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Re: 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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