[R-390] URM-25 RF output measurement

[Bob Camp]

Hi

This is actually two questions in one.

The first one deals with how to set up the IF gain adjustment. There 
has been quite a bit of work done showing that the procedure in the 
manual does not result in the most sensitive radio. It does result in a 
radio that can be rigged up in a diversity system. Since most of us are 
not running diversity setups there are better ways to set the gain. 
Chuck Rippel certainly has made the most posts about this so I think 
you might want to check his web site for the details.

The second question is how do I make sure I know what my URM-25 is 
actually doing? Needless to say this has been another of those things 
that long threads have gone on about. Here's one way to do it, there 
are *many* others some of which may be better:

1) Always run the generator into an attenuator. This way you are always 
sure of what the output impedance of the generator is. Different people 
use anything from 6 db up to 20 db for the purpose.
2) Take a radio you reasonably trust (an R-390 is always a good 
candidate here ...) and run the attenuator in the generator from one 
end of it's range to the other while watching the radio's AGC voltage. 
The if the AGC jumps abruptly you may have an attenuator with a 
problem. Often simple cleaning is all that's required to fix it.
3) Grab a 20 db pad you trust and switch it in and out of the line from 
the generator to the radio. Since the external and internal attenuators 
add to each other a simple "get back to the same AGC voltage" approach 
will let you check the calibration on the generator's attenuator. Just 
like in the step above what you are really looking for are dead spots.
4) If you have a low level power meter you trust you may be able to get 
the URM-25 to drive it, if not then try step 5:
5) The output of a CMOS gate running with a light load goes almost from 
ground to the supply voltage. Feed it with a simple crystal oscillator 
and  that's close enough to make a pretty good RF voltage standard 
provided you keep the frequency down to a few MHz. A couple of one 
percent resistors from the junk box should let you go from a K ohm or 
so down to around 50 ohms. Just for fun let's say it's 1K down to 50 
and that gives you 250 mv peak to peak. Anything much less than this 
and the ground bounce on the CMOS gate may become a problem. 250 peak 
to peak has a 88 mv rms component in it at the fundamental frequency if 
I remember correctly. A pair of 20 db pads should get this down to the 
range that the radio and the URM-25 can reasonably deal with it.

Once you have one frequency on one band checked out then you can be 
pretty sure the attenuator is running fine. All that remains is to be 
sure you have level on all the bands. I do not see any need to try to 
check the attenuators range at any more than one point.

It takes almost as long to describe all that as it does to do the 
tests. Once you are done, except for leakage out of the generator 
cabinet and the accuracy of the attenuators you used your URM-25 pretty 
much right on for level calibration.

You can get pretty exotic with the CMOS gate voltage standard.

The Boonton calibrators have something like a 12th order low pass 
filter between the gate and the output. That's overkill for what you 
are doing with the standard and it adds the tolerance of all the filter 
parts into the mix.

Padding the output part way, then putting it into a transformer to 
break up the grounds followed by another attenuator followed by another 
transformer also is said to help the ground bounce problems.

Shielded box inside a shielded box construction probably is a good 
idea. That way you do not have a leakage path around everything that 
messes it all up. Copper foil flashing material  from Home Depot cuts 
easily and can be quickly soldered into all kinds of cute little shield 
boxes. Some people like brass stock from the hobby store.

  If you have a good oscillator the output will be a 50/50 duty cycle 
square wave. The odd harmonics will fall off in a predictable fashion 
up through at least the first few. Since the radio will tune the 
harmonic just as well as the fundamental this gives you a way to check 
the setup.



On Jul 6, 2004, at 11:13 AM, Paul H. Anderson wrote:

>
> I need to align a nice 67 EAC.  To set the receiver gain pot, I have to
> set my URM-25 to 150 microvolts output (then adjust the gain until the
> diode load voltage is -7 volts).  How can I tell if my URM-25 is 
> actually
> putting out 150 microvolts?  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?
>
> Thanks for any suggestions...
>
> Paul
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