In article 43c63f8a.7487750@news-server,
(Michael Thorpe) wrote:

On 11 Jan 2006 14:14:01 -0800, wrote:

http://www.lazygranch.com/images/radio/cwfilter.gif
I did this test as follows:
1) radio in AM
2) AGC turned off
3) RF generator connector to antenna input
4) distortion analyzer connected to earphone output

Pick a frequency. I used 10Mhz. Set the RF generator and radio to that
frequency. Leave the radio dial alone. Sweep the RF generator manually
and measure the audio level from the distortion analyser. Plot the
results. This was a crystal filter, so you will need a RF generator
with 1Hz steps.

This is an in-situ measurement, i.e. this is not the same as just
testing the IF filter by itself. You need the distortion analyzer to
get the audio signal level independent from the noise.


Many thanks, this is interesting. What I am still unsure about is how
you determine the -6dB and -60dB points using the distortion analyzer:
When you say "measure the audio level", what exactly do you measure,
the audio signal fundamental (i.e. filtered at the modulation
frequency)?

So in fact you are only using the distortion analyzer to filter the
audio and to measure the level? I don't have a distortion analyzer,
but I guess I could simply use an oscilloscope or an audio spectrum
analyzer (PC sound card based).

rkhalona gave you the right definition of shape factor and a web page
that explains it.

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

http://www.radio-electronics.com/inf...ty/selectivity.
php

The audio passband is often but not always defined at the IF stage with
a IF passband filter. The -6dB and -60dB is referenced to the filter
center IF frequency of 0dB. The filter is usually symmetrical and the SF
number refers to how fast the slope changes. Steeper is better. The -6dB
down point is usually considered the bandwidth of the filter.

With what you have the easiest thing to do would be to use the PC audio
spectrum analyzer. On a strong signal playing music use a max hold
function on the analyzer software and the passband of the filter should
become apparent. Look at where the high end response rolls off 6dB down
relative to what looks like a flat audio spectrum from low to middle
audio frequencies. Where the high frequencies roll of 6dB down from the
flat audio response range will be 1/2 the filter bandwidth.

For this to work the station you have are receiving must be have a wider
modulation bandwidth then you are using for an audio bandwidth filter so
this may only work for narrow filters.

If this does not work well enough then you will need an RF generator
that you can modulate at a frequency higher than the largest filter you
want to measure and use the PC analyzer as described above.

--
Telamon
Ventura, California