[R-390] URM-25 RF output measurement

[Jack Antonio]

I'd like to add a couple of things to the URM-25 thread.

I've used a scope to check output levels, but,
remember the scope input is high impedance, so make
sure the sig gen output sees a 50 ohm load, and also the
scope reads peak to peak, not RMS. I wouldn't use this
method for absolute accuracy, but at least it will let you
know you are in the ballpark.

I've also used the Tektronix 2710 spectrum analyzer at work to look at
the sig gen output levels. Playing with looking at really
weak signals with an analyzer is an education in itself.

Also, I've owned two URM-25s,  a D model and F model. The
output attenuator in each is basically a bunch of resistors in
a super fancy rotary switch arrangement, the last resistor being
a precision resistor in the neigborhood of 50 ohms.(49.9? 50.1?
can't remember exactly). In both of my URM-25s, this resistor was
burned open.  Then, to add insult to injury, back when Fair was
selling the URM-25 output attenuators, I ordered one, and this resistor
was open in that one as well!

Hope this helps someone.

Jack

Jack Antonio WA7DIA
scr287 at sbcglobal.net
----- Original Message ----- 
From: "Roy Morgan" <roy.morgan at nist.gov>
To: "Paul H. Anderson" <paul at pdq.com>; <r-390 at mailman.qth.net>
Sent: Tuesday, July 06, 2004 1:10 PM
Subject: Re: [R-390] URM-25 RF output measurement
> 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.)