At Chinese AD8307 power measurement module #1 I documented the first phase of checkout of a low cost AD8307 module.
There are two requirements for accurate power measurement:
- input impedance of the power meter must be very close to 50+j0Ω (say input VSWR<1.2); and then
- gain from the SMA terminals to the AD8307 input terminals must be independent of frequency.
Though the module was clearly junk in terms of criteria 1 as supplied, it was possible to modify it to present a low VSWR 50Ω input impedance, and that was documented in the last chapter.
This article carries on with criteria 2 above, the amplitude response.
The application requires an adjustment of the AD8307 calibration to 20mV/dB with -90dBm intercept, meaning it will produce 1800mV at 0dBm input and have a slope of 20mV/dB.
Though the original circuit shows the necessary components R3 & R4, and R5 & R6, they are not fitted and must be fitted to the board. I have used 50kΩ 20t trimpots for R3 and R6, and 33kΩ for R5 and 47kΩ for R4.
The technique used to calibrate slope and offset is that described at (Duffy 2014).
Slope and offset calibration, and log conformance / scale linearity at 10MHz
Above is a sweep from -65 to -6dBm after calibration of slope and offset. The linear fit to the blue curve shows slope is 20mV/dB and intercept 1.8015 for 0dBm means the offset is -1.8015/0.02=-90.08dBm. Log conformance is 0.2dB.
The module was swept from 1 to 500MHz and response at 0dBm captured.
Above is the response from 1 to 50MHz. Response is down by 1.5dB at 55MHz, but within 0.5dB to 30MHz so quite suited to the intended application, a HF common mode current meter.
- The module as supplied was cheap Chinese junk, it had 35dB slope from 10MHz to 1MHz, input VSWR from 1.6 to extreme over the range 1-500MHz.
- Reworking the input circuit delivered very good input VSWR to 240MHz.
- Amplitude response with the reworked input circuit is within 0.5dB from well below 1MHz to 30MHz.
- Flat response at VHF – UHF would require an equalised input circuit and appropriate PCB layout.
- Duffy, O. Dec 2014. VK3AQZ RF power meter (RFPM1)