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Re: Comb Filter (bad assumption)



I'll take a stab at this, though I have not seen one of these units.  My
guess is that the comb filter is nothing more than a multi-section bandpass
filter.  If so, then tuning only one section will have small effect on the
total bandpass curve.  Typically to get a wider pass band some sections are
tuned to different frequencies to widen the curve.  But to move the band
down in frequency will require retuning the entire filter for maximum
performance [minimum insertion loss].  You need a weak, constant signal
source, and a good signal level meter to observe the effect of your tuning.
 Ideally you have a swept-freq. generator and spectrum analyzer [not in
your average ham shack].

You are right to be hesitant, as if you blindly start tweeking things you
will quickly make a deaf receiver.  A 100 MHz xtal osc feeding a microwave
diode will produce a signal at 2400 MHz [and 2200, 2300, 2500, 2600, etc].
You could use your IF receiver and turn off the AGC and connect a RMS
ac-voltmeter [analog scale..not digital readout].  Start with one screw and
turn it slightly to see the effect.  Essentially you are looking for max
signal.  If you tune all stages for max signal the bandpass will be narrow,
so do it on the frequency you will operate on.  This will render the
converter unsable on any other frequency.  If you need 2400-2448 MHz then
you must tune some combs at different frequencies througout the bandpass.
This is near impossible without a spectrum analyzer.  Not for the faint
hearted.  GOOD LUCK!

Ed
A solution may be finding a kind ham with access to the needed equipment
that is willing to re-tune the filters.  I do not have this equipment.  I
saw and adv. for a surplus signal genrator for only $750.  Probably can
find a spectrum analyzer for $800-1200.  Probably cheaper to buy a ham
converter ready to go?  Has anyone though to simply bypass the filter?

>To: "amsat-bb" <amsat-bb@AMSAT.Org>
>I know nothing about comb filters, so here is my bad assumption.  If all of
>these TranSystem comb filters are tuned up for 2500 MHz would it make sense
>they could have a very similar adjustment to repeak at 2400 MHz.  In other
>words, it main not fully peak your unit, but everyone would see improvement
>by turning the first screw an 1/8 of a turn CCW or something like that.
>
>My unit has been working so I hesitate to open it.  How many stages are
>there in the filter?  What is the maxium noise figure / gain improvement a
>re-tune can provide?
>
>73,
>Joe
>ka0yos@amsat.org

>> > > The three (or two) socket-head cap screws on each filter bear down on
>> > > the ungrounded end of the filter fingers with a bit of insulating
>> > > material (Teflon?) interposed, thereby forming a capacitor. 


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