Dear Ken, Wolf, LF Group,
I have been playing around with the insides of a Halcyon PFS-1
Droitwich-derived frequency standard over the last few days, and some
observations might be of interest.
The Receiver part of the PFS-1 is an off-air frequency standard that
generates 10MHz, 5MHz and 1MHz outputs from the received 198kHz carrier, and
is quite similar to the Quartzlock 2A device on Ebay. It has an external
ferrite rod antenna, a single crystal 198kHz filter, limiting amplifier and
a PLL which locks a 10MHz VCXO to the 198kHz signal. Most of the time it
stays locked, but sometimes it becomes unlocked for a few minutes at a time,
for reasons unknown. The 10MHz output is switched off when this happens.
The rest of the PFS-1 is a DDS synthesiser that covers 0 - 16MHz in 100uHz
steps. It has an internal 20MHz OCXO module, which is phase locked to the
10MHz reference from the receiver when that is present, and is otherwise
free-running, at a frequency which is calibrated via a fine-tuning trimpot.
The OCXO is a HCD71 module, which has specified ageing rate <1E-8 per day.
The OCXO module in isolation seems to do much better than that, but when my
PFS-1 was free-running, I found the stability was rather worse. After poking
around inside, I think this is partly due to the trimpot used (a 20k, 22
turn component - these things are not great in my experience, not very
stable and with a lot of backlash in the adjustment), and partly due to the
circuit connected to the OCXO frequency control input, which is the PLL
amplifier/loop filter, with the addition of some switching that, when the
external reference is removed, sets the frequency control voltage using the
trimpot, another resistor and a 78L09 regulator IC.
Bearing in mind that 1 part in 10^8 requires the 0-6V frequency control
voltage to be held stable to within 1mV or less, this needs quite a good,
stable, bias voltage and pot to achieve the specification. I have now
modified my unit to use the 6V output from the OCXO module and a 10k, 10
turn helipot on the front panel, with a CMOS switch to select the PLL output
when operating. This seems to have improved the free-running stability
significantly.
It is interesting to compare the free-running OCXO output to the 10MHz
locked receiver output on an oscilloscope. Once the OCXO is trimmed, the
phase of one waveform "walking" relative to the other shows frequency
differences down to the parts-per-billion level quite easily. It seems the
198kHz carrier has some kind of cyclical drift in phase which occurs over a
period of a minute or two (the receiver PLL seems to filter out the faster
phase modulation data), so the direction of "walking" changes from time to
time.
For using the PFS-1 as a frequency source for high stability signals over
periods of hours, it seems to me it would be better to operate the
synthesiser part in "unlocked" mode, and periodically trim the OCXO
frequency using the receiver part. Otherwise, there are bound to be glitches
in phase when it switches between off-air reference and OCXO, as it does
from time to time. Also the observed wobbling phase of the 198kHz carrier,
plus any ionospheric effects around darkness hours, would be reflected in
the synthesiser output. The same possibly applies to the Quartzlock 2A.
Cheers, Jim Moritz
73 de M0BMU
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