Posted on Wednesday, January 12, 2011 • Category: Test and Measurement
A pair of AD8307 RSSI (Received Signal Strength Indicator) chips are used to produce DC voltages that are proportional to the logarithm of the Forward and Reflected power levels. These DC voltages can then be subtracted in an Op Amp to produce a voltage proportional to SWR which is essentially independent of power level. The Forward DC voltage also drives a separate Power meter. I have modified Paul's design to use a "Stockton" directional coupler, and to provide a peak-reading capability. The meter automatically provides an accurate readout of SWR for any power level between 10mW and 1000W. I arranged for the Power meter to cover the range 100mW to 1000W, but it is easy to change the circuit to set upper and lower power limits to any values in the range 100uW to 1000W.
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