RF Probe




About: I'm Chandra Sekhar, and I live in India. I am interested in electronics, and building small one-off circuits around tiny chips (the electronic kind).

Probe for measuring in the Radio Frequency range built inside an earphone jack, designed to have minimum capacitance and complete shielding.

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Step 1: The Parts

You will need:

A diode. Germanium point contact, OA79 or 1N34 is the preferred and traditional choice. But a modern substitute will be a schottky (gold bonded) diode. These have a low forward voltage, below 250 millivolts or so, versus the 600 or so of the silicon diode.

A few capacitors, chip type, value not critical, around 1 nf to 100 nf. (or 1000 pf to 0.1 microfarad)

One resistor, 1 Megohm.

One stereo earphone jack.

One pin from a turned pin IC socket. This forms the 'hot' end of the probe, so that various pins can be inserted for probing. A short piece from a needle for exploratory work. A piece of wire soldered to the circuit board when using it for a tuning up session, requiring it to be in place for a long time.

The circuit is traditional, as below:
   C1          R<--||-------/\/\/\/\-------------o +      |                |     ---               |      ^                |     /-\ diode        === C2      |                |      |                |<--------------------------------o -

Step 2: Connect It Up

I used a small sliver of circuit board to hold C1. The turned pin socket was fitted to one end, the capacitor C1 soldered to it and then the resistor and diode leads formed to shape and soldered to the capacitor. It is shown next to the stereo earphone jack in order to see whether it will fit. Adjust sizes, get a smaller capacitor etc until it does.

The socket, capacitor and supporting board will go inside the barrel of the jack.

Step 3: Take the Jack Apart

A bit of filing, twisting and pulling will result in the jack coming to pieces. It is composed of a large number of precision formed metal and plastic pieces.

We need just the outer shell and the cover - It is best that the cover of the jack is metal, otherwise the shielding will not be perfect.

Step 4: Form the Body

The body of the probe is made from an old ball point pen. A piece of brass tube was found to serve as a join between the shell of the stereo jack and the ball pen body. The rubber grip from another pen was then slid over the metal to insulate it from accidental contact with circuit parts.

A molded mono earphone lead was used to get the signal out from the probe.

Step 5: Put It All Together

I used teflon tape to insulate the turned pin socket and capacitor, which had to go inside the barrel of the jack. A dab of petroleum jelly kept its bore from filling up as superglue was applied to hold it in position.

A HAM friend of mine measured about 5pf of capacitance for this probe, which is pretty good.

In use, a sharp probe is inserted into the turned pin socket. The ground lead is wire wrapped around the barrel of the jack. This is necessary if accurate measurements are to be made in a noisy environment.

I hope this will be of use to the HAMs among us.

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    19 Discussions

    Jimmy Proton

    8 years ago on Step 2

    i never knew what those clear things with the colors on them are until now, why do i only see them on tvs and monitors?

    4 replies

    What kinds of things do you take apart? I find so many signal diodes (1N4148's typically) in trashed electronics I don't bother desoldering them anymore. (Besides, they're cheap enough that using a new one in a project is less troublesome than using a damaged scrap bin diode.)

    Me too :3
    I prefer early 90's or late 80's era devices, before everything went surface mount.
    If you take recent products apart, you'll be quite disappointed at the component yield. You've probably seen few discrete signal diodes because manufacturers cram everything they can into ICs to reduce component counts.
    On the other hand, an early 90's handheld four-in-a-row style game I recently took apart (lack of backwards current protection fried a LED ) has 34 discrete transistors in it!
    Try going through free piles after garage sales or the free section on your local craigslist boards.

    Old CRT TV's probably have the best stuff in them. Whenever I take apart new things I put the boards in the toaster oven or i destroy them.


    12 years ago

    I thought the first picture was a person trying to pull their bike away from a huge cylindrical crusher..

    3 replies

    Reply 12 years ago

    lol i seriously rofl'd it does look like that


    Reply 12 years ago

    It isn't, because the socket at the tip, the series capacitor and the ends of the resistor and diode are held securely inside the tip of the audio jack. You may use hot melt glue to hold them in position. I wrapped it in teflon tape (available at plumbing stores) and secured it in position using superglue. Epoxy would also work.


    Reply 12 years ago

    I think that epoxy would be best, hot glue expands, and you tend to get a but too much, super glue is runny and I always get it on my fingers (my tongue too once, don't ask).


    12 years ago

    Okay, for those of you who want a more in-depth understanding of how it works... c1 is a dc-blocking capacitor. It's there to make sure you're only measuring the AC voltage in the circuit. d1 is there to convert the ac into dc pulses. c2 is a filtering capacitor, so that you're reading the peak voltage (or close to it) rather than the average, which will be quite a bit lower. It also allows any remaining ac a nice path to ground. R makes sure that you don't draw too much current through your meter, destroying it. One thing to keep in mind is that you'll probably want a shielded cable, to keep from picking up stray AC from the board you're testing. RG 174 coax would be nice if you've got some. Connect the shield to the - side of the circuit, and the center conductor to the + side. Slap a BNC connector on the end, rather than a phone plug, and you can use the probe with an oscilloscope, too.


    12 years ago

    This is a classic from the AARL. The neatest ones were built, IMHO, out of lipstick tubes. The non-linearity of the diode, not to mention the integrator, limits the accuracy of this technique, so nowadays, something built out of today's high-speed op-amps might be possible. A bolometer would be more complex, and slower, but far more accurate. This is a very useful instrument, where it is indicated. "Rectify and integrate" is also the principal behind simple AM detection, although with a shorter time-constant, typically.


    12 years ago

    this may be a stupid question. but i dont know what it does. so what does it do?

    2 replies

    Reply 12 years ago

    Most voltmeters don't accurately measure AC (alternating current) at high frequencies (RF = Radio Frequency). At RF frequencies, the components in the probe convert the AC value to an equivalent DC value that a voltmeter can accurately measure. There is a good writeup here ( http://www.io.com/~n5fc/rfprobe1.htm )


    12 years ago

    Nice instructable, neelandan.... one of those pieces of test equipment you don't think you need until you have one!