order to enjoy all the featuresdescribed in this manual.

The sensor does not oscillate

This could be due to the amplitude characteristic of the sensor. Whenthe quality factor Q is too small to generate self-oscillation. In this case,use the Lock-In mode

This could also be due to the phase characteristic of the sensor. Whenthis characteristic has more than one 90° transit within the lock range,the lock in may switch between the frequencies at which this transitoccurs. This may for example happen when two different torsional vibra-tion modes are coupled to a normal vibration mode.

The approach process stops immediately after starting it

Check that the polarity of the Detector output is set correctly.

The Detector unlocks during approach

If possible, operate your system in Self Exciting Oscillation or the Con-stant Excitation mode during approach. If this is not possible, you canwrite a software procedure that uses the COM-interface of the easyPLLplus software to automatically search for a resonance frequency after thesystem has unlocked (see easyPLL plus Detector programmers manual).

TROUBLESHOOTING

46

Theory of Operation

To explain how the easyPLL plus works, first the function of the compo-nents of the easyPLL plus system is explained. Then the interplay ofthese components in the various operating mode can be described, andfinally, the relative advantages of the frequency modulation (FM) modesare compared to those of the more conventional Lock-In mode.

Components of an easyPLL plus systemAn easyPLL plus system consists of at least a sensor, the Controller andthe Detector. The main parts of these components are described here.

The sensorThe sensor is the central component of the system. It consists of threeparts: the excitation that turns the excitation signal into a mechanicalvibration, the sensor itself and the detector that converts the sensorsreaction into an electrical signal.

Typical elements used are a piezo actuator for excitation, a silicon canti-lever with integrated tip as sensor and a laser deflection system with asplit photodiode as detector.

In OutSensor

In

OutLaser

Piezo actuator

Cantilever

Photodiode

SamplePhase=90˚

Phase=-90˚

When the sensor is approximated as a harmonic oscillator, the ampli-tude and phase response are given by:

THEORY OF OPERATION COMPONENTS OF AN EASYPLL PLUS SYSTEM

47

[ ] [ ]2

0

220

0

)/()/(1)(

Qffff

AfA

+−=

[ ]

−= −

0

201

/

)/(1tan)(

ff

Qfffφ

With Q the so called Q factor of the sensor. The bandwidth of the reso-nance peak is inversely proportional to the Q factor, resulting in a slowerresponse time of the amplitude for sensors with high Q factors:

tresponse = Q / 2 f0

The phase shift on the other hand changes very quickly.

The easyPLL ControllerThe Controller consists of two main parts that are connected togetherand to the various in- and output connectors of the Detector in a differ-ent way, depending on the setting of the Mode switch:

- An analog phase shifter. This produces a signal that is phase shiftedwith respect to the reference signal or the ‘Detector in’ signal.

- An Amplitude measurement that consists of an RMS-to-DC converter.

- An Amplitude controller, that consists of a PI-controller and a variablegain amplifier (multiplier). The amplification of the amplifier is eitherdetermined by the set point, or by the PI-controller, which changes thegain of the variable gain amplifier, so that the amplitude signal pro-duced by the Amplitude measurement is equal to the set-point.

A block diagram of all the components in the Controller is given infigure Block diagram of the Controller.

The easyPLL DetectorThe Detector consists of three main parts that are connected togetherand to the various in- and output connectors of the Controller in a dif-ferent way, depending on the setting of Operating Mode:

THEORY OF OPERATIONCOMPONENTS OF AN EASYPLL PLUS SYSTEM

48

Drive out

40kHzLow Pass

RMS

DC

AmplitudeDissipation Detector in

diff.Amp

Ref in

diff.Amp

Amp.

x10

Drive Amplifier

onoff

Filter

Mode

Drive out

x 1x 0.1

x 0.01

X

1 Self Osc.

1

2 Const. Ampl.

2

3 Const. Drive

3

Signal out

Phaseshifter

0˚..180˚

Phase

+0˚+180˚

+0˚+60˚

PI controller

x10x 100

x 1k

0V .. 10V

SetPoint

x1 .. x11

P-Gain

onoff

I-Gain

Block diagram of the Controller

- A voltage controlled digital oscillator (VCDO), that produces the ref-erence signal, and an internal signal, that has a digitally set phase shiftwith respect to the reference signal.

