AD8307 Power Meter - Mk2

I previously discussed the Mk1 version of this AD8307 power meter which had a x10 enhancement. One day the meter movement got damaged when I was trying to re-glue the scale. That was when I rebuilt it into the case you see above. I took the opportunity to avoid drilling a hole for the pot and to streamline the whole setting up process in use by revisiting how the reference voltage for the differential amplifier is generated.

I was never happy with the meter being driven off-scale until the pot had been adjusted. This time, I used a small micro to generate the reference voltage with a D2A based on a R2R ladder network. I wrote some code so that at power up, a form of reset, the micro started increasing the reference voltage until a threshold was reached. Then it simply stopped. Now I had automated this part of the process my Q measurement procedure became:

1. find the peak response, 
2. flick over to meter to the x10 setting, 
3. turn the power off and back on, thereby resetting the micro and generating a useful reference voltage
4. then to check the Q I look for a response 30dB less than the peak shown on the meter.

This made it even easier to measure -3db frequencies when looking at inductor Q, crystal parameters or trying to get that last 0.1dB improvement when adjusting filters.

Since the R2R network is only 4 bits, the reference voltage to the differential amplifier has 16 steps and I chose the upper resistor on the network to compress those 16 steps into the range needed.

 

Naturally you're welcome to the code and board design. Get in touch and I will shoot it across. I still haven't worked out how to make it available from my blog.

Regards

Richard VK6TT