Sampling RF Power

Posted on Feb 5, 2014

Perhaps you might want to provide a little ALC feedback to your driver, or use signals from a directional coupler to drive power monitors or SWR lockout circuits. I use this modest little circuit to do these things; the basic idea is to use a microwave diode pair as a detector/voltage doubler to sample the RF signal, and produce a useful DC level to feed

Sampling RF Power
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those other devices. At the front end is a chip resistor attenuator (R1, R2 and R3) to help put the detected RF at the most useful level for the diodes; then follow the whole thing with a low-pass filter. A trimmer resistor across the output provides some adjustment of the output level, as well as a load to make the response of the diodes reasonably linear (more on this in a bit). The output has some bandwidth, which can be helpful for monitoring video or audio (AM). The circuit is wideband and useful throughout the VHF range to well above 3. 5GHz as shown; the useful range can be shifted down to HF and below by changing C1 to a larger value (1000pf). The circuit is set up to produce a negative output signal, which is the most commonly used polarity, though a positive signal can be produced by reversing the diode connections. For power monitoring, it`s best to find a range of power where the diodes are most linear; for ALC feedback, this is not necessary, nor is it for necessary for SWR lockout circuits (both of these require only a relative output level). For power meters, custom meter scales are a must for absolute accuracy, but you can get close by adjusting the load (I used 5k here) and driving the circuit in a range of power that produces the best linearity. I didn`t spend a lot of time fine-tuning the load, so more experimentation might produce even better results. This test data was measured in steps up to about 2. 5...

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