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October 11th 05, 01:20 AM

wrote:
On Mon, 10 Oct 2005 20:48:51 +0100, John Wilkinson
wrote:

Hi,
I have bought a 45MHz crystal filter with a bandwidth of +/-10KHz,
according to the spec.
Now does this mean a real bandwidth of 20KHz?

Likely the case. Some spec total bandwidth other specify 6db edge
as distance from the filter center frequency.

If so when a receiver states a bandwidth of 7KHz, is that +/-7KHz, ie
14KHz?

If they didn't put the +/- there then its 7khz total.

If I am designing a good communications receiver from 6KHz-30MHz to receive
AM, SSB and CW and want 2 good filters, what are the bandwidths I should
consider?

AM wide 12-16khz
Am narrow 6-10khz

SSB anywhere from 2-3khz with many around 2.4khz wide Any wider
than 3khz will be poor in crowded bands. I happen to prefer 2.1 to
2.3khz.

CW I've seen 1.4khz all the way down to 200hz most consider
400-600hz adaquate.

In all cases the skirt selectivity usually bandwitdth measured from
the 6 to 60db points are important indicators of filter quality and
any value of 2 or less is good enough and 1.4 would be excellent.
The idea is you'd ike to be able to put the offending signal outside
the bandpass and well attenuated.

What some builders do for CW is use the CW filter and use an peaked
audio filter to narrow the audio band pass. Not quite as effective
but often cheaper.

Allison
KB1GMX

That 45mhz would make a good 'roofing' filter to use ahead of a 455khz
if filter. Normally using a first if of 45mhz and a second if of 455khz
would result in 'second order' images leaking in, but with a
good roofing filter ahead of the second mixer the problem is solved.

Some years ago, I bought a bunch of 9mhz if filters at the Dayton
Hamvention. They are 3.2khz BW (at the 6db points) filters made by
CF Networks for the Gonset Sidewinder transciever. I think this was
a vhf rig, which would account for the wider bandwidth. However, these
are 8 pole filters, with a shape factor of 1.08 @ 15db down (rises
to 1.23 @ 45db down). With this shape factor, these filters probably
have similar rejection of off frequency qrm as would a 2.4khz filter
with a 2.0 shape factor. Since I have a bunch of them, I could use
2 or even 3 of them in the IF stage (one after the mixer, one before
the detector, and one between stages). This would decrease the
apparent shape factor even more. The nominal carier frequencies for
these filters were 8998.0 khz and 9001.7 khz. With the 3.2khz
bandwidth this put the cariers 250hz outside the stated bandwidth
(-15db point for these filters).

I wonder just how these filters would actually work out in a rig
today. That 3.2khz bandwidth DOES seem a bit wide, but the crazy
shape factor seems to make it worthwhile to try.

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