[R-390] Measuring Sensitivity

[roy.morgan at nist.gov]

Quoting John Kolb <jlkolb at jlkolb.cts.com>:

> 
> If the signal generator is outputing 1 microvolt across the 1/2 ohm 
> resistor, the receiver
> will see 1 microvolt at it's input with any reasonable receiver input 
> impedance.

John and others,

This is right - that is why I suggested the 100:1 attenuator with the output
taken from the half ohm resistor.
 

> I believe that 50 ohm output signal generators are calibrated so that 
> the output reads correctly when the generator is loaded with 50 ohms. 

MOST generators do operate this way.  The HP 606, 608, the URM-25, and if I
remember right, the Measurements Model 80 included.  For other generators of a
professional sort, just read the manual, or assume that it needs to be loaded
by 50 ohms to produce indicated voltage across the load.  

The General Radio type 1001A, however, does NOT operate this way. It produces
the voltage indicated by panel settings when the output is OPEN CIRCUIT.  If
loaded with 50 ohms on the 100 Millivolt setting, or with the 40ohm series unit
and a 50 0hm load, it will deliver HALF of what the panel indicates.

The following is right, if the a generator intended to indicate correctly with a
50 0hm load is connected directly to the receiver with no other load.  (I'm
sure that is what John meant.)

>  So the receiver would see 1 uV
> at frequencies where it's input was 50 ohm, but almost 2 uV at freqs 
> where it's input impedance is 700 ohms. 

Roy
Who has too many signal generators.


Roy Morgan
13033 Downey Mill Road
Lovettsville VA 20180