[R-390] Initial progress and Questions

[Roy Morgan]

At 01:54 PM 8/17/03 -0700, you wrote:
... I'd like to take some baseline
>measurements of sensitivity  ...   What is the preferred
>method of getting the generator signal into the balanced input

Dennis,

Duplicate the setup called for in the R-390A manual if you want to be 
"authentic".  Make believe your Model 80 is a URM-25.  It has the same 
output requirements, I believe, but do check this.

>  ...  I can build a pi network with a 50 ohm input and a 125 ohm
>output and a balun to balance it to the radio, but it this necessary
>or overkill?

It is overkill, unless you really want to know what's going on at the input 
terminals under balanced conditions.  Likely a simple toroid with carefully 
counted windings would do the job... BUT:  The actual input impedance of 
the radio varies with many factors: band, frequency within the band, 
adjustment of the input circuits, et. al.  Soo, you'd need to arrange a 
swamped termination/input circuit combined with the balanced toroid to 
really know the input voltage with any certainty.

Example: the output attenuator acccessory device supplied with the GR 1001A 
Standard Signal Generator had a division ratio of 100:1* and an output 
impedance of one half ohm*.  Readings indicated on the instrument were 
divided by 100 to get the actual voltage.  The one-half ohm value made 
variations in receiver input impedance have no or little effect on the 
voltage developed.
* I may remember these values incorrectly, but the basic idea is correct.

There can and has been much discussion on the actual voltage presented to 
the R-390A receiver input terminals by using the manual procedures and test 
equipment (URM-25).   In my opinion, the methods specified give sensitivity 
numbers that are somewhat optimistic. But, the method did accomplish the 
goal of determining if a receiver was within "specifications" or if it 
needed work.

>   The generator is a Measurements Model 80 with
>calibrated attenuator and 50 ohm output.

If I remember correctly, the Model 80 needs a cable terminated in 50 ohms 
in order to create at the termination the voltage indicated by the panel 
controls.  This is the case with the URM-25, HP 606 series, and many other 
lab grade signal generators.  This is not necessarily the case with the GR 
1001A.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
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