Universal frequency reference
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Universal Frequency Reference. Presented first at Gippstech 2012 V1.11 Glen English VK1XX [email protected] Frequency reference system. Provides reference for any radio Low noise fundamental output 1Hz – 150 MHz Provides 30 mHz steps with 125 MHz clock

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Universal Frequency Reference

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Universal Frequency Reference

Presented first at Gippstech 2012

V1.11 Glen English VK1XX [email protected]


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

GPS

Frequency counter

XO

CPU

LPF and driver

DDS


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

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

9.9 MHz

0.5Hz

1Hz

GPS

/2

19.8 MHz

9.9 MHz

GPS data

Divider/1,2,4,8,16

Divider/1,2,4,8,16

CPU+counter

XO

serial

19.8MHz

Multiplierx 1,4,5,6..20

13.2MHz

LPF and driver

DDS

~118.8MHz


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 TCXOs


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 MHz (0.1ppm/sec)

100 MHz

(0.01 ppm/sec)

GPS

10 MHzXO

X10VCO-PLL

CPU/counter

LPF and driver

DDS


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


DDS tutorial :

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


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