Saturday, March 7, 2015

Prj135 RF RMS Power Meter (Part3)

I decided to construct the second level control board using a power divider rather than sampling network to the RMS power detector as in the previous post.  Using a sampling resistor loads the output less but is susceptible to errors based on the load impedance variance. There is a choice between using a two resistor and three resistor configuration.  A detailed treatment of why a 2R divider works better than a 3R divider for level ratio applications is well described in “Choosing the Right Power Splitter: Two-resistor or Three-resistor; B. R. Smith, National Conference of Standards” (Currently available from the Keysight website).  The modifications to the original circuit are shown below.
2R 6dB Power Splitter with Matching Network for LTC5587
A picture of the end result is shown below.  You can see the stacked capacitor over the underlying cut trace between the 50 ohm resistor and the SMA post.  In the lower right you can see the matching network to the power meter.
Unit #2 using a 2R divider (upper right between U103 and SMA) and 50 ohm matching network at meter (lower right to the right of the LTC5587 at U105).
The mV/dBm response of the meter was measured using the same techniques described in the previous post. The following graph shows scans for the sub 1GHz region.
Power Meter Response 40MHz - 1000MHz.
The results are amazingly good and near Y [dBm] = (M=32.5 mV/dBm) * mV + (B=-43 dBm). The measurement error of the instrumentation equipment and setup is at least +/- 2 dBm.  The following is the same data over the 1GHz - 2GHz region.
Power Meter Response 1000MHz - 2000MHz
The results in this band are floating a bit high compared with the previous band but still within measurement error and device specification.  The following captures the response in the 2GHz - 3GHz region.
Power Meter Response 2000MHz - 3000MHz
Again, the results are bit higher than the 1GHz band but within expected results.  The amplifier is only stated to 2700MHz so you see the level reduction starting here.

The slope and intercept are both a little higher than the nominal value from the datasheet.  They actually look closer to some of the higher temperature data listed.  My guess is I didn't get the QFN paddle soldered sufficiently well to make a good thermal junction and the part is running warm.

A couple of different 50 ohm matching configurations were tried. No matching (device direct) worked well in the sub 1GHz region and increased sensitivity, however, above this the results were not usable.  A 2.2nH serial inductor with no shunt capacitor was tried.  This produced good results through 2GHz (not much difference than the above except the >1900MHz results were closer to the B=-43dBm line).

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