[50mhz] Preamp and receiver question

[Carl]

Find a beacon in a quiet location and measure the SNR with the 2 radios 
using a meter across the audio output and AGC off.
The biggest number wins.

Carl
KM1H


----- Original Message ----- 
From: "John Geiger" <aa5jg at fidmail.com>
To: "ICOM Reflector" <icom at mailman.qth.net>; <Yaesu_FT-450 at yahoogroups.com>; 
<50mhz at mailman.qth.net>; <vhf at w6yx.stanford.edu>
Sent: Wednesday, June 29, 2011 12:52 PM
Subject: [50mhz] Preamp and receiver question


>I have a receiver question for those on the list who know a lot more about
> this stuff than I do, at least from the electronics standpoint.  I know 
> more
> about receiver performance and less about how we get there.
>
> I have an Icom 706-had it over 3 months now which is approaching a record
> for me HI HI.  On 6 meters I have some noise source to my southeast (right
> at the Carribean) that really raises the noise level on it.  It hits S8 or
> so if the beam is pointed in the right direction.  I didn't remember
> the FT450 I had being that noisy, so I had a friend bring his FT450 over,
> and it was showing a noise level of S2 or so in the same direction.
>
> Now I know that different variables are in play.  Checking the QST review
> for each radio, the 706 (preamp on) has a 3db advantage in sensitivity. 
> The
> MDS for it is -139dbm vs -136dbm for the FT450 (preamp on).  That should 
> be
> 1/2 S unit difference or so in picked up noise.  The S meter calibration 
> is
> very different.  For the 706 with the preamp on on 6 meters, a S9 signal
> takes 3.3 microvolts, which is insanely liberal, but that is what it is.
> For the FT450 under same conditions (preamp on, 6 meters) it takes a 33
> microvolt signal for S9.  That is a factor of 10, which should show up as 
> a
> 10db difference on the meter-correct?  If that is correct, we have a 13db
> difference in readings between the 706 and 450 (3 db for increased MDS, 
> 10db
> for the meter difference).  That should be 2 S units or maybe 3-4 given 
> the
> non linear calibration of most meters.  But I am seeing a 7 S unit
> difference or so.
>
> So here is my big question-can the preamp device in the 706 (or any other
> radio for that matter) become overloaded and start to generate increased
> noise?  I think it can, correct?  If you put enough signal, local noise,
> whatever into the receiver, won't the preamp at some point become swamped
> and start to create noise on its own?  Could that be where this extra 
> noise
> is coming from?
>
> You really can't run the 706 original on 6m without the preamp as it is
> pretty deaf without it.  The noise blanker on either rig doesn't do much 
> on
> this noise, so it isn't that the NB in the FT450 is taking it out while 
> the
> 706 isn't.
>
> 73s John AA5JG
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