Universal Frequency Reference

Presented first at Gippstech 2012V1.11 Glen English VK1XX

glen@pacificmedia.com.au

CT

Frequency reference system

• Provides reference for any radio• Low noise fundamental output 1Hz – 150 MHz• Provides 30 mHz steps with 125 MHz clock• Locked to GPS, auto holdover• Low Power (0.5-1.5W depending on power

supply and output ) and 60 x 80 mm• Can be controlled/setup from PC

Implementation

• Any GPS provides 1 pulse per second• Uses a DDS (direct digital synthesiser)• Free running TCXO or OCXO provides clock• Frequency of XO not critical• Many XOs do not have external V ctl- not

required.

Basic Block diagram

XO Frequency counter

GPS

DDS LPF and driver

CPU

How DDS works (simplified)• Consists of a binary counter and an adder• The counter has a maximum value• The RF output is connected to the highest bit

(MSB) of the counter.• A clock is input which every time there is a

positive-going transition, a fixed value is added to the counter.

• The amount added to the counter every ‘clock’ determines the how often the counter rolls over its maximum value

DDS counter

• 4 bit binary counter being incremented with value of 3 every clock.

• 0000,0011,0110,1001,1100,1111,0010,0101,1000,1011,1110,0001,0100,0111,1010,1101

• 4 bit binary counter being increment with value of 1 every clock

• 0000,0001,0010,0011,0100,0101,0110,0111,1000,1001,1010,1011,1100,1101,1110,1111,0000,0001,0010,0011,0100

DDS cont

• Example• Counter with max value of 100• If a clock adds a value of 5 at 1MHz, what will

be the rollover rate per second?• = (clock freq * step) / counter max (eq1)• = (1,000,000 * 5 ) / 100• = 50,000 times per second.

DDS cont2

• This DDS :• can be clocked up to 400 MHz• Has a rollover value of 2^32=4,294,967,296• Allows for very precise frequency steps if used as a

synthesiser• Using (eq1)• 125e06 * 100,000 / (2^32) = 2910.383046 Hz• 125e06 * 100,001 / (2^32) = 2910.41215 Hz• Cosine lookup table is connected to the counter so

that the DDS generates sine as well as square waves.

Frequency control

• Precise DDS frequency steps allow us to use any source frequency for any output frequency

• DDS has clock multiplier to further enhance flexibility.

• But no control over frequency of source oscillator ? How do we lock this to the GPS ?

Frequency Counter

• We count how many cycles of the fixed XO occur between 1PPS from the GPS

• If 63,000,005 oscillator cycles are counted for each 1pps GPS pulse, the frequency must be 63,000,005 Hz

• Now we know the frequency of the XO

CPU calculation• Think of DDS as a fractional divider (for the

moment)• For 10 MHz output , we must program the DDS

steps for (63,000,005 / 10,000,000)• Which is 6.3000005. which we can do….• The XO frequency is measured every 2 seconds

and the new ‘divisor’ (step) is applied to the DDS • Enables drift in XO to be compensated for• Averaging of different lengths are provided to

enhance precision

Implementation

• I figured this out when building WSPR DDS based exciters- I had odd frequency XOs available

• PCB costs about $50 of bits depending on the type of oscillator used.

• Better results with better quality oscillators -can work with $1 oscillator if does not change too much per update cycle. Proto used $4 125MHz TCXO.

• Care taken to ensure no feedthru noises from digital controller into oscillator.

CPU job :Count clocks per GPS 0.5 pps pulse

Update moving average

Calculate actual XO frequency

Calculate new Frequency Tuning Word

kFreqActualClocFoutFTW 32^2

Write to DDS

Outputs

• PCB has:• 100mW RF driver• Opto isolated closures• Serial port for config/ctl• DAC output for audio tone generation • Can accept any oscillator 5 to 125 MHz input

Detailed Block diagram

XO

DDS LPF and driver

Divider/1,2,4,8,16

Multiplierx 1,4,5,6..20

CPU+

counter

Divider/1,2,4,8,16

/2 GPS

19.8 MHz

9.9 MHz

9.9 MHz

19.8MHz

~118.8MHz13.2MHz

1Hz0.5Hz

serial

GPS data

Jitter Notes

• Jitter performance of output limited to jitter performance of source XO

• DDS output inherently has jitter equal to the DDS clock on output – this is why we low pass filter

• On board filter design important to reduce jitter• Use highest DDS clock (by using on-chip

multiplier) to ease filtering requirements• Jitter important when reference is multiplied up

to 10 GHz.

