[Hallicrafters] re: urm-25d

[Roy Morgan]

Glen and others,

It's good to use equipment recently calibrated equipment, but if you don't 
have any handy, it costs a LOT to get calibration done.  There may be other 
methods that will help us out.  Here is one suggestion:

Measuring RF voltages at moderate frequencies (low HF, for instance) and at 
levels of a volt or two is relatively easy to do.  If you only need to be 
reasonably sure of the voltage to, say, 5 percent, then you can build a 
simple detector circuit. Once you have an RF voltage at a volt or two, then 
you find a way to attenuate the signal down to the level you need.  By 
comparisons between a moderately reliable low voltage and other levels in 
the vicinity, you can be pretty sure of your results.

Attenuators can be bought or scrounged at hamfests.  (New variable wide 
band, accurate ones cost hundreds of dollars.)  There are little fixed rf 
attenuators available now specified quite closely and not very costly 
(Start with the Vishay company). The attenuator in the URM-25 can be 
checked at line frequencies or dc to see if the resistors have been 
damaged.  Your common modern DMM is plenty accurate for this.  You may be 
able to get the attenuator apart enough to measure the network at different 
points if you detect trouble.  This can be done, of course, from the output 
connector at DC with an ohm meter.

The URM-25 uses a system of a diode voltmeter/detector (at 2 volts I 
think)  to set the calibration level, and the stepped attenuator to reduce 
the output by 10 db levels from there.  The oscillator or buffer is varied 
in output by changing the B+ on it, with the voltmeter indicator giving you 
"calibrated" output levels.

The voltmeter/detector useful at HF frequencies can be little more than a 
diode, capacitor and a resistor in a box, all carefully put together.  The 
URM-25 circuit can be a starting point. You may be able to send a home made 
one to someone who can "calibrate" it, or more reasonably, check it's 
operation, over a limited range of frequencies and levels. This then 
becomes your "standard", and combined with a step attenuator, you have a 
system to create rf voltages at the microvolt level with moderate assurance 
that you are getting what you think you are getting.

If you have a URM-25 or similar generator, here is what I suggest:

1) Understand the output setting system and how it works.

2) Understand termination requirements for your generator, including 
generator load, cable effects if mismatch is present, and receiver load 
effects on actual generator output.

3) Examine the detector/voltmeter circuit in the generator and replace any 
suspect components, such as 50 year old germanium diodes, any paper 
capacitors and meter movements and mulitpliers that may have aged.

4) Do some calibration of the meter system using low frequencies and/or DC 
if you can.

5) Check the attenuator for damage.  Apply D-Oxit is similar to the switch 
contacts if you can.  Use DC or low frequency AC measurements to check the 
attenuator operation.

6) Get and use cable terminations and pads intelligently to ensure proper 
rf output.

7) Ponder the effects of receiver input impedance on the generator output 
system.

8) Then measure your receiver(s).

Lastly, let us all know what conclusions you came to.

Here are a couple of questions to ponder:

1) If I set my generator to 2.0 micro volts and the actual rf input voltage 
at the receiver input might be as low as 1.9 micro volts or as high as 2.1 
micro volts and I'm not sure, what difference does it make?

2) If I think I have 2.0 micro volts at 1 megacycle, how sure do I need to 
be that I can also get 2.0 micro volts at 20 megacycles?

With reference to Glen's question: "...how do I know that I can believe
that It's really putting out what the x-multiplier says it is?" ---  you 
cannot know for sure.  You can take steps to be more sure of what it is, 
but lots of "more sure" costs lots of money and trouble.

At 07:52 AM 3/18/2004 -0800, you wrote:
>The easiest way is to compare the receiver sensitivity
>by using a signal generator that has been recently
>calibrated by a reputable calibration service.
>
>Glen, K9STH
>
>
>--- JFran10657 at aol.com wrote:
>
>Very frustrating, how do I know that I can believe
>that It's really putting out what the x-multiplier
>says it is?
>
>=====
>Glen, K9STH


- 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 --