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The VU2SWX BiQ rf probe

Revised version - Simplified with LED only.

This is a simple rf probe, being self contained with LED indication of rf power with modest
power levels (a few milliwatts). Since it uses hand capacitance for isolation the coupling capacitor
traditionally seen at the input is dispensed with. This results in a very simple circuit.

A photo of the dual diode together with a diagram of the pinout is below:

The components are assembled on a two-strip wide piece of general purpose circuit board, cut and sanded down so that it will fit inside the pen.

The completed circuit. After testing, this will get inserted into the body of a defunct BiQ pen.

A capacitor is placed directly at the terminals of the dual diode, mainly to avoid harmonics getting radiated.

The exploded view shows how it all goes together. A pointed wire is sealed into the nozzle of the pen to act as the probe tip.

The earth terminal, and the return for the rf current that lights up the LED, is via the hand of the user. To increase the value of the capacitance between the user's hand
and the 'cold' ends of the dual diode, a strip of aluminium foil is wound around a plastic tube (empty refill) and connected to one side of the LED. The completed probe is below:

These modifications were done as a result of feedback from VU2SWX on using the probe. He said he used it exclusively to tune up Tx circuits, using the LED for indication,
and that he had NEVER connected it up to a meter. Since this probe uses silicon diodes their forward voltage added to the voltage required to light the LED comes to a few volts.
So this probe will not be useful in low power or Rx circuits. The classical germanium diode based probe should be used in such cases.

The Original VU2SWX BiQ rf probe:

This is a dual mode probe, being self contained with LED indication of rf power with modest
power levels (a few milliwatts) and using a meter for indication of smaller levels.

Since it uses silicon diodes, the relatively large threshold voltage (around a volt for two diodes) is
overcome with a "bias box", supplying the probe with the forward voltage from a button cell and
preset pot.

If the rf level is above about two volts or so the LED lights up and peaking, tuning etc can be
performed using this indication.

For detecting smaller levels of rf, the probe is connected to a bias box and meter. The preset is
advanced until the meter indicates about ten millivolts. This indicates that the diodes are conducting.
For subsequent measurements, this value can be subtracted from the indicated reading to obtain the
peak-to-peak value of the rf at the probe tip.

The loading presented by this probe is low until the LED begins to light up. That load may be decreased
further by increasing the value of that 1 Megohm resistor to 10 Meg, but then the meter reading tends to
jump around due to pick up of noise.

When the LED has lit up the probe presents a heavy load to the circuit being probed and any tuned circuit
will be heavily damped. So certain precautions are necessary. If you apply the probe to the LC circuit
which you are trying to tune for resonance, the setting will be slightly off due to the additional capacitance
presented by the probe. One workaround is to probe the output while adjusting the input, of an amplifier
or frequency multiplier stage.

This rf probe was named the BiQ rf probe because it used a transparent plastic body of a
ball point pen which looks like a pen which sounds somewhat like BiQ Ball point pen.
This came about because I found a few dual diodes in a small outline transistor (SOT) package
and was playing with them, trying to find if they were good to use at rf. They were, and
VU2SWX said he would like an rf probe using them. Thus the VU2SWX BiQ rf probe came into being.

This is the body of the pen, minus cap and innards. A piece of copper clad board was cut to size and then
filed down to make it fit inside.

This shows the layout of the components on board. The two diodes are inside the SOT package,
the capacitor at the output is a chip type and the coupling capacitor in series with the pin is mounted
on the other side of the board.

This shows the other side of the board, showing the coupling capacitor ( a 100n capacitor wouldn't fit,
so a smaller one of 10n had to be used) and the socket for the ground lead. This socket is a single pin
from a turned pin i.c. socket, inserted and soldered into a hole in the board.

A side view of the board shows how the components are placed on the board

The completed rf probe. The board was inserted into the pen, filing it to make it fit and then fastened with
superglue. A hole in the side was drilled for access to the ground socket. enamelled wires were used to bring
out the connections to the meter. A socket was fabricated, again, from two pins from a turned pin ic socket
and placed at the other end.

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