Dear James and LF group,
I have one of these units but was unable to find out any info on the
internet.
I figured the DDS section OK and used it to set up the receiver for T/A
tests some time ago.
I am no longer active on LF so am looking for a home for the PFS-1
I can be contacted on <[email protected]>
Regards
Peter, G3LDO
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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