Simple RF Probe for my DMM

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Simple RF Probe for my DMM

Post by Ragwire on Tue Dec 23, 2014 8:49 pm

I made this for easy signal chasing, but primarily for a quick check of my CB tranceiver output across a dummy antenna. (A non-inductive 50 ohm power resistor network).

There are a lot of simple RF probe designs out there, and most call for a 4.7 megohm resistor for the voltage divider, but because of diode voltage drop, and some reverse leakage I suppose, I have found, experimentally, that a 3.3 megohm resistor and an extra filter capacitor after it, seem to make the RMS value of the radio frequency voltage much more accurate to the DC to the meter. This is assuming a more or less standard 10 megohm DC meter resistance for a digital multimeter.

It is surprisingly accurate from about 200 mVRMS to 20 VRMS or so up to 30 MHz. Below that voltage it is pretty non-linear, but it can still be used accurately if a correction table or graph is made up ahead of time.

The housing is 1/2" (~13 mm) outside diameter with metal foil tape for a shield. The cable is also shielded to prevent stray "hum" voltage or RF field from materially affecting the reading.

The endcaps I just shaped out of an epoxy putty, but I have also made them of wood and other things in the past. They are painted with black laquer.

cheers
Rob











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Re: Simple RF Probe for my DMM

Post by 75X11 on Tue Dec 23, 2014 9:16 pm

If it were just a few years ago, you'd be making some money publishing project articles for the DIY electronic magazines. That is just about as elegant in form and function as I have seen!
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Re: Simple RF Probe for my DMM

Post by Ragwire on Wed Dec 24, 2014 10:20 am

Thanks!
I like it when people ask me where I bought something that I actually made. To me, half the fun is trying to make these things durable and accurate, and the other half the fun is the painting, decaling, etc.

Rob
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Re: Simple RF Probe for my DMM

Post by 75X11 on Wed Dec 24, 2014 1:41 pm

It is something to consider that the mighty Hewlett-Packard started out in a simple 1 car garage and built equipment people could rely on. I admire those that can do much the same these days when most have given up on thinking they can do for themselves.
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Re: Simple RF Probe for my DMM

Post by Ragwire on Thu Dec 25, 2014 10:04 am

I thought about selling some of my probes online, but I would have to decide which was the most perfect design, and set up to make a bunch of them all alike with minimal hours of hand fitting, etc. so they could be sold at a reasonable price.
Each unit would come with it's own tested correction graph or table (possibly decalled right onto the handle) for extended accuracy at low RF voltage levels where the diode(s) is/are in square law operation.

Some of the units I made for my own use have this and that allows me to put it right onto a high impedance RF radio circuit and read down to, literally, a few millivolts with good accuracy by correcting the meter reading to the chart or graph. At higher levels, they can be read 1:1.

These can be handy when someone does not own a scope or is measuring RF that exceeds the frequency response of the scope that they do own.

Rob

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Re: Simple RF Probe for my DMM

Post by Ragwire on Mon Dec 29, 2014 3:18 pm

This one is a bit more sophisticated. It is still a passive probe, but can be calibrated in both output and linearity. Its minimum sensitivity is about 2.3 mVrms input, and in testing so far, has an essentially a flat response from high audio frequencies to about 30 MHz I have not gone beyond that, but it should be good to 100 MHz or so.

The main difference with this RF probe is the fact that it is just about dead on 1:1 RMS input to DC output from 150 mV to past 15V. Again, it should continue to be linear, but I have not tested its voltage response beyond about 26 dB.



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Re: Simple RF Probe for my DMM

Post by Ohwenzelph on Fri Apr 21, 2017 4:16 pm

What is the circuit for the second more sophisticated and sensitive probe and how do you adjust it?
Thanks

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Re: Simple RF Probe for my DMM

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