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M1BYT > TECH 10.08.03 22:55l 59 Lines 2635 Bytes #999 (0) @ WW
BID : 0B1648M1BYT
Read: DB0FHN GUEST OE7FMI
Subj: Re: Off air lock for ref osc.
Path: DB0FHN<DB0RGB<DB0MRW<DB0SON<DB0SIF<DB0CWS<DB0ZDF<DB0LJ<DB0RES<ON0AR<
VK3TE<ZL2BAU<GB7FCR
Sent: 030810/2042Z @:GB7FCR.#16.GBR.EU #:9132 [Blackpool] FBB-7.03a $:0B1648M1B
From: M1BYT@GB7FCR.#16.GBR.EU
To : TECH@WW
G8MNY Opined with considerable skill:-
> Here is my method of locking the master osc in a PLL signal generator to BBC
> 625 line TV syncs. (BBC Ceasium ref better than 1/10^9), obtained from any TV
> or video. This can be better than other lock sources there ie less fading etc.
>
> NB. It is of no use for anything other than analogue UHF TV main station or
off> air fed repeaters. Digital sources are NOT accurate as they only use local
> timebases. In the UK only BBC has accurate syncs, CH4 is quite accurate, ITV
> are poor, CH5 are no use as it is usualy from a digital sat feed!
>
> This design will work with any 1MHz clock that has (or can have) a varicap to
> trim its frequency. So it can easily be used on a frequency counter if a 1MHz
> clock can be derived or extracted & will greatly improve the accuracy &
> temperature problems of simple Xtal timebase.
Hi,
I designed a much simpler version using just a PLL IC some 20 years ago.
It uses the same 15,625Hz pulse, but derived directly from the
deflection circuit of an old battery portable B&W TV, generating a
stable 1Mhz reference as output.
All I do is plug the unit into the 15,625 output of the TV, ensure the
TV is tuned to the BBC. It suffers a little jitter at shorter period
settings of the frequency counter, but is superbly accurate at longer
gate times of 10sec.
Note the TV MUST be tuned in and firmly locked to a signal (no rolling
frames). No signal means the timebase will just free wheel.
Its obviously no good at all for providing an external reference to the
counter, but perfect as a means of checking the calibration is spot on.
I will say though that after my first calibration of my Racal, I have
never needed to re-adjust it again.
If anyone fancies building my version and doesn't mind moding a TV, I'll
trace out the circuit.
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º 73 from Harry, M1BYT @ GB7FCR.#16.GBR.EU º
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º International Museums Weekend 19th+20th June 2004 º
º www.ukradioamateur.org/imw/index.html º
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º The ARC's FL/RAE courses + Newcomers Intro to º
º to amateur radio:- http://www.ukradioamateur.org/ º
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º QTH: Garforth, W. Yorks.. º Timed 20:42 º
º harry.m1byt@tiscali.co.uk º 10 Aug 2003 º
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