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From: "g6tmk" <g6tmk@thersgb.net>
To: rsgb_lf_group@blacksheep.org
References: <01C26537.C1466D20.g4jnt@thersgb.net>
Date: Fri, 27 Sep 2002 10:39:16 +0100
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Subject: LF: Re: RE: Narrow CW filter using 32kHz clock crstals?
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Hello Andy,
In your recent posting about the 33kHz crystal filter, can you clarify what
the characteristic impedance is ...
It is very unusual to specify an impedance in terms of a frequency, so I
feel there must be some mistake.
Thanks
Hugh M0WYE

----- Original Message -----
From: "Andy talbot" <g4jnt@thersgb.net>
To: <rsgb_lf_group@blacksheep.org>
Sent: Thursday, September 26, 2002 8:36 AM
Subject: LF: RE: Narrow CW filter using 32kHz clock crstals?


> Its was I ...........
>
> 1)  Take a handful of watch crystals, from the same batch  (buy 10 or 20
from
> Farnel / Maplin ??)
> 2)  Connect  5 in series to form the top elements of a ladder filter; you
now
> have 6 connection nodes, taking 1 as the input, 6 the output
> 3)  Connect shunt capacitors to ground as follows, from node 1  47pF,
nodes
> 2,3,4 and 5   82pF,  and node 6  47pF.   This gives a ladder filter with a
> characteristic impedance of around 50kHz, a 3dB bandwidth in the region of
> 2.2Hz and 10 dB loss.  Centre frequency is 32765.5Hz - the series
resonance of
> a 32768Hz device  cut for  || resonance.    The loss is high enough to
smooth
> out significant ripples so I made no serious attempt to get a decent
response
> shape - it was rounded - but with a lot of patience it could be improved
no
> doubt.
> 4) All attempts to use the crystals in thier parallel mode, with top
coupling
> capacitors lead to very high losses that would have needed a
characteristic
> impedance of meg-ohms to get operational.
>
> I didn't think much of the xtal filter idea idea and abandoned it for a
DSP
> solution using the 56002EVM.
>
> Andy  G4JNT
>
> -----Original Message-----
> From: Tracey Gardner [SMTP:tracey.gardner@ntlworld.com]
> Sent: 2002/09/25 18:49
> To: rsgb_lf_group@blacksheep.org
> Subject: LF: Narrow CW filter using 32kHz clock crstals?
>
>  << File: ATT00001.txt; charset = Windows-1252 >>
>
>
>