[R-390] URM-25 RF output measurement

[Chuck Ochs]

Let's see now--Musta been checking tranceivers and keyed the mic<G>.
Chuck N1LNH
----- Original Message ----- 
From: "Jack Antonio" <scr287 at sbcglobal.net>
To: <r-390 at mailman.qth.net>
Sent: Tuesday, July 06, 2004 4:46 PM
Subject: Re: [R-390] URM-25 RF output measurement


> 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.)
>
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