Message-ID: <4D8716A2.1020506@o2.co.uk>
Date: Mon, 21 Mar 2011 09:13:06 +0000
From: Peter Dodd <g3ldo@o2.co.uk>
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References: <D747086FAE0644ABA22E9087CEE71EAF@lindavideo> <D2E1F496D4EA447DBFB9A7A1E1EBF115@home2361108df7> <4D83CAEE.2060305@freenet.de> <3D88D5AFC0094751A8CA041C9700D50E@JimPC>
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Subject: Re: LF: Re: Frequency Stability
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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I am no longer active on LF so am looking for a home for the PFS-1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 <g3ldo@o2.co.uk>

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
>
>