Commercial RF Millivoltmeter probe?


Floyd Sense:
I want to pick up an older commercial RF Millivoltmeter, such as HP or Boonton, but of course they just about all come without probes.  Anyone know the design of these probes so that I may construct one?  The connector appears to have two pins plus grouind.  I know how to make a simple RF probe to use with a DC voltmeter, but is that what these meters used?  I suspect not, otherwise how would one be able to measure millivolt level signals that would not cause the diode in the probe to conduct?  

Tom Rauch:
Usually all of those things I've seen use a special Schotkey diode (it has to have known characteristics) and dc bias voltage that sets the diode just at conduction limits.

Even though I have access to hundreds of special diodes in my stock, I always bite the bullet and buy the original parts. Otherwise I've never seen the calibration be correct.

73 Tom

Floyd Sense:
OK - that makes sense.  The real mystery to me is where in the heck all the old probes went.  There are hundreds of old HP and Boonton voltmeters for sale, but almost none with a probe.  

Steve Katz:
People must have hanged themselves with them!

But I agree, if you don't use the "factory" probe, chances are slim that you'll ever get the instrument calibrated, even roughly.

The Boonton probes used custom-made diodes (by ITT) which Boonton kept under lock in a safe, with the combination only known by three people (honestly!) when I was there back in the seventies.  I wasn't one of the three.  For engineering development (bench) purposes, I'd have to beg the C.E.O. to unlock the safe and give me a few...

The bypass/filter cap was also special, a discoidal device with no resonances through 1.2 GHz, when used in that probe.  The diode had to be matched well to the bias and chopper used in the meter, which allowed it to exhibit almost straight line log characteristics over 1200 MHz of spectrum and about 60dB of input signal range.  The 50 Ohm screw-on adapter, which created a good termination for a lot of work, was also very good from DC to 1200 MHz, and the whole assembly (termination + RF probe) was flat within <0.5 dB over that spectrum.

Although they don't do mail-order (that I know of), I see Apex Electronics in Sun Valley, CA always has these probes for both the HP and Boonton RF MVM's.  If you have a local electronic surplus dealer you might find the same, locally to you.  Otherwise, the "usual suspects" might help: Fair Radio Sales, Surplus Sales (of Nebraska), etc.

Good luck!



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