[50mhz] Preamp and receiver question

[Cecil Acuff]

----- Original Message ----- 
From: "John Geiger" <aa5jg at fidmail.com>

.  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.

---------------------cut-----------------------------------------

Yep my same question...are we comparing apples to apples with the same IF 
bandwidth?

I'm assuming also that the numbers you quote are specs from a test run by 
the ARRL or some other lab and not measured for the two samples.  No way to 
know if the two radios are up to spec.

Doesn't look like the 706 is too deaf.  -136 with preamp off which is the 
same as the 450 with the preamp on.  I think you are seeing the difference 
between the rig sensitivity and meter inaccuracy at play.  You could add 
attenuation and see if there is a point where there is a drastic drop in the 
noise level on the 706...but if it tracks pretty accurately down it's 
probably the real deal and not the preamp overdriven.  (add 10db of 
attenuation and see if the meter shows aprox 10db of decrease...etc.)  I'm 
guessing you will find the same impact on a received desireable signal 
between the two radio's.  Might be a good test.

Just some thoughts..

Cecil
K5DL