RF Probe Circuit

[Al Waschka]

Mark A Moss wrote:
>
> On Thu, 6 Aug 1998 17:07:32 -0700 (PDT) Joe Foley <redmenaced at yahoo.com>
> writes:
>
> >> I believe the question was: "What is the purpose of the circuit of
> >your
> >> earlier post?" (Description given below)
> >>
> >> To Probe >-----]<----][----<To circuit
> >>                D1    C1
> >>
> >> This circuit is used to convert an RF voltage to a signal that can
> >be
> >> handled by test equipment designed for use in the audio range (such
> >as
> >> VTVM's or signal tracers.)  The diode rectifies and averages the
> >incoming
> >> signal.  This results in a signal that varies with the modulation on
> >the
> >> RF signal.  The capacitor is mostly to isolate the circuit from the
> >test
> >> equipment and vice-versa.
> >++++++++++++++
> >Ah! Thank you, I understand now.  I knew what the components did, I
> >just didn't understand why it was done.  You say the diode "averages
> >the signal", is this a zener or a similar function?
> >
> >Thank you for your patience,
> >
> >Joe
>
> I believe the principle is that the diode cuts off half of the signal
> (either the positive or the negative half depending on the polarity of
> the diode).  Because a diode is not perfect, especially at a frequency of
> several MHz,  is has an effect of averaging the signal because it can't
> switch into and out of conduction fast enough.  That is my understanding
> anyway.
>
> If anyone can explain this better or more correctly, please feel free.
>
> Mark Moss, KC8DEI
> Kalamazoo, MI
> U.S.A.
> Grid EN72eh
> www.qsl.net/kc8dei
>
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My 2 cents:

The purpose of the circuit is to allow a VTVM to be used to measure RF
voltages.  The capacitor provides a DC block so that the meter is
measuring only the RF part of the voltage present.  Without the
capacitor, if you tried to measure the signal on the plate of a tube you
would get a combination of the DC and AC voltages.

The diode rectifies the RF (converts it to pulsating DC).  Most VTVMs
did not have the frequency response in AC mode necessary to measure RF
voltages.  Rectification to DC allows the meter to measure the voltage.

This is a very simple circuit.  Most I have seen also add a resistor in
series which calibrates the probe so the voltage measurements mean
something, i.e. RMS or peak.  Some add an additional capacitor from the
output to ground to filter any remaining RF component.  This is
especially helpful for use with more recent DVMs.  Another has the diode
going to ground to short the positive cycle to ground rather than in
series.  That is the Heath RF probe circuit, i.e. incoming capacitor,
diode to ground, series output resistor.

73,

Al K5TAN

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