Hi Guys,

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One method of AC voltage measurement that is easy to build and calibrate is the shunt diode detector. With suitable components and layout, a frequency response up to the low GHz, flat to ±1dB is possible. The low frequency response is determined by the input capacitor’s value.? Calibration is achieved using a an audio oscillator, amplifier and a true RMS DMM, with an accuracy of ±0.5% to calibrate the AC/DC transfer curve. If the detector’s designed low frequency response doesn’t extend into the audio region. temporarily add a larger capacitor to extend the low frequency response into the audio region.

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I have built several germanium diodes probes using diodes similar to the OA81. These have a PIV of 70V, giving a max RMS input of 24.5V, which equates to 12W into 50Ω (40.8dBm). This would give repeatable readings down to around -10dBm (0.3mV). For higher voltages an attenuator would be required. For RF work, this could be implemented as part of a test load using a tubular 50Ω resistor. The diode is connected to a point, say 10% up from the cold end, which gives several advantages. It reduces the diode detector’s loading of the signal, improves the return loss and? increases the power capability up to 1.2Kw.

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