[R-390] R-390A sensitivity measurements

[Gord Hayward]

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.