RF Voltage Quadrupler Probes Used In Older Equipment

[Dawn]

I've been trying to find a information on these probes. They apparently were common with some service grade VTVM's, TVM's and early DMM's where the consumer service would not invest in a true RF voltmeter. These probes had their loading and capacitance limitations and often were supplied with a nomograph at the rated frequencies. The problem is that many service stage gain measurements primarly during the 60's and 70's relied on the voltages derived from these type of probes for consumer equipment rather due to limitations on scopes and the very expensive cost of a RF voltmeter. Annodated voltages were often referenced to these. Among them, the Sencore DP-213 RF probe tip converter was used as a metric in many consumer service manuals. They must have had a very low impedance and loaded the stage even at IF and lower frequencies such as CB, but none the less were spec'd when doing stage gain measurements or point voltages in service docs. I'm good with both a Helper and Millivac RF voltmeter on the bench. However much of the older equipment relied on measurements with these probes that was annodated in the documentation of the time. Motorola used such a multiplier probe in one of their early electronic voltmeters as well although I don't know if it was a quadrupler. I'm dealing with some 70's restorations that spec'd the Sencore DP-213 as their reference. It was some sort of slip over attachement to VTVM and TVM's of the time. The loading effects would not be consistant with a conventional RF Voltmeter. I've seen some circuits that used 1n34 diodes and some SM caps from the 50's as early VTVM specialized probes. I'm sure by the 70's, other diodes such as Shotkey diodes were used. Does anyone have any info on these probes specifically the Sencore? That was the one heavily used in domestic Stereo FM, TV, and CB service documention during that period. Anyone have any docs on this probe?

I'm working and doing restorations using the results of these probes in the documention and wonder if anyone has any docs on the probes.

[Mr. Coffee]

I'm sorry I do not have any documentation, but I have made and used these.
To the best of my experience and understanding though, it is not true that they excessively load the RF circuits under test.
When the capacitors charge after initial connection, so long as the meter being used--such as a VTVM or DMM--has sufficient DC input resistance, there is virtually no current draw from, or into, the quadrupler. The only loading is the reactance of the probe tip, diode(s) (which is/are usually a very low capacitance point contact type, and the ground leads. The ground leads are usually only 3 or for inches long and connect to the probe handle ground shield for this reason (rather than a long ground back to the meter. The only thing past the first diode is DC, and it is charged almost instantly to the peak voltage of the RF. From that point on, the only current draw is that which the VTVM draws.
I personally have not found any significant change in output voltage to input voltage for ordinary radio frequencies, and I suspect the nomograph, if supplied, is more for the voltage nonlinearity of these passive devices when reading low RF voltages such as less than 150mV or so.
These devices--even homebrew, with a little care--can easily output a predictable output voltage into a sensitive, high impedance meter when reading only a few millivolts RMS at several MHz. However, the diodes are then operating in the "square law" region, where the diode is not fully turned on, but is conducting with a higher reverse resistance than the forward resistance and the output DC voltage is a function of the square of the input AC voltage. This is the way the diodes operate when they are well below their normal knee voltage. Therefore, the output voltage to input is very low and nonlinear as the signal increases. Above a certain AC (RF) input, they again become linear--or very close to it.
A scale can be applied to the face of an analog dial to compensate for this, but few people would mark up their meter face, and in either case, they would need the nomograph or some such conversion chart to do it in the first place.
Typical crystal diode or vacuum tube RF voltage doubler probes will operate relatively flat into the hundreds of MHz, if properly designed.
The best I could do is perhaps post a list of input vs output voltages at low signal levels if that might help interpret the service data, but I have no way of comparing my data to data obtained from an unknown device, of course.

cheers
Rob

Edit: After further testing side by side with both probe types, I did find that though both types are useful, the quadrupler probe does load my radio circuits more than the single diode doubler type. The single diode type may in fact be the more useful one for signal tracing of this kind, after all.

[Mr. Coffee]