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Universal Frequency Reference

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Universal Frequency Reference

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Universal Frequency Reference

Presented first at Gippstech 2012

V1.11 Glen English VK1XX glen@pacificmedia.com.au

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.

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

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

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