Saturday, February 21, 2015

Prj135 RF RMS Power Meter (Part 2)

Following on from the previous post, with the attenuator populated and working the next step was to populate one of the amplifiers and the power meter.  A picture of that step is below.

LTC5587 RMS power meter with integrated ADC at lower right.  Power coupling using a sampling resistor.
The schematic and layout accomodates multiple configurations of coupling between the output and the power meter.  It also allows the board to be used as a standalone power meter itself.  I wanted to use the meter with low frequencies so a coupler was not a great first choice.  There are two resistive sampling or dividing configurations.  The first uses a large resistor to limit the power sent down the meter leg of the output while the second uses a power divider to split the output into two paths - one for measurement and one for output. The first configuration tested uses the sampling resistor configuration.  The block diagram is shown below.
Resistive power sampler power meter configuration
The voltage measured by the meter is 20log10( Xin/(Xin+Xf) ) lower than that at the output so the power meter reading must be adjusted for this amount depending on frequency.  This kind of configuration is used for large signals and narrow bands.  The coupling can drop significantly, particularly as Xin of the meter drops.  In this case the coupling resistor, Xf, is 300 ohms while the meter input impedance is nominally 200ohms with a shunt 1.6pF capacitance.  Using the datasheet S11 values one can compensate the readings based on frequency.   It still leads to a hefty reduction (7 - 14 dB).

The device used has an integrated ADC with an SPI interface.  This configuration uses the meter with a 3.3V reference voltage so all readings are converted to milliVolts (reading * 3.3V/4096 codes) then use the mV/dBm conversion from the specification.

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)
There are a couple of things to note with my measurements.  First, there is wobble in the measured RF output due to uncertainty and variation in the spectrum analyzer envelope.  Second, I cannot get to the high end of the input due to the output amplifier saturating. (The coupling network reduces the reading by at least 7dB and the output amplifier P1dB is roughly 8dBm).  Having said this, the output is amazingly good - falling within the dBm = M*X + B where M is the conversion of dBm to mV and B is the zero intercept.  The reference lines on the graph reflect M=32mV/dBm and intercepts of -40, -42, and -44 dBm.  The next figure captures the same data at 880MHz.

Power measurement at 880MHz similar to previous diagram.  Top is from datasheet, bottom is measured on Prj135 board.  
The datasheet does not include a 1575MHz dataset, however, this is a frequency band of interest given the prevalence of GPS SAW filters.
Power measurement at 1575MHz.
The reading at 2145MHz shows a similar conformance to the 32dBm/mV, however, the digital readings are a few dB higher.  I believe this is due to leakage around the attenuator and directly into the power meter given the jumpering over of the missing first stage amplifier and the lack of shielding.  The original goal was 1575MHz and below for this configuration so I won't spend a lot of time investