[50mhz] [VHF] Preamp and receiver question

[Russ Pillsbury]

Hi John.

> 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? 

No.  Voltage ratios are not the same as power ratios.  If you were measuring
a factor of 10 power ratio, then it would be 10 db (dbDiff = 10 *
Log(p1/p2). So 10 * log(10/1) = 10).  But for a voltage ratio the equation
is dbDiff = 20 * log (v1/v2), so the gain (or difference) for a voltage
ratio of 10 is 20 * log(10/1) = 20 db.

Note that these equations work out to nice simple numbers only because you
are using ratios of 10 (and log(10) = 1).  But if you were using a ratio of
2/1, then if it is a power ratio, the result will be 3 db, or 6 db if it is
a voltage ratio.  Notice that a properly calibrated S-meter should be 6 db
per S-Unit.  Thus each S-Unit represents a doubling of power.  Most ham
receivers s-meters are not properly calibrated though, more common is 3 db
per s-unit, and I have seen 2 and even 1 db per unit.

Hope this helps,
73, Russ K2TXB 

> -----Original Message-----
> From: owner-vhf at w6yx.stanford.edu 
> [mailto:owner-vhf at w6yx.stanford.edu] On Behalf Of John Geiger
> Sent: Wednesday, June 29, 2011 12:53 PM
> To: ICOM Reflector; Yaesu_FT-450 at yahoogroups.com; 
> 50mhz at mailman.qth.net; vhf at w6yx.stanford.edu
> Subject: [VHF] 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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