[R-390] R-390A sensitivity measurements

[Mark Huss]

First off, use the Balanced Adaptor that is normally used for the Balanced
input.  This shorts out one side of the input transformer, and presents the
other pin with a '62 ohm' input.  Since this input changes with tuning, you
will have error unless you correct impedance match at each frequency of
measurement.  According to Lankford, the input impedance of an R-390A varies
from 90 to 250 ohms over frequency range.  For measurement purposes, a
reading of 0.3uV could actually be 0.2 to 0.4uV.  Without a lot of work you
are never going to get much better than that.  If you are interested in
doing better than that, build an attenuator with a 50 ohm input, and a
variable impedance output.  Connect the generator to the receiver at the
frequency of interest.  Tune radio to same frequency, increase generator
output until you can measure amplitude on oscilloscope or AN/URM-26 VTVM
across receiver input.  (you have to use unbalanced feed for this to work).
Adjust output impedance of attenuator to maximum.  Measure voltage.  Divide
this by half, then adjust attenuator impedance until voltage reading is
half.  Measure components in attenuator and calculate input impedance,
output impedance, and attenuation.  Reinsert attenuator into circuit and do
sensitivity test at that frequency.  Adjust result using the calculated
attenuation.  Repeat the whole thing at the next frequency.
	Unless you want bragging rights, and can be sure they are doing it exactly
the same, it doesn't seem to be worth the effort.

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]On
Behalf Of John KA1XC
Sent: Tuesday, March 01, 2005 11:59 PM
To: R-390 reflector
Subject: Re: [R-390] R-390A sensitivity measurements

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.
>
>
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