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