Highfields Amateur Radio Club
Constructors Corner.

Without access to a Spectrum Analyser or an Oscilloscope it can often be quite difficult to get circuits to work. This circuit is for a basic 10K probe that can be used to detect the presence of RF and even give some indication of the level of RF. It will not tell you the frequency (you need a good RF frequency counter for that), but at least you will know if the circuit is oscillating or not.

This probe is useful for any low level RF work, and simply connects to your multimeter. The voltage shown will not be accurate, since this is a rectifier probe, but the measurements are good enough for you to be able to determine where the RF stops, or if a stage is not giving the gain you think it should.

Parts List:

C1 = 10pf. C2 = 1nf. Both cermic disc type.

D1 = 1N34 or similar Germanium point contact detector diode or a "Zero Bias" silicon diode.

R1 = 10K 5% (¼W should be plenty).

A length of coax (enough to give you a reasonable distance between meter and item being tested) The coax can be anything that you have to hand. In fact, high capacitance cable that is useless for anything else can be put to good use.

2 plugs to suit your meter, 1 red (+) the other black (-).

A short length of stranded hook up wire (30cm or so).

A crocodile clip.

An old, large bodied pen (a large marker pen is ideal, as long as C1 & C2 will fit in).

A copper tack or small piece of brass rod, sharpened.

Some heat shrink sleaving (large enough to fit the O/D of the coax or insulation tape.

Method:

1: Disassemble the pen and remove the innards. Make a hole in the top so it will accept the coax and the hook up wire. Slide the top of the pen onto one end of the coax so that the coax would be protruding from the writing end if the pen were put back together the joining part of the pen is toward this end of the coax. Place the pen body and tip cover to one side, you'll need them shortly.

2: Strip about 50mm of the sheath off of the end of the coax that has the pen top on it, tease out the braid and twist together, tin the end with some solder.

3: Remove the inner insulation from the coax leaving about 5mm. Ease the braid back to the edge of the sheath and either wrap tape over the end of the coax or cover with a piece of heat shrink to stop short circuits (see detail). Place to one side.

4: Assemble and solder the copper tack, C1, C2, D1 and R1 as shown in the layout diagram (in Layout, lower down) ensuring that the finished conctruction will fit inside the pen casing.

5: Trim the stripped ends of the coax and solder to the completed circuit from 4.

6: Feed the hook up wire through to hole in the top alongside the coax and solder to the rest of the circuit. You can cover with tape or heatshrink, leaving the tack uncovered, for rigidity and to stop short circuits.

7: Carefully slide the completed circuit into the pen casing making sure that the copper tack protudes from the hole where the writing tip used to be. You can secure with some epoxy resin (Araldite® or similar) and slide the top down, holding the coax and wire, so that it clips (or screws, depending on type) into the top of the casing.

8: Prepare the other end of the coax to fit the plugs for your multimeter, the positive goes on the centre conductor, the negitive to the screen. You can use heatshrink or tape to make it neater or even another piece of hook up wire soldered to the screen with the negitive plug on.

9: Finally fit the crocodile clip to the end of the hook up wire at the pen end, keep this lead reasonably short for best performance.

Layout:

Shown on the right is the general layout without the pen casing, tape etc.

To Use:

Connect it up to your multimeter, clip the crocodile clip to the ground of the circuit you want to check and probe away. With DC-Volts selected you can get a reasonable indication of the RF voltage level.

To use the RF Probe for signal tracing in a malfunctioning RF circuit or a homebrew circuit, connect the aligator clip to a convenient "ground" or "common" point in your circuit. Often this is the chassis. Most of the time, you'll be probing at the base/gate, emitter/source, or collector/drain of a transistor, one either side of a coupling capacitor or transformer, or at the input or output of an IC. Because the circuits' RF must overcome the diode's barrier potential (of 0.25V, for a 1N34A), voltages much less than that won't read at all, and voltages less than about a volt won't read very accurately. Typically, RF and post-mixer-amps in receivers don't have enough RF voltage, unless you inject a very strong signal at the input.

If you want the probe only for HF circuits then you could increase C1 a little (to say 47pf) so the sensitivity becomes more uniform at lower frequencies.

For use with lower frequencies (a few MHz only), C1 can be increased further in value, but I would not go above 100pF.

Warning!
High voltage circuits must be treated with the utmost respect, and a 500V cap is recommended for C1 unless you know that you will never use it on a valve transmitter or receiver circuit.