Re: RF probe for millivolts?

On Dec 28, 1:41 pm, lagagnon <lagag...@xxxxxxxxx> wrote:
I need to make some trimmer cap replacements to an IC-735 and then do
a PL adjustment after replacing them. To do this properly (according
to the service manual) I need an RF voltmeter capable of measuring
the range of 40-400mV.

I have a DMM with an input impedance of 10M ohms. I was hoping to
construct a simple RF probe to the DMM and thus measure RF volts.
However, the way I understand it, most of the suggested RF probe
schematics (eg - the one in the ARRL Handbook) would only work in the
range of volts, rather than millvolts, due to the voltage drop on the
detector diode.

I have found a solution in the RSGB "Test Equipment for the Radio
Amateur" book, which unfortunately requires building 4 transistor
array to amplify the signal. A bit more work than I want to get
involved in unless necessary!

So my question is if anyone knows of a simpler solution for an RF
probe that can measure RF millivolts without an amplifier array?

Any suggestions greatly appreciated.

Larry VE7EA

It's a common misconception that detector diodes (or any other
semiconductor diodes, for that matter) don't turn on at all below some
magic voltage. That's a lie! The conduction current versus bias
voltage is a smooth curve that passes through 0 current at 0 volts
(assuming you don't have light falling on the junction ;-), but
there's more conduction at +1 millivolt than there is a -1 millivolt.
I've built single diode detectors that I've used to detect RF as low
as about 100 MICROvolts. It's QUITE EASY to see the output for a 40
millivolt input even if you use a cheap 1N4148 silicon diode as a
detector and a not very fancy DVM to read it out. Try it! The thing
is, the output at low voltages will be a voltage that is linearly
proportional to the _square_ of the input voltage. That transitions
to being linear with input voltage as you get to RF voltages in the
range of "one diode drop," or about half a volt for standard silicon
diodes. That square thing means sensitivity drops rapidly as you get
to low RF voltages: cut the RF in half and the DC output drops to 1/4
what it was.

If you can get an RF Schottky diode, or a germanium detector diode,
you'll have greater sensitivity, but if your volt meter goes down to
below a millivolt (e.g., a 3-1/2 digit DVM with a 200mV range), you
should be able to easily see the response to 40mV of RF with just a
1N4148-based probe.

To be sure I'm not trying to blow smoke some bad place, I just tried
this, with a 1N4148 as detector. Using a _very_ cheap DVM with 1
megohm input resistance, I got 0.1mV output with about 30mV RF input.
With a better DVM with 10 megohm input resistance, 15mV of RF is
easily detectable at 0.1mV output. This was at 50MHz, but holds down
at lower frequencies too.

Cheers,
Tom
.

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