Universal Frequency Reference
Presented first at Gippstech 2012
V1.11 Glen English VK1XX
glen@pacificmedia.com.au
CT
Frequency reference system
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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
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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
XO
Frequency
counter
CPU
DDS
LPF and
driver
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
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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
FTW 
2 ^ 32 Fout
ActualClockFreq
Write to DDS
Outputs
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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
Divider
/1,2,4,8,16
19.8 MHz
XO
0.5Hz
CPU+
/2
1Hz
GPS
9.9 MHz
Divider
/1,2,4,8,16
counter
GPS data
serial
19.8MHz
Multiplier
x 1,4,5,6..20
~118.8MHz
DDS
13.2MHz
LPF and
driver
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
1ppm
freq
• Internal correction of some cheap TCXOs
temp
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
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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
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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)
10 MHz
XO
100 MHz
(0.01 ppm/sec)
GPS
X10
VCO-PLL
CPU/
counter
DDS
LPF and
driver
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
DDS tutorial :
http://www.analog.com/static/importedfiles/tutorials/450968421DDS_Tutorial_rev12-2-99.pdf