Universal Frequency Reference
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Transcript of Universal Frequency Reference
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Universal Frequency Reference
Presented first at Gippstech 2012V1.11 Glen English VK1XX
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CT
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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
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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.
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Basic Block diagram
XO Frequency counter
GPS
DDS LPF and driver
CPU
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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
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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
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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.
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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.
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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 ?
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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
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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
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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.
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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
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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
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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
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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.
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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.
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Oscillator limitations
• Internal correction of some cheap TCXOsfreq
temp
1ppm
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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
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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.
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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 .
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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
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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.
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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
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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.
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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)
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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
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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
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AR
http://www.analog.com/static/imported-files/tutorials/450968421DDS_Tutorial_rev12-2-99.pdf
DDS tutorial :