[50mhz] Preamp and receiver question

[N6FD]

John,

Usually an overloaded device will not generate noise on its own.  
However, it will cause intermodulation with stuff on other frequencies, 
so that noise will appear in other places in the band.  This can usually 
be detected by turning the preamp on and off.  The intermod signal will 
completely disappear with the preamp off.  If it is a real signal, it 
will still be there at a reduced level.

73, Erich N6FD

On 6/29/2011 9:52 AM, John Geiger wrote:
> 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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