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Re: Lessons Learned: Downconverters & Preamps



Jerry,

Thanks for this information.  I have the CalAmp 31732 d/c and have been
mulling over the DB6NT v. DEM preamps.  Looks like the 1.8dB NF and ~34dB
gain (untested) of the 31732 will work better with the DEM preamp.

Joel B. Black
jbblack@dbtech.net
k2sat@amsat.org
k2sat@arrl.net
http://web.dbtech.net/~jbblack

----- Original Message -----
From: <K5OE@aol.com>
To: <amsat-bb@AMSAT.Org>
Sent: Monday, October 08, 2001 11:53 AM
Subject: [amsat-bb] Lessons Learned: Downconverters & Preamps


> I tested a couple of combinations of preamps and downconverters, and the
results were not surprising (fortunately).  In summary, a low-gain preamp
(DEM) feeding a high-gain downconverter (3733) seems to offer the "best"
combination for my station.  It allows me to keep the rig's internal preamp
off (low front end gain) and have no S-meter reading, but I can still hear
the d/c is turned on.  Here's the hardware:
>
> Rig:  Kenwood TS-2000 fed from a Hamtronics receive converter to the
receive-only HF input.  Default RF gain is at max, but rig's preamp is off.
AGC is on, set about middle.  If bandwidth set at 1800 Hz (200-2000).
>
> Preamps:
>  - DEM 13ULNA, 0.6 NF @ 18 dB gain (spec)
>  - DB6NT , 0.7 NF @ 40 dB gain (spec & tested)
>  - Conifer 1033, 1.15 NF @ 12 dB gain (tested)
>
> Downconverters:
>  - Drake 2880, 5.5 NF @ 22.5 dB gain (tested)
>  - TSI 3733, 1.15 NF @ 32 dB gain (tested)
>  - Norsat 2500, 4 NF @ 25 dB gain (guess)
>
> DEM + Drake, good S/N, but weak audio (needs rig preamp)
> DEM + 3733, best S/N, strong audio w/o rig preamp
> DEM + Norsat, better S/N, but weak audio (needs rig preamp)
>
> DB6NT + Drake, good S/N, with S2 background noise
> DB6NT + 3733, bad S/N, too much audio, S5-6 background w/o rig preamp
> DB6NT + 2500, best S/N, just a little background noise
>
> Conifer + Drake, good S/N, audio a little weak
> Conifer + 3733, not tested
> Conifer + 2500, better S/N, just right with rig preamp
>
> The tests were done with all the hardware sitting on the floor of the
shack and a low-level signal source in a coffee can about 5 meters away :-)
>
> It is not surprising the high gain preamp (DB6NT) feeding the high-gain
downconverter (3733) was a poor combination.  However, the DB6NT looks like
a good choice to perk up your simple-mods Drake or Norsat 2500 and get the
most from the system.  Unfortunately, the basic unit is not WX-proof.
>
> Even the weakest of these preamps and d/c combinations will allow you to
work lots of weak signals.  I started out with a Drake and Conifer and the
difference between that pair and the ones I am using now is only
incremental--it is not night and day.
> 73,
> Jerry, K5OE
>
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