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AIDC 3733 Downconverter NF and gain questions answered

Greetings from K5GNA;

Let me clear up a few items:

1.  Comb Filter - I have measured the comb filter to be reasonably flat (3dB) 
from 2000 to 2700 MHZ., except as noted in # 3.  There is a definite loss of 
gain through the converter when you approach the LO frequency of 2278 MHZ, 
but this is to be expected.  The IF amp doesn't work too well below about 50 
MHZ, so there appears to be no gain through the comb filters and preamp (and 
downconverter) within the 50 MHZ or so of the LO frequency.

3. Tuning the comb filter.  The best way to tune a preamp is with a noise 
figure meter, tuning on gain will give a different noise figure reading than 
if tuned on noise.  Try this on an FM radio -- tuning an FM radio for best 
signal indication, or best quieting for the 20 dB quieting method, will not 
give you as good a sensitivity as tuning it with a Sinad meter.  The Sinad 
meter, here, approximates what a noise figure meter does as far as tuning the 
whole unit for best noise figure.  Unless you disturb the front end comb 
filters, I wouldn't mess with the front end comb filters.

If you must, the comb filters are tunable by tightening the allen head 
screws.  The block is mounted on a rubber pad and compresses against the high 
ends of the comb filter.  The high end of the comb filter is wrapped in 
Teflon, so more pressure adds capacitance and less pressure is less 
capacitance. The three screws let you vary tuning among the various elements, 
as you are moving the plate in 3 different positions.

2. Notch in response at 2400  -- there is a 5-8 dB notch in response at 2400 
MHZ or so on some of the converters.  The little piece of rigid coax that is 
on the output of the preamp section (before it goes to the second comb 
filter) appears to be the culprit. It connects near the rear comb filter and 
runs up above the circuit board to near the front comb filter. It appears to 
be a 2.4 GHZ stub. I cut one at the connection to the preamp output, and the 
gain is now flat across the range.  Try unsoldering yours, I have plenty to 
experiment on.

3. Converter gain - I have been seeing about 5 dB less gain than it says on 
the converter also.  It is probably an IF gain number, the 30 DB gain 
downconverters I have use less FR stages on the circuit board.  However, when 
the 37 dB gain downconverters are run with less than a couple of hundred feet 
of lossey coax, the normal 2 meter (or 123 MHZ) receivers show S-7 or S-8 on 
just noise.  To operate the receiver properly, it needs about 10 dB or so of 
attenuation to get the noise into the receiver down to a reasonable level.

4. Revisit the notch at 2400 MHZ - as is, the notch at 2400 is not that 
great, and the loss is at the mixer end of the preamp.  In other words, the 
noise figure is not affected very much (the noise figure formula shows 
reduced NF before the gain, not after it), and you only lose the extra gain 
and not much else in many cases. Remember, there is way too much gain in most 
systems, and it needs to be attenuated (see # 3).  I would hope some of the 
other guys with good equipment will verify my findings, before anyone with no 
test equipment tries it.

5.  Crystals - the crystals cut for a Drake 2880 are 50 KHZ high. THEY ARE 
NOT RIGHT.  I have sent a crystal to Jan Crystals and they promise to give me 
the full spec's on it tomorrow.  I will post it here when I find out.

6.  Measuring LO frequency - I originally hooked my HP 8660 synthesizer into 
the input and measured the output frequency on a counter.  This worked well 
and gave a good output to the counter.  After my 8660 has become terminally 
unlocked (it is time for an alignment) I hooked my HP 5342 directly to the 
pad that feeds the mixer from the LO.  I disconnected it, left the leads 
about an inch away and it was still reading frequency.  I measured -25 dBM 
into the counter with the leads nearby.  My next mod is a BNC completely cut 
off at the bottom and mounted in the cover, near the output pad, so I can 
measure frequency.

7.  Noise Figure - we have measured 1.3 dB consistently at 123 MHZ output.  I 
am open to inputs on this one.  Maybe the difference is one is measuring DSB 
instead of SSB noise?  Remember, the front end of these guys will accept both 
high side and low side frequencies.

TNX & 73,

Bob Seydler

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