RF Probe Circuit

[Mark A Moss]

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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