- A phase detector, consisting of a multiplier and a low-pass filter. Themultiplier multiplies the Input signal with the internal reference signal,resulting in a signal that contains a sin wave with the sum and difference

THEORY OF OPERATION COMPONENTS OF AN EASYPLL PLUS SYSTEM

49

frequency of it’s two input frequencies fInput

and fRef

. The low-pass filterfilters out the sum frequency. The output voltage, U

d , of the phase de-

tector varies with the difference frequency according to

Ud = sin(2π(fInput - fRef) t + φΙnput - Phase shift)/Kd ,

with φΙnput the phase shift of the Input signal relative to the Ref output.When the Input frequency is the same as the Internal frequency, and thephase difference between the two singals is small, the voltage at the dFBNC is given by

UdF

= sin(φInput

- Phase shift)/KdF

.

The voltage at the Output BNC is then

UOutput = (φInput - Phase shift)/KOutput + UOffset .

The sensitivities KdF and KOutput of the output signals in deg/V are dis-played in the status message area of the easyPLL plus software panel ofthe corresponding output.

- A PI-Controller that can regulate the output frequency of the VCDOusing a control voltage UdF. The control voltage is stable when the inputand the reference signal have the same frequency and 0° phase differenceφΙnput - Phase shift. When both frequencies are the same, and a 0° phasedifference has been reached, the loop is 'locked' This function principleis also called Phase-Locked-Loop (PLL).

To demonstrate the phase locking capability, lets assume that the inputand the reference frequency are the same, but the phase difference islarger than 0°. U

d becomes negative and the controller will increase the

reference frequency until the reference has caught up in phase with theinput, and the U

d becomes zero.

To demonstrate the frequency synchronising capability, assume that thephase difference is 0°, and f

Input becomes larger than f

Ref. Again the out-

put Ud becomes negative and the controller will increase the f

Ref until the

reference is again the same as fInput

. In the following, we assume that thePLL is locked, and f

Ref =f

Input.

The signal UdF is output at the BNC connector dF. The relation betweenUdF and fInput is:

UdF = (fInput - CenterFrq + OffsetFrq)/ KDeltaF .

THEORY OF OPERATIONCOMPONENTS OF AN EASYPLL PLUS SYSTEM

50

CenterFrq and OffsetFrq are entered in the easyPLL plus software. Thevalues of U

dF K

DeltaF and the responsivity K

DeltaF are displayed in the status

message area of the easyPLL plus software as DeltaF. The value of KDeltaF

and thereby the maximum detuning of the reference frequency genera-tor depend on the setting of Lock range: The frequencies in the Lockrange are mapped to the +-10 V output range.

In addition to the dF connector, the Detector has an Output connectorthat is meant to be used as the error signal for a Z-feedback loop. Itsoutput voltage, UOutput, is given by

UOutput=(fInput - CenterFrq + OffsetFrq + Offset)/KOutput .

The responsivity KOutput

of the output is displayed in the status messagearea under Output.

A more detailed block diagram of all the components in the detector isgiven in figure Block diagram of the Detector.

Operation of the easyPLL plus modesIn the following, the implementation of the frequency modulation (FM)modes (Self Exciting Oscillation, Constant Amplitude and Constant Ex-citation) and the Lock-In method are discussed. Each component andits interaction will be explained using simplified block schematics.

Self Exciting Oscillation modeIn the Self Exciting Oscillation mode, the Controller is configured tofeeding the sensor signal back to the sensor excitation. With suitablesettings, this feedback will cause the sensor to oscillate at its resonancefrequency. The Detector converts changes in resonance frequency to ananalog voltage that can be measured, or used for Z-feedback. Simultane-ously, the Dissipation output of the Controller provides informationabout the dissipation of the tip-sample interaction.

To achieve the Self Exciting Oscillation, two conditions must be ful-filled: First the multiplied amplification of the sensor element and thefeedback element must be equal to 1. Second, the total phase shift of allelements in the excitation loop must be 0°.

THEORY OF OPERATION OPERATION OF THE EASYPLL PLUS MODES

51

PI-Controller

PC Interface and Control

Voltage ControlledDigital Oscillator

Input BNC

Offset voltage fromeasyPLL Manual Controller

Parallel Port

Low PassFilter

LockDetector

Low PassFilter

Auto GainAmplifier

dF BNC

Amp.

Ref BNC

Am

p.

Output BNC

Amp.

