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Date: Tue, 01 Jan 2002 21:10:25 +0000
To: rsgb_lf_group@blacksheep.org
From: "Walter Blanchard" <blanch@pncl.co.uk>
Subject: LF: Frequency Standard
In-reply-to: <5.1.0.14.0.20020101114249.01df2650@pop.tiscalinet.it>
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Re Marco's comments:

MSF is a simple absolute frequency standard accurate to 2 in 10-12.  Add 
about 20 dB amplification to a 60 kHz ferrite rod antenna  and there'll be 
plenty of carrier to see on a scope (in the UK anyway).  Make up a divider 
to get your 5 or 10 MHz standard down to 100 or 10 khz and use it to lock 
the timebase.  (Locking direct on MSF doesn't work on most scopes because 
MSF pulses every second and the scope loses phase in the breaks).  Then 
fiddle your osc until it stops drifting.  Takes a long time if you want to 
get into the 1 in 10-10 class (about the best any xtal osc will do 
short-term even if double-ovened) because one cycle drift at 60 kHz takes 
about three hours at this stability level.

HBG on 75 kHz could also be used directly with the right dividers but  DCF 
on 77.5 would be a bit more complicated. You could have a go at using Loran 
on 100 kHz if you're into this sort of thing.

That'll give you an excellent absolute frequency standard but what about 
drift in your Tx (and Rx) oscs?  To take full advantage of 60-sec dots and 
Argo/Spectran you need to drift less than 0.02 Hz in 180 secs dash length. 
That's 1 in 10-10.  You need a double-ovened proportional-control reference 
osc to do this, even short-term. The usual "high-stability" ham options are 
only good to  1 in 10-7.

Stewart - careful with those Piezo/HCD/HP oscs; they have different 
pinouts. Also, the Piezo ones are fast warmup and need about 0.5A at 24v to 
get going while HCD/HP need only 0.2A.  The HCD are somewhat sensitive to 
attitude (g effects) although OK if kept in one position and are trickier 
to adjust than Piezo/HP.

Marco - it's the rubidiums that get poisoned quicker, average life about 5 
years.  The cell in my Efratom unit is on its last legs after 7 years. 
Caesiums get noisy after 10 years.  90% of the GPS sats that have died did 
so because all four Rb/Cs went down after 10 years or so.  Old Rb/Cs units 
are worth buying only for the xtal oscs in them.

Walter G3JKV.