RF PROBE

RF PROBE

This configuration is made so that it's a simple detector to detect both positive and negative half of the RF signal. The resultant DC voltage will be peak to peak at C1 with respect to the input RF voltage

C1 – 0.001uF

D1 – IN34A or similar Germanium diode

Rx - 4.7 ohms.  R = 4.7 Meg for Zin = 11-Meg;
                                   R = 4.3 Meg for Zin = 10-Meg;
                                   R = 430 K for Zin     = 1-Meg

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Rx For best accuracy, size the resistor to match your DC Voltmeter's input impedance:

 The value of the resistor Rx

 The peak value is about 1.414 times the RMS voltage. Rx has to drop this excess voltage so the meter indication is accurate. If we know the input resistance of the meter, we can calculate the value of Rx as follows.

10,000,000 X 1.414 = 14,140,000
Rx = 14,140,000 - 10,000,000 = 4,140,000 Ohms = 4.14 Meg Ohms
4.7 meg ohms is the value chosen in all circuits since digital Multimeters have input resistance of 11 meg ohms.

RF probe and the DMM resistance.  This ratio is 10/(10+4.7) or .68 which is less than 5% from .707

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