How exactly does one see 60db down?

Brian Denley 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

Michael:
If you don't have a spectrum analyzer, you can use an oscope (setup for
x-y) while driving the filter (and x ) with a sweep function generator.
Remember that if you look at the IF output prior to the detector, you will
get a curve going positive and negative. If you look at the response after
the detector, it will look more like the IF filter shape that you expect.

--
Brian Denley
http://home.comcast.net/~b.denley/index.html

wrote:
How exactly does one see 60db down?



Very carefully!

--
Brian Denley
http://home.comcast.net/~b.denley/index.html

In article ,
"Brian Denley" wrote:

wrote:
How exactly does one see 60db down?



Very carefully!

Things start getting pretty dark after 40dB down so be very careful!

--
Telamon
Ventura, California

On Fri, 13 Jan 2006 05:10:44 GMT, Telamon
wrote:

In article ,
"Brian Denley" wrote:

wrote:
How exactly does one see 60db down?



Very carefully!

Things start getting pretty dark after 40dB down so be very careful!

--
Telamon
Ventura, California

Many thanks to everyone who invested their time and effort to help me
with my filter shape measurement problem. Your advice has been very
valuable to me and I am now back on track as a result. I will report
what I'll find.

Michael

I'm not sure why the XY display, but seeing 40db is about the limit.
You really need test equipment to get any meaningful results. Network
analysers that cover a few MHz aren't that expensive. You can also just
use a RF generator and power meter, which is a cheaper solution since
you don't need phase measurements to see attenuation.


Telamon wrote:
In article ,
"Brian Denley" wrote:

wrote:
How exactly does one see 60db down?



Very carefully!

Things start getting pretty dark after 40dB down so be very careful!

--
Telamon
Ventura, California

wrote:
I'm not sure why the XY display, but seeing 40db is about the limit.
You really need test equipment to get any meaningful results. Network
analysers that cover a few MHz aren't that expensive. You can also
just use a RF generator and power meter, which is a cheaper solution
since you don't need phase measurements to see attenuation.



If you DON'T have a spectrum analyzer (most don't) or if you DON'T have
expensive test gear, a decent Oscope with x-y and a sweep generator can be
used as a poor man's spectrum analyzer. 50 years ago, there was no other
way to do it, in fact. If you look at, say, a Hammarlund receiver manual,
that's exactly the recommended way to adjust the IF filter response curve.
You don't have to go down to 60 db to see a bad shape factor.

--
Brian Denley
http://home.comcast.net/~b.denley/index.html