[R-390] URM-25 RF output measurement

[Bob Camp]

Hi

So now I'm both in hot water for Lutefisk threads and in trouble for 
being to technical. I just can't win this week .....

Every time I have gotten the "can I trust my signal generator" bug the 
attenuator has been the part that I trusted the least. Admittedly the 
waveguide beyond cutoff style used in the URM-25 and it's cousins is 
less prone to trouble than the switch based gizmos used in more modern 
equipment. The attenuator check is pretty simple and generally it gets 
me back to worrying about other things.

The CMOS signal generator is fairly easy to build, but to your point if 
you don't get it quite right then it's more trouble than it's worth.

There is one other way to do the whole thing. It's nice because it 
takes out all the errors at the low signal end of things. That's 
normally where you really care about it being accurate anyway. The 
stuff I came up with last time was all aimed at the high level end of 
the range.

There are a number of ways to generate a fixed level of RF noise at 
R-390 type frequencies. One easy way is to pick up a used GR tube based 
noise source. You might also go looking for a tube for it as well.

Since you have a R390 handy you have a radio with known IF bandwidths. 
Once you know the bandwidth *and* the noise level into the radio you 
can calibrate the radio in terms of micro volts. Poof! you have an 
accurate RF micro volt meter. Just the thing to check out your URM-25 
with. Since you have the full attenuator in on the URM all the 
tolerance build up from the accuracy of your substitution attenuator in 
the other procedure goes away.

Note that no animals were harmed in any of the above proceedings ....

	Enjoy!

		Bob Camp
		KB8TQ


On Jul 6, 2004, at 4:10 PM, Roy Morgan wrote:

> 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!
> 7130 Panorama Drive, Derwood MD 20855
> Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> roy.morgan at nist.gov --
>
> _______________________________________________
> R-390 mailing list
> R-390 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390
>