RF Probe and Watt Meter

Posted on Feb 6, 2014

This RF probe can be used at High Frequency (HF) or Ultra High Frequency (UHF) on both 50 and 75 ohm coaxial cables. In addition the RF voltage can be measured under load or no-load conditions which allows the circuit to double as an RF Watt meter. The RF probe can be used for oscillators and small transistors for powers up to 2 Watts. The circuit

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is a simple half wave rectifier. In this circuit it works at radio frequencies (RF) and converts any RF signal to a DC voltage, in addition S1, allows a resistive load to be switched in or out of circuit. S1 is a single pole, double throw switch with a Centre off position. The centre position is no load, and left and right positions 1 are for 50 and 75 ohm measurements. First, a small section on measuring RF voltage, current and power, then I`ll describe how to use this simple test instrument. Digital and analogue multi meters can already measure AC voltages so why can`t they be used at radio frequencies The reason is that they can only measure with accuracy a limited frequency range. My Maplin meter measures frequencies up to 400Hz with 1% accuracy, and up to 20KHz at 4%. This also requires that the waveform is a sine wave. At frequencies above 20KHz, accuracy is not reliable. To measure radio frequencies (RF) a simple diode detector circuit is all that`s needed. The detector in this probe is an OA91 germanium diode, but any germanium diode will work. Germanium diodes have a low forward voltage drop (about 0. 2V) and are preferred to silicon diodes which have a higher (0. 6 - 0. 7V) voltage drop. The diode rectifies the RF signal and converts it to a DC voltage, which can be read by a multimeter with good accuracy; the 1nF capacitor is there to smooth the rectified DC signal presented to the meter. When measuring any AC...

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