Lock-In

Oth

er m

odes

120Hz400Hz

1300Hz

PolaritySwitch

pos/

neg

OutputAmplifier

TipGuard

on/o

ff

pos/

neg

CenterFrq

OffsetFrq

LockRange

OperationMode

Mixer

LockedLockedLED

Hi

Range LED

Lo

Mixer

X

+

<10V<1.7V<0.31V<54mV<9mV

x0.1x1x10x100

PhaseShift

TipGuardResonse

Amplitude

0V..10V

Block diagram of the Detector

THEORY OF OPERATIONOPERATION OF THE EASYPLL PLUS MODES

52

The feedback element consists of two parts:

• A variable phase shifter which compensates the phase shift of all theother parts in the loop.

• An amplitude controller that keeps the vibration amplitude constantby varying the amplification of the excitation signal.

dF

X

Amplitude

Dissipation

Amplitude Measurement

Detector as PLL

Sensor

Amplitudecontroller

Phase shifter

Self Exciting Oscillation mode

When the sensor is oscillating, it will be close to its resonance frequency,because the resonance conditions are most likely to be fulfilled there.The precise vibration frequency depends on the phase shift setting. Inorder to excite the precise resonance frequency, the phase shift must beset so that the amplification of the excitation signal (dissipation) is mini-mal, because the sensor responds with maximum amplitude to its reso-nance frequency.

With high quality factor sensors, the self oscillator will start to oscillateby itself as a result of thermal noise. With systems of low quality factor(like measurements in liquids), starting the self exciting oscillation is aproblem, as is high frequency noise. These drawbacks are solved in thePLL Resonance modes.

Constant Amplitude modeThe setup for the PLL controlled resonance with constant vibration am-plitude uses a modified Phased-Locked-Loop (PLL) circuit. The Detec-tor in this case also supplies the excitation signal (Ref ) for the sensor, aswell as the analog voltage proportional to the resonance frequency change.


53

dF

X

Amplitude

Dissipation


Detector as PLL

Sensor

Amplitudecontroller

Ref

Constant Amplitude mode

The PLL in the Detector will now change its output frequency until thephase shift between its input signal and excitation output is equal to thephase shift set in the control software. The optimum phase compensa-tion can be deduced from the excitation amplitude, or the steepness ofthe phase-frequency characteristic. The resonance frequency is reachedat minimum driving amplitude and/or at the steepest point in the phasecharacteristic.

The advantage of using the excitation signal from the Detector is that it(usually) has a higher spectral purity than the sensor signal. Moreover, itis able to excite the sensor even at low Q factors because it does not neednoise to start the cantilever vibration. The disadvantage of using theexcitation signal from the Detector, is that the resonance condition islost when the resonance frequency shift goes outside the Lock range ofthe Detector.

Constant Excitation modeBecause amplitude control is neither mandatory nor desired for certainmeasurements in the PLL operation mode, the amplitude controller inthe Controller can be turned off.

The PLL is simultaneously the excitation generator and the frequency tovoltage converter for the Z-controller. The amplitude controller providesdissipation information of the tip-sample interaction.

THEORY OF OPERATIONOPERATION OF THE EASYPLL PLUS MODES

54

dF

X

Amplitude


Detector as PLL

Sensor

Amplitudecontroller

Ref

Constant Excitation mode

Lock-In modeWith low Q factor sensors, it can be usefull to work in the Lock-In mode(also known as Tapping mode, Intermittent contact mode or Dynamicforce mode), and use the measured vibration amplitude or phase forfeedback. To operate in this mode the amplitude controller is switchedoff, and the Amplitude output of the Controller and/or the dF output ofthe Detector can be measured.

The Detector now simultaneously acts as a Sine generator that excitesthe sensor, and as a Lock-In Amplifier that measures the phase responseof the sensor.

dF

X

Amplitude


Phase Detector

Sensor

Amplitudecontroller

RefSine

Generator

Lock-In mode


55

Choosing an Operating mode.To explain why the frequency modulation (FM) modes are usefull, weshould first discuss the conventional measurement mode (Lock-In mode,also known as Tapping mode, Intermittent contact mode or Dynamicforce mode).

Lock-In mode or FM-mode ?In conventional AFMs, measurements are performed with constant exci-tation frequency. A sinus generator is used to excite the sensor (canti-lever) and the resulting vibration is measured.

The force interaction between the sensor and the sample causes the reso-nance frequency to shift, and the resulting change in amplitude and thephase response of the sensor can be measured using a Lock-In amplifier.

Either the phase, or the amplitude can be used for topography feedbackwith a Z-distance controller of the scan electronics.

The advantages of this approach are its simple design and application.Nevertheless it has severe drawbacks:

• To increase the force sensitivity, sensors with higher quality factors (Qfactors) are used. With these sensors, the response time of the ampli-tude to resonance frequency changes is increased. Thus, the maxi-mum possible scan speed is decreased. The response time of the phasedoes not suffer from this problem, but the phase shift is not clearlyassignable to changes in tip-sample distance and hence is difficult touse for controlling of the tip-sample distance.

• When the change in the resonance frequency is large compared to thewidth of the resonance peak, the feedback signal saturates, and evenchanges sign in the case of amplitude feedback. This problem be-comes more severe when using high Q factors.

• Both the amplitude and the phase signal are a mixture of two pieces ofinformation: firstly the tip sample interaction (distance) and secondlythe quality factor of the resonator. Thus a variation of the tip-sampledistance or a change in the quality factor can’t be distinguished; bothmay alter the phase response (see figure Lock-In mode signals).

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Lock-In mode signals: Changes in resonance frequency and Q factor influence both the measuredamplitude and the measured phase.

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With the FM modes, the sensor always vibrates at its resonance fre-quency, and changes in the vibration frequency and vibration/excitationamplitude are measured.

In the FM mode of operation, the phase response of the sensor is used tocontrol the sensor’s vibration frequency: The vibration frequency is al-ways adjusted so that the phase shift corresponds to the phase shift atresonance. In a second order harmonic oscillator, this phase shift is 90°(see figure Resonant oscillation). This is independent of the sensors qual-ity factor, because the resonance curves at different quality factors allintersect at the resonance frequency at a 90° phase shift (see figure Reso-nant oscillation). Moreover, the phase shift, and resonance frequencychange instantaneously when the tip-sample interaction changes.

The FM modes thus eliminate several drawbacks of the conventionalLock-In method:

• The scan speed can be high even with high quality factors, because thesystems response speed is independent of the sensor’s quality factor.

• The resonance frequency shift does not saturate for large tip sampleinteractions.

• There is a better separation between topography and other sampleproperties. The resonance frequency shift is a measure for the con-servative tip-sample interaction and the sensors amplitude dampingis a measure for the dissipative tip-sample interaction. When one sig-nal is used for topography feedback (usually the resonance frequencyshift), the physical interpretation of other signal is more straightfor-ward.

For these reasons, use the Lock-In mode only for measurements with lowQ factor sensors.

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FM-mode signals: Changes in resonance frequency only influence the tracking, changes in Qfactor only influence the amplitude/dissipation

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Which FM mode should be used?The choice between the resonant oscillation modes depends on severalfactors. On the one hand, Constant Amplitude and Constant Excitationmodes offer the best signal to noise (S/N) ratio. On the other hand, theSelf Exciting Oscillation mode is more robust against large changes inresonance frequency and amplitude when used with high Q sensors.Therefore, we suggest choosing the operation mode depending on thesensors Q:

• Q > 100000: The S/N ratio difference between the Self Exciting Os-cillation and Constant Amplitude and Excitation modes becomes neg-ligible.

• 500 < Q < 100000: Constant Amplitude or Excitation mode offer abetter S/N ratio.

• Q < 500: Constant Amplitude or Excitation mode should be used,because it becomes difficult to sustain self exciting oscillation whenthe Q factor is low.

The relative advantages of the controlled vibration mode and the con-stant drive mode are still a matter of scientific debate. The measurementresults for controlled vibration mode are easier to understand theoreti-cally, whereas the operation of the constant drive mode is simpler tooperate, and thus more reliable. The controlled vibration mode is gener-ally preferred in European countries and the USA, whereas the constantdrive mode is preferred in Japan.

Literature

1) Introduction of using FM-Method: T.R. Albrecht, P. Grütter,D. Horner, and D. Rugar, J. Appl. Phys. 69, 668 (1991).

2) Dynamic force microscopy by means of the phase-controlled oscilla-tor method - a comparison between Self Exciting Oscillation modeand Constant Amplitude mode. U. Dürig, H.R. Steinauer andN. Blanc, J. Appl. Phys. 82, 3641 (1997).

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3) Phase Locked Loop - Design, Simulation, & Applications, RolandE. Best, McGraw Hill, ISBN-0-07-006051-7.

4) A direct method to calculate tip-sample forces from frequency shiftsin frequency-modulation atomic force microscopy. F.J. Giessibl,Appl. Phys. Lett. 78, 123-125 (2001).

5) Introduction of using AM-Method: Y. Matin, C.C. Williams,H.K. Wikramasinghe, J. Appl. Phys. 61, 4723 (1987).

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