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Comb Filter (bad assumption)



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


----- Original Message -----
From: "Vince R." <vincesr@ij.net>
To: "amsat-bb" <amsat-bb@AMSAT.Org>
Sent: Monday, April 02, 2001 11:15 PM
Subject: Re: [amsat-bb] AIDC-2733 gain measurement


> John,
>
> Do all the screws get about the same tension or does one
> experiment with various combinations of torque?
> Do you know the thread and size of those little
> capscrews? I was going to remove one but got nervous
> when I noticed that they had "sealer" on them!
>
> Also, would it make any sense to have a crystal cut for
> a different capacitance to bring it within range of the trimmer?
>
> Vince
> KD7AI
>
> ----- Original Message -----
> From: "John Harrington" <johnh@ih2000.net>
> To: "Mahlon Haunschild" <mahlonhaunschild@home.com>
> Cc: "greg" <greg@valkyrie.net>; "amsat-bb" <amsat-bb@AMSAT.Org>
> Sent: Monday, April 02, 2001 11:30 PM
> Subject: Re: [amsat-bb] AIDC-2733 gain measurement
>
>
> > Hello, all.  Yes, you can easily tune the comb filter- BUT- you
> > need a stable, weak signal source to do it properly.  A satellite
> > signal is NOT suitable, because you can't peak anything on a
> > signal which has rapidly-varying strength and doppler.  If you
> > don't have a suitable beacon in your area, or a synthesized
> > (stable) signal generator,  you can build a little microwatt
> > beacon with a 50MHz canned crystal oscillator and a diode to
> > generate harmonics.  There are several designs floating around
> > on the web.
> >
> > Mahlon is correct in that you can tune by ear for a very close
> > approximation of best noise figure.  I just checked that out
> > a couple of weeks ago with the very bright people on the microwave
> > list.  Not having easy access to a noise figure meter, for years
> > I have used an FM detector and tuned for maximum quieting.  For
> > this method to be reliable, you must insure that no stages in your
> > receiver are saturated.  This means you should use a weak signal,
> > barely detectable, and tune for maximum quieting.  If you tune
> > using the S-meter alone, it is possible that the point of maximum
> > gain and best noise figure will not coincide.
> >
> > Once you are set up with a weak, stable source and an FM detector
> > (flip your mode switch to FM), tune the comb filters with an Allen
> > wrench in the socket-head screws.  You may get quite an improvement;
> > I suspect the filters were tuned at the factory with a sweep generator
> > to a bandpass response.  Since you are only interested in a narrow
> > range of frequencies, you can peak the filters without the compromise.
> > Some 3733's have a tuning screw or two missing, if it tuned better at
> > the factory without it.  Try a tuning screw at the missing location
> > and you might see an improvement in your non-bandpass tuning.
> >
> > I have had several of the front-end comb filters apart, as I find it
> > easier to replace the dipole with an N connector if you get the filter
> > cover out of the way.  All have tuned up nicely upon reassembly.
> >
> > 73  John  W5EME
> >
> > Mahlon Haunschild wrote:
> > >
> > > Greg,
> > >
> > > 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.  I'll ask
> > > John Harrington to add the construction details for these filters, as
I
> > > have not actually had one apart.  He has, and I understand that he has
> > > successfully re-peaked them for 2400 MHz (John, are you listening?).
> > >
> > > You might need to go to Your Favorite Hardware Store to get an extra
> > > screw to use in the blank hole.
> > >
> > > You'd need a noise figure meter to do this right, but a sufficiently
> > > weak 2.4 GHz signal source and a Mk 1 Mod 0 ear (not the tin one!)
might
> > > suffice.  You'd want to adjust for minimum noise figure, which is
> > > equivalent to max. gain with min. noise.
> > >
> > > I haven't actually tried this, but I know that others have.  Your
> > > mileage may vary.  Caveat emptor.  Carpe diem!
> > >
> > > regards,
> > >
> > > Mahlon - K4OQ
> > >
> > > greg wrote:
> > > >
> > > > how do you tune that filter ?
> > > >
> > > > GREG N8TDL
> > > >
> > > > > Art,
> > > > >
> > > > > In stock configuration, the AIDC 3733's frequency response is
> specified
> > > > > only down to 2500 MHz, so your results as stated are inconclusive.
> Did
> > > > > you measure its conversion gain anywhere else?  Did you re-tune
the
> two
> > > > > comb filters to 2400 MHz before you measured it, or are they still
> tuned
> > > > > for the stock bandwidth?
> > > > >
> > > > > regards,
> > > > >
> > > > > Mahlon - K4OQ
> > > > >
> > ----
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>
> ----
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