Limitations

• It is basically a frequency counter.• Longer counting times will yield more precision.• Compared with counting for one second , If the

number of cycles over 10 seconds are counted, there is 10x the precision, as the ‘error’ produced is 10x what it would have been over 1 second.

• Or average the 1 second results over 10 seconds (take avg of 10 numbers) , -same though bias in the number crunching must be removed.

Oscillator limitations

• Internal correction of some cheap TCXOsfreq

temp

1ppm

Moving averages

• Currently a moving average is used –• for each GPS 1pps pulse, the last n counts are

added together and divided by n. • Update is therefore on the fly, but incapable

of tracking changes faster than the filter length because current estimate is made up of last n values.

• Thermal drift limit is imposed on the XO• This goes for all disciplined oscillators

Accuracy and Precision

• Averaging improves error precision• Accuracy is based on 1pps GPS output• Count 1,000,000 cycles over 1 second• = 1Hz precision (1ppm)• Count 10,000,000 cycles over 1 second• = 0.1 Hz precision (0.1ppm)• Faster counters yield improved basic

precision.

Improving precision

• Higher precision per counter gate time (1 pps) yields better drift tracking capability.

• Averaging improves precision but takes time• Sure we can get 0.00001 ppm if we wait a long

time.• Some applications required good precision hold

and absolute frequency accuracy is unimportant.• Some applications required high accuracy – IE

blind netting on 10 GHz .

XO Thermals

• Averaging with drifting XO just takes average of the frequency over the drift. Moving average is behind the time.

• Yes more precision due to averaging.• But drift over averaging period reduced

accuracy.• 10 MHz 1PPM XO (0-70C ) : if drifts 5 deg C• Drifts 0.0714ppm. A country mile

Drift calcs

• 0.0714ppm. (5deg C)Not a country mile if over days.

• If 10 MHz counter clock, 0.1Hz precision per 1 second gate.

• = 0.1 ppm• Desired precision 0.01ppm = 10 sec

averaging/counting.• Max thermal drift over 10 seconds is 0.7deg C.

Solution to drift problem

• 2nd order predictor• The future events can be predicted from the

previous events• Useful for warm up / warm down drift• Non linear change with time variations OK• Not useful for random drift

Drift 2

• Solution to short term random drift• Higher counter frequency• 30MHz counter clock = 0.0333 ppm/ sec• Vs 10 MHz clock = 0.1 ppm/sec• Averaging over long periods provides further

precision but system can respond to short term drifts at high precision.

More basic precision by add clock multiplier

10 MHzXO

GPS

DDS LPF and driver

CPU/counter

X10VCO-PLL

10 MHz (0.1ppm/sec)

100 MHz(0.01 ppm/sec)

Next version

• 48 bit DDS will provide 1mHz control steps at 10 GHz

• Higher counter speeds (32 MHz)/slave osc.• Predictor improvement.• Need to port 128 bit math lib to micro.• On board GPS receiver opt. (adds about $50)• High Z square wave output.• More flexible power supply

Extras

• Also functions as a stand alone FSK style beacon – WSPR implemented.

• Can connect to PC to provide steps smaller than CAT control provides for doppler tracking.- FT817 10 Hz CAT steps example.

• Radio will follow the reference frequency blindly.

• Fast to get going (20 seconds after gps aq.)• Can do chirps, FM, PSK, FSK

AR

http://www.analog.com/static/imported-files/tutorials/450968421DDS_Tutorial_rev12-2-99.pdf

DDS tutorial :