wrote in message
...
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

Allison's advice is right on the mark.
I recently built an amateur only band HF receiver and chose 6/2.5/0.5 for
the three modes.
I think what Allison meant to say about the peaked audio filter was to use
it in conjunction with the SSB filter for CW selectivity.

73,
Dale W4OP