A brief study of RF probes for signal tracing with a VTVM (or DMM)

A brief study of the response of my crystal R.F. probe as used with my VIZ, WV-98C V.T.V.M. with its 10MΩ input impedance.  (And yes, it works with my DMMs, too, and they actually appear to be a bit more accurate at very low RF voltages.)
I thought this might be handy for some of you fellow VTVM users for chasing radio frequency signal through tube radios. First, let me describe my “homebrew” RF probe. This is the schematic, courtesy of N5ESE at http://n5ese.com/rfprobe1.htm




I have made a few of these, and this is one of the prettier ones. I used thin walled brass tubing for the grounded handle underneath all the electrical insulation, and a 630V DC blocking capacitor. The tip is steel. The diode is a 1N34A I get by the package for cheap on “ the Bay” or from Radio Surplus Supply…I think I have the name right(?). The cable is shielded coaxial audio cable. The connector is the old style Amphenol Model  # 80-75-MC1F, Single Conductor Microphone connector. Tube Depot sells these, I think.  The shielding is to keep stray voltages off the line but it is only carrying DC current and is pretty well isolated from the circuit under test by the resistor in the probe handle.

If you look at the typical curve of a 1N34A germanium diode, you’ll see that there is a great deal of non-linearity in the area below 0.2 to 0.3 volts (below the knee-voltage) but that does not mean it cannot rectify AC into DC below these voltages, they just become less accurate.


The probe:



The meter:



The measurement setup:


I have a 470KΩ resistor across the output of the signal generator to help stabilize the load. The scope and RF probe are constantly all hooked up across this resistor during all measurements. The generator is set to 2MHz during the entire procedure, as well. This was chosen because my generator puts out a healthy voltage below 4MHz and because it approximates the highest frequency typically found in AM broadcast band receivers. The scope was using a 10X probe, the meter was set at 0.5VDC range.
Signal generator measurement—all voltages listed are RMS values

Starting with the generator at full output:
Scope = 297mV Meter =  272mV

Turning down the generator at random increments:
Scope = 226mV Meter = 200mV
Scope = 141mV Meter = 112mV
Scope = 85mV Meter = 66mV
Scope = 42mV Meter = 12mV
Scope = 14mV Meter = Needle just moves off zero, but movement is clearly visible.

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RF Probe/VTVM  measured RF in a GE, T143A, 5 tube AM radio receiver tuned to strong local station of 50kW, about 14 miles away:  (Again, RMS voltage values, or really DC into the meter)
(I believe the pin numbers to be correct, but I make mistakes.)

12BE6 Convertor pins:
#1 Oscillator grid = 7.2
#2 Cathode = 0.34
#5 Plate = 1.5
#7 Antenna fed control grid = ~0.01V(?) just moves from zero when any strong station was tuned in.

12BA6 IF amp. Pins:
#1 Control grid = 0
# 5 Plate 0.1V off station, 2.75V tuned in. Needle fluctuates slightly with station modulation.

12AV6 Detector/1st Audio amp pin:
#5 Detector anode = 0V off station, 0.7V tuned in. Needle fluctuates slightly with station modulation.


Well, I hope this information might help someone who uses or is contemplating buying or building a basic RF Probe for use with their high impedance voltmeter for RF work, by giving an example of what they can do, and what they might read in a normally operating radio.

Yes good info for me, I have RCA 510A TVM (transistorized Voltmeter) and no probe, so I will sooner or later build one, if I can not find an original. May consider trying it on the HP 410C too!

Good job. Thank you so very much.............. Nice set up.........

Very well done! Valuable information! Thanks for sharing!

Thanks guys! I hope it is useful sometime.
I think these are a handy, overlooked, little tool.

R.i.F. This would be a good additional probe type for that 510, I think. I keep this one rubber banded to my wv98c. I tried the regular AC settings and normal probe for below 3MHz, since it's within the bandwidth of the meter, but the AC impedance is just too low and the results were terrible until I built this here RF probe.

Well, fellas, I had so much fun making and testing this here RF probe that I made me another one. This one has a retro look and uses a vacuum tube in the probe handle for the rectification.
Wire similar to the other one otherwise. This tube model indicates peak radio frequency voltage though, without the 4.7M ohm series resistor in it. I used a 220K ohm for a bit of extra lead isolation, so, technically, it feeds about 98% of peak voltage into 10M ohm meters.

The double lead is shielded coaxial cable. One half feeds the filament. Grounds are common.
The performance is similar to the crystal probe but just a bit more sentitive, perhaps, on the low signal/high impedance end of things. Plus it keeps your hand warm. You do have to let it warm up and them re-zero the meter due to the bit of DC contact voltage generated by the vacuum tube. It must be re-zeroed when voltage range is changed on the meter, too.


Under construction from 3/4" nominal copper plumbing pipe:




Finished and tested:

Excellent work! It has a nice period look.

Oops....went to edit and lost the picture. Here it is again.

Thanks! I like to mess around in the shop and I don't have any radios on the operation table at the moment.

This last probe with the vacuum tube has now been modifed to indicate peak-to-peak voltage on the meter by using each diode section independently in a voltage doubler configuration.

This is the schematic.
(I do not believe it contains errors, as I wired the probe from it, but errors are always a possibility in a non-proof read document.)

Made a new scale on the VTVM today, calibrated to P-P for what the actual P-P is, and what the vacuum tube voltage doubler RF probe puts out:

That is great work! (For just messing around  )

Thanks. I will use it, actually.

Wow. Some neat work Ragwire.
I assume you printed the probe label on a clear transfer?

Polaraligned wrote:Wow.  Some neat work Ragwire.  
I assume you printed the probe label on a clear transfer?

Thanks, yea, I got some sheets of clear water slide decal for inket printers at a local hobby store. Testors brand. Also the clear overspray in the can that keeps the ink from running in the water that you dip it in to release the decal from the backing paper.