In article 43c63d58.6926500@news-server,
Michael Thorpe wrote:
On Wed, 11 Jan 2006 21:23:09 GMT, "Dale Parfitt"
wrote:


"Michael Thorpe" wrote in message
news:43c4e407.4277000@news-server...
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...

Although I have never given it much thought, I don't believe one can fully
characterize a filter while it is installed in the receiver- certainly,
parameters such as insertion loss would not be easily measurable. Perhaps
programs like Spectran may give some rough insight.
My method is classic- the filter is removed, properly terminated and
connected as the DUT on a scalar network analyzer.
If you have a vector network analyzer available you could also observe group
delay- an important factor in narrow filters.
See Paul Kiciak's (N2PK) site for his clever VNA- ideally suited for
filters.

Thanks, Dale, for your input. I would have no problems measuring
filters when taken out of a receiver. But this is not always possible,
for example with modern DSP software-defined receivers. There you have
no choice, the filter is no longer just a component you can desolder,
so you do need to measure the entire receiver. Parameters such as
insertion loss is immaterial in such cases - all that matters is the
filter shape.

But when the whole system is put together, it's no longer "filter
shape factor", it's some other parameter, "adjacent signal rejection"
or something like that.

Mark Zenier
Googleproofaddress(account:mzenier provider:eskimo domain:com)