[R-390] R-390A sensitivity measurements

[John KA1XC]

Maybe for the sig gen, but from the radio's point of view it is a total
abandonment of the concept of  "matching" :^0
And on top of that error you're also adding the noise contribution of the
buffer amp, so you are no longer measuring the S/N of the radio, but the S/N
of the radio + the buffer amp. You'll measure something but it won't be
meaningful.

John

----- Original Message ----- 
From: "Gord Hayward" <ghayward at uoguelph.ca>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, March 01, 2005 11:26 PM
Subject: Re: [R-390] R-390A sensitivity measurements


> I did the impedance match with one of National's really fast unity gain
amplifiers,
> the LH0063. I run the sig gen into the high impedance fet input, with a 52
ohm
> terminator and drive the antenna input directly with the amp low impedance
output
> with a 0.01 uF cap to kill any DC offset from the amp. The voltage at the
amp input
> is the same as the voltage impressed on the radio antenna connector. I
used 20 dB
> attenuators on the terminated amp input so I could check the actual sig
gen output
> with a scope. The scope results look OK when I use a humongous signal and
when I
> cut the gen output way back I get reasonable sensitivity values. Does this
sound
> like a reasonable approach to the matching?
>
> Cheers, Gord (VE3EOS).
>
>
> >Thanks for the note, Bob.   I went with the instructions in the URM-25
> >manual, which clearly intend that there be a 50 ohm load on the generator
for the
> >output voltage to be equal to the meter reading.   I was reminded of this
when i
> >put my scope on the HP generator to check the output- way off until I put
a
> >50 ohm termination on the etup, then the generator output was right on.
So
> >I'm convinced that the generator needs to be terminated properly.   Of
course
> >this is not a problem for 50 ohm receivers.   For the R-390, it troubles
me that
> >we don't have an impedance match between the generator and the radio, but
we
> >do know, I submit, the voltage across the receiver terminals.   If you
wanted
> >to determine the power going into the receiver, I would use the 125 ohm
value
> >and the measured voltage.   Probably the best way is to devise a lossless
> >transformer to go from 50 ohms to 125 ohms.   I haven't tried to do that
yet, but
> >I'm working on it.
> >Regards, Ed
>
>
> -- 
>
> Gordon L. Hayward, Ph.D., P.Eng.,
> Associate Professor, Biological Engineering,
> School of Engineering, University of Guelph,
> Guelph, Ontaro, N1G 2W1.
>
>
> _____________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/faq.htm
> Post: mailto:R-390 at mailman.qth.net
> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>