[R-390] R-390A sensitivity measurements

[JMILLER1706 at cfl.rr.com]

One thing I found with my HP8640B sig gen is that if you do not use the 50 ohm terminator, the output voltage meter would not measure correctly.  If you depend on the output level meter as a reference point for sensitivity measurement, without the proper impedance match your calculaions could be off considerably due to meter inaccuracy.

----- Original Message -----
From: Cecil Acuff <chacuff at cableone.net>
Date: Wednesday, March 2, 2005 11:48 am
Subject: Re: [R-390] R-390A sensitivity measurements

> I've been contemplating a balun to go from the balanced input to 
> the 
> antenna/generator anyway.  Question is do I do a 2:1 or a 1:1.  
> Keep the 
> radio at it's native 125 ohm nominal impedance or take it to 
> around 50 ohms 
> nominal which is what we are doing using current wisdom on the 
> twinax 
> connector to coax fabrication.
> 
> Cecil...
> ----- Original Message ----- 
> From: <DJED1 at aol.com>
> To: <ghayward at uoguelph.ca>; <r-390 at mailman.qth.net>
> Sent: Wednesday, March 02, 2005 8:59 AM
> Subject: Re: [R-390] R-390A sensitivity measurements
> 
> 
> > What we old microwave guys are used to is having an impedance 
> match 
> > between
> > components, so maximum power is transferred.   For newer 
> equipment, we 
> > almost
> > always work in a 50 ohm system, and measure in dBm.   For older 
> radios, 
> > where
> > the impedance is not 50 ohms, measurments were not so clearly 
> defined. 
> > This
> > is true for the op-amp also, because its output impedance 
> probably does 
> > not
> > match that of the receiver.   That's why I'm troubled by using 
> resistors 
> > to fudge
> > the sensitivity measurement-it terminates the generator 
> properly, by part 
> > of
> > the signal power is lost in the resistor.   I think we need a 
> good 
> > low-loss
> > transformer to properly feed the R-390. Anyone have such a thing?
> > Ed
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