|LTC5587 RMS power meter with integrated ADC at lower right. Power coupling using a sampling resistor.|
|Resistive power sampler power meter configuration|
The setup used to evaluate the calibration was a bit more involved than I originally expected. I do not have a well stocked industrial lab - basically a SA0314 and 7L12 spectrum analyzers and some home made RF sources. To evaluate the calibration, a programmable frequency source is swept over a narrow range while the attenuation on the board is stepped. This produces a table of measured output level by frequency step. The spectrum analyzer max hold is used to capture this envelope across the sweep. This is then correlated with the digital readings from the power meter. The reading is adjusted with the above attenuation factor for the sampling network. Below is one of those sweeps compared to the manufacturer measured curve.
|Power measurement at 450MHz. Top is from datasheet, bottom is measured on Prj135 board. The 3 lines in bottom graph are expected linearized readings (32dBm/mV with zero intercept at -44, -42, and -40dBm)|
|Power measurement at 880MHz similar to previous diagram. Top is from datasheet, bottom is measured on Prj135 board.|
|Power measurement at 1575MHz.|
One of the problems with this type of sampling network is that any impedance mismatch of the load skews the measurement. A better approach is to divide the power using a 6dB resistive divider. That will have to wait for another Digikey part order. All in all I am very pleased with the LTC5587 performance and ease of use.