On Tue, 13 Jan 2009 12:49:08 -0800, paolo67 wrote:

Thank you for your answers.

OK so AGC must be off. What made me confused is the fact that the
manufacturer explicitly indicates that this alignment must be done with
AGC auto.
With AGC off, the gain can be increased up to a point where the curve
gets clipped and wider, this is the point then. A scan width of 1sec
seems to be the limit for an optimal representation.

The receiver has a proper IF output, it is just before the AM detector
diode but not immediately after the filter, in between there is a three
stages, fixed bandwidth, high gain IF amplifier.

With AGC off the output level now varies depending on which IF filter is
selected: the manual says that the input level should be regulated for
0dB at IF output, but this is assuming AGC on. Although I always try for
maximum gain, each filter seems to produce its own maximum output level,
and differences are as large as 15dB+ with no obvious logic, so no the
wider the filter the higher the level. Does this make sense? How can I
detect which is the "reference" level to look for or, alternatively,
what is an "acceptable" difference among the filters?

Thank you and best regards
Paolo

There are a lot of factors that go into designing a filter, and for an IF
filter the insertion loss is one of the easiest to overcome -- so I'm not
surprised that they should vary. An acceptable level of attenuation
through an IF filter is anything that gives you adequate signal on the
other side, so you could have a filter that absorbs quite a bit and is
still OK to use.

If by a "1 second scan width" you mean that the total scan time is one
second, then you could _maybe_ get away with using a 10-second AGC. _I_
don't like it, but the manual is aimed at busy techs, so maybe it makes
sense...

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html