Message-ID: <003d01c26643$4e792780$045bfea9@RSGB613192>
From: "Andy talbot" <G4JNT@THERSGB.NET>
To: rsgb_lf_group@blacksheep.org
References: <DDC408CAE72CD511827A0002A5131CD6D9F782@exc_wil08>
Date: Fri, 27 Sep 2002 17:29:32 +0100
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Subject: LF: RE: Narrow CW filter using 32kHz clock crystals?
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This filter is shown in the LF Experimenters Handbook on page 25, as part of
the outline design for a narrow bandwidth LF receiver.  The frequency
response I measured is given on page 26

If you view the output on a scope and hit it with a signal generator, as you
tune over the passband, the response first appears as a bouncing trace which
grows to its maximum over a few seconds.  A step change causes it to ring
for a few cycles - but bear in mind that all filters (except Gaussian
designs) will ring.  This just happens to be so narrow that the ringing is
clearly visible on a scope - it is a rather surreal experience actually
seeing the transmient response of a crystal filter in real time !

See you spotted the deliberate mistake on the spec,   50kHz impedance

Quite correct, it should of course read 50 k ohm

Andy  G4JNT


----- Original Message -----
From: Ashlock,William <William.Ashlock@edwards.boc.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: Friday, September 27, 2002 3:04 PM
Subject: LF: RE: Re: Re: RE: Narrow CW filter using 32kHz clock crstals?


> Hello Andy, Hugh, and all filter lovers,
>
> I'm concerned about the transient response since this is the REAL measure
of
> a good filter. How does it respond to a step input? It could ring for
> minutes!
>
> Hugh, please post the results of the spectrum test when you complete it.
>
> Thanks,
> Bill A
>
> >Hello Andy
> >I think I've answered my own question here.
> >I built the filter and it doesn't work with 50 ohm input/output
impedance,
> >but it DOES work with 50k ohms. Narrow isn't it ! I can just about hit
the
> right frequency with >a signal generator and see some sine wave on the
> scope. I haven't had a chance to put it on the >spectrum analyser yet.
>
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