[R-390] please help

[Bob Camp]

Hi

So now we are back to the “noise in what bandwidth?” issue. Since it’s a 
floor spec (not a sensitivity) you must get the noise bandwidth of the filter 
used. Note that this is only the same as the 3db bandwidth if the filter has
infinitely steep sides. For a normal filter, the noise bandwidth will always
be larger than the 3db bandwidth. 

Taking 50 ohms as the resistance (no, that’s not correct, see the message below). You could 
be talking about a 0.2 uV sensitivity for a 20 db S/N ratio, if 0.02 uV is the “floor” 
(or total noise).  That makes sense for a narrow filter, not much sense for 
a wide filter. 

One thing that happens with all this stuff: People are not very careful about the
details. They grab a piece of gear, do a reading and go off and yack about the 
result. 20 years later we still have the yack. None of the details have survived. 
A signal generator that is *capable* of a 0.2 uV sensitivity measurement is a relatively 
rare bird. A cabling setup and bench that will do it is even more rare. Almost all 
of the errors are on one side of the measurement. 0.6 uV “for real” will come up as 
0.2 uV “as tested” most of the time. There are very few ways for 0.1 uV “for real” to
test out as 0.2 uV.

If you want to be “king of the hill” in a standard sensitivity measurement, the drop dead
simple answer is still the same. Implement a 1 Hz wide filter. Your numbers will be 
20 db (10:1) better than any poor idiot who is using a 100 Hz wide filter. As long as
nobody come along with a 0.01 Hz wide filter, you will reign as king. Since an audio 
filter is just as good as an IF filter in this regard, it’s a sub $10 sort of thing to implement. 

Bob


> On Oct 8, 2016, at 1:10 PM, Jacques Fortin <jacques.f at videotron.ca> wrote:
> 
> I disagree...
> -141dBm is .02µV in 50 ohms.
> If another receiver input impedance is specified, the related voltage will
> change accordingly.
> I hope that all agrees that 0dBm equals 1 mW.
> I also doubt that a R-390 (A) input impedance gives 50 ohms at all
> frequencies, all the time...
> This is why some states a receiver sensitivity as being a  number of EMF
> volts from a specified source Impedance (like 1µV EMF in series with 50 ohms
> for 10dB of S+N/N), because this puts the receiver input impedance
> variations out of the picture.
> 
> 73, 
> Jacques, VE2JFE
> 
> -----Message d'origine-----
> De : R-390 [mailto:r-390-bounces at mailman.qth.net] De la part de Bill Riches
> Envoyé : 8 octobre 2016 12:22
> À : wb3fau55 at neo.rr.com; R-390 at mailman.qth.net
> Objet : Re: [R-390] please help
> 
> -141 dBm = .2 uv
> 
> 73,
> 
> Bill, WA2DVU
> Cape May
> 
> OK guys,  some years  back I read someone quote a noise floor of -141 DB for
> a "golden" R-390A. Please, if someone would,  convert that to microvolts.  I
> think it was in ER magazine 
> 
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