[R-390] URM-25D recap.

[Roy Morgan]

At 02:15 PM 6/24/02 -0400, JM/CO wrote:
>Yes, an "RF" voltmeter is required here.

URM-25 Calibrators,

I agree, but there is more.

>   An excellent meter for
>this purpose is the Hewlett Packard model 410-C,

Indeed it is, but what if such a thing is not available?  All is not 
lost.  Lets make some assumptions:

1) High accuracy is not required.

2) The voltmeter circuit in the URM-25 measures accurately enough for our 
purposed across the frequency range of the generator.

3) If an AC meter is available that can measure well enough at a few 
hundred KC or even much lower, then we can use it to set the URM-25 
calibration.

4) The manual for any meter that is available will give some idea of the 
accuracy to expect at a few hundred KC or lower.

If we can accept these ideas, then the procedure would be as follows:

1) Get whatever AC meter is available, and figure out what accuracy you can 
expect from it.

2) Calibrate the URM-25 with that meter.

3) Call the project done.

A couple of alternate methods come to mind:

Method A:  Find some recommended AC voltage detector circuits useful at the 
frequencies of interest.  Build one and assume the measurements are close 
enough.  This method may well involve very few components (one diode, one 
resistor, one capacitor and a way to mount them), and a DC voltmeter of 
moderate accuracy.   If a circuit reported to have good performance is 
duplicated carefully, the results for these purposes should be quite good 
enough.

Method B: Build such an AC voltage detector and mail it so someone who does 
have the likes of an HP 410C for calibration by comparison at some 
frequency(ies) of interest. Upon its return, use it to do the calibration.

Notes:

1) I seem to remember that the output calibration of the URM-25 depends on 
setting the signal generator section output, at the input to the 
attenuator, to 2 volts rms.  (I believe that the GR 1001A and the HP 606 
are similar.)  Making an ac voltage detector that works well at 2 volts and 
over a limited range of low to moderate frequencies would be a rather 
simple job.  The URM-25 output voltage specifications is based on at last 
the following points:
  A) Calibration of the 2 volt point is done as specified,
  B) The  RF attenuator is working properly.
  C) The generator output is terminated properly.

2) Making very accurate RF voltage measurements over a wide range of 
frequencies is quite involved.  Careful study of the specifications of the 
HP RF voltmeters and similar general use laboratory quality instruments 
will give an indication of the accuracy attainable in production equipment. 
A look at the following URL, which explains the RF voltage calibration 
services available at NIST, reveals that at 100 MC and above, peak to peak 
detectors can be calibrated to 1.2 percent.  No one in the country can do 
better. (Think about that next time someone reports the sensitivity of a 
radio receiver to two or three significant digits.)
<http://ts.nist.gov/ts/htdocs/230/233/calibrations/Electromagnetic/Voltage.htm#534>

3) An ac voltmeter that works well up to frequencies beyond the range of 
the URM-25 is simply not necessary.

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
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