AD8307 Power Meter

Here is what my current AD8307 power meter looks like. I built this into a "box within a box" to enhance shielding. Inside the case is the battery, a lipo cell, and it's charging regulator. When I want to charge it I plug it into a 12V power supply. The remaining RF circuitry is built into a shielded compartment at the rear of the front panel bnc. It's basically 99% air inside the case!

I use a pretty much standard circuit for the AD8307. I think it was Wes Hayward, W7ZOI
and Bob Larkin, K7PUA that first promulgated this in QST, June 2001. I dispensed with offset and calibration adjustments since I was quite happy with AD8307 as it stands.

However, it occurred to me one day that being able to zoom into the response would be really handy when looking for the 3dB down point when trying to measure inductor Q with my fixture, here. While it would be easy to add a dc amplifier, the input signal would have to be adjusted so that the amplifier did not saturate. My modification was to add a differential amplifier based on an op amp so that rather than adjusting the RF levels, one could twist the pot until the reference voltage to the differential amplifier allowed for the amplifier to avoid being saturated.

The circuit for the first iteration of this is shown below.

My method for sweeping inductors in my test fixture was:

1. find the peak response, 
2. flick over to meter to the differential amplifier output, 
3. adjust the pot to bring the meter to approximately 0dBm, and 
4. then to check the Q I look for a -30dB response on the meter.

This was much less guess work on my part as to where the 3dB down frequencies were. And if you were trying to peak something it was much easier to see the peak.

I made some further improvements and will blog those soon.

Regards
Richard VK6TT