At the time I did my test, I had a "hole" in my generators. The audio
gear went as high as 100k, and the RF gear went as low at 1Mhz, but the
IF was 455Khz. Hence I tested the whole audio path.

The impedances in the radio itself will effect the performance of the
filter, so there is something to be said for in-situ measurements.

Oh yeah, note that Abramoff didn't donate any money to the Democrats.

rkhalona wrote:
Michael Thorpe wrote:
Amongst all these political propaganda posts, I wonder if there is a
little room for a technical question related to radio please?

If so, here goes:

How does one measure the IF filter shape factor in a shortwave radio?

I am technically minded and do have a signal generator, etc., but I am
not sure of the exact procedure. Do I get the audio level at the peak
frequency, then detune both sides off the peak for -3dB of the
demodulated audio, note down the width between these points, then
detune to obtain for -50dB of the demodulated audio, get the width at
these points, and then the ratio of the two widths is the filter shape
factor?

I would appreciate a link to the exact procedure if there is one
(don't seem to be able to find this in Google) or an expert advice
please.

I would also appreciate advice as to what actual values of filter
shape are considered poor, good, excellent, etc...

Thanks in advance,
Michael

You need to generate a swept frequency response of the IF filter

Take a look at the following article on filter selectivity:

http://www.radio-electronics.com/inf...electivity.php

The shape factor is traditionally defined as

BW(-60dB)
SF =---------------
BW(-6dB)

(see the third figure in the article illustrating a typical filter
response and the meaning of the two bandwidths)

For an ideal filter (see second figure), this bandwidth ratio would be
equal to ONE (1.0).
The closer the shape factor is to 1.0, the sharper the selectivity. For
example, if you look at the specs for the Eton E1 on the following link
(Monitoring Times review)

http://www.monitoringtimes.com/html/...k-etone1xm.pdf

the shape factor for the 7 KHz filter would be:

SF = 12 KHz/7 KHz = 1.71 (not a very sharp filter, but not bad for a
radio of this type)

The other two filters have even worse shape factors. With DSP
receivers, these filters can be made extremely tight and shape factors
can be very close to 1.0.

Hope this is helpful. Perhaps Pete can chime in with a detailed
measurement procedure.

RK