[R-390] URM-25 RF output measurement

[Roy Morgan]

At 11:13 AM 7/6/2004 -0400, Paul H. Anderson wrote:
>  ... How can I tell if my URM-25 is actually
>putting out 150 microvolts?

Actually, you can't. You can be more or less confindent after some 
testing.  Bob Camp's advice on building a new rf generator or level setting 
device sounds like a find but challenging project.

>  Do I need a calibrated RF meter with a known
>input impedance and make sure that is matched to the output of the URM-25?

I see two ways to do this, both depending on the assumption that your 
URM-25 attenuator is working as it should.

1) Follow the calibration procedure in the URM-25 manual. Basically, you 
set the oscillator output to two volts and then depend on the attenuator to 
do the rest.  the URM-25 front panel meter measures that two-volt level 
during normal operation.

2) Get or borrow a millivoltmeter of some sort.  A good oscilloscope may do 
in a pinch.  Measure the URM-25 output level at some reasonable level like 
100 millivolts (being sure to terminate the thing correctly) and then 
assume the attenuator is working on other (lower) settings.

If you then think clearly about what the signal generator is doing, what 
the termination or external attenutaor "pad" is doing, and what the load of 
the receiver is on that setup, you will come to some conclusions about what 
the actual rf voltage is at the receiver terminals.  (note that the input 
impedance of the balanced antenna input can vary from 50 to 700 ohms.)

Roy


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