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Date: Sat, 19 Mar 2011 17:52:08 +0000
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From: Andy Talbot <andy.g4jnt@gmail.com>
To: rsgb_lf_group@blacksheep.org
DomainKey-Status: good (testing) 
Subject: Re: LF: Re: Frequency Stability
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You need to take into accout the frequency period over which you need
stability.=A0=A0=A0 A locked frequency reference used for LF has very
different requirements over one used for stabilising moicrowave
sources - even though the headline stability,=A0 typically 10^-9 may be
similar for both ends of the spectrum.

Firstly consider microwaves 10 and 24GHz in particular wher enerly
everyone uses GPS locked sources.=A0 Short terms stability over seconds
to tens of seconds is necessary there, as otherwise SSB and CW =A0will
drift and.=A0=A0 A Simple GPSDO like the one on my website is good enough
just, but the 10 - 20Hz of frequency variation can be heard as a
steady up and down drift in tone frequency.=A0=A0=A0=A0=A0 A better GPSDO=
 is the
VE2ZAZ design, using teh 1 PPS to freqeuncy steer (not lock!) an
inherently good to start with OCXO.=A0 I have two of these, one runing
24-7 as a master reference, and another set for faster lockup and
resulting lower spec for /P operation.=A0 Both give better thatn 10^-10
over just about any measurement period I chhose to test over.

But for LF the requirements are very different.=A0=A0 Short term stability
over seconds is irrelevent if the signalling interval is much longer
than this, provided it remains locked at all times.=A0=A0 So now you can
sacrifice short term stability and hence no longer need an OCXO as the
primary source.=A0=A0 In fact, the 1 PPS signal from the GPS can be used
directly to steer a simple TCXO.=A0 The note may sound horribly drifty
at HF/VHF but provided the average stays locked with a time constant
in teh tens of seconds to keep within TCXO drift. it will be fine for
LF/VLF.

So a quite simple GPSDO can be made up like this, without having to
rely on a specific GPS module with a 10kHz output.  In RadCom October
2002 I did a "GPS Locked Frequency Standard for LF Signalling=A0"=A0=A0Tha=
t
did what it was supposed to do on 137kHz =A0for very long duration
signalling, although I wouldn't recommend doing it like that now with
its obsolete PIC and a bodged bit of software.=A0=A0 There is a=A0far bett=
er
PIC with internal comparator / timer - the 16F628 and can be used in
an identical way to the VE2ZAZ design=A0but with a TCXO and making it a
lot simpler.

G3PLX even=A0constructed an analogue PLL using the 1 PPS signal from a
GPS module and a TCXO - not a uProc in sight!

Off air radio sources may give you semi-decent short term stability
during daytime and the middle of the night, if interference is not
present, and teh antenna tuning/ phase isn't altered, and, and, and...
Their stability over hours is of dubious value; and this is the very
requirement for LF.

So, conclusion.=A0 I would avoid off air such as MSF and Droitwich
locked standards for very narrow band long duration working.=A0=A0 Go for
GPS locked stability using the 1 PPS output from any low cost GPS
module.=A0 Either build a VE2ZAZ GPSDO, or have a go at a simple PLL
that is probably as chirpy as anythiing over several seconds, but will
remain reliably locked and stable over minutes to hours to days.

See www.g4jnt.com for a lot of the work I've done on frequency
standards and frequency locking over the years.

Andy






On 19 March 2011 17:09, James Moritz <james.moritz@btopenworld.com> wrote:
>
> 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 gen=
erates 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 100u=
Hz 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 otherw=
ise free-running, at a frequency which is calibrated via a fine-tuning tri=
mpot. The OCXO is a HCD71 module, which has specified ageing rate <1E-8 pe=
r 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 use=
d (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 tha=
t, when the external reference is removed, sets the frequency control volt=
age 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 mod=
ified 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 si=
gnificantly.
>
> 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 dif=
ferences down to the parts-per-billion level quite easily. It seems the 19=
8kHz 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 faste=
r 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 ove=
r periods of hours, it seems to me it would be better to operate the synth=
esiser part in "unlocked" mode, and periodically trim the OCXO frequency=
 using the receiver part. Otherwise, there are bound to be glitches in pha=
se when it switches between off-air reference and OCXO, as it does from ti=
me 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
>