On Tue, 13 Jan 2009 09:04:00 -0800, paolo67 wrote:
Hi all,
I'm Paolo from Italy, this is my 1st post. I'm trying to align the IF
filters of the R&S EK-07. From a general standpoint, I would like to
understand what the recommended procedure should be. I just followed the
manual:
- a sweep at the antenna input, centered at the tuning freq. I generate
this sweep from the combination 141T/8443A - the 300kHz IF output and
the scan ramp output of the sweep at the scope in XY mode
- auto AGC
The filter has six fixed positions, the narrowest is 300Hz (crystal
filter) and the widest is 12kHz. The - only - tube is biased by AGC. I
observe large modifications of the symmetries depending on the
combination sweep rate/AGC time constant: if there's enough time for the
gain to get stable, I get more symmetric curves. There are three
positions for the time constant: 0.1, 1 and 10 seconds. Attack and decay
times are unknown. I'm missing something: what setting should I choose
to perform the alignment?
Thank you and best regards
Paolo
Where are you picking off the signal strength from the IF? The really
"right" way to do this is to measure the IF signal strength with the AGC
_off_ and the sweep rate slow enough for the IF filters to settle (which
you can check by lowering the sweep rate until things stop changing). If
the radio doesn't have a diode (like the AGC detector or the AM detector)
that you can use for measuring signal strength, make up something
yourself.
I could see someone taking a short cut and having you measure the signal
strength by monitoring the AGC voltage. I certainly don't recommend
this, but if that's what the radio manufacturer wants to do you may
possibly want to do it before going back and doing it right. In this
case, before you go back and re do it right, you'd want the AGC set to as
fast as it would go, and sweep slowly enough so the AGC settles. And
redo the measurement right (did I say you should redo the measurement
right?).
--
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