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:[email protected]]
Sent: 2002/09/25 18:49
To: [email protected]
Subject: LF: Narrow CW filter using 32kHz clock crstals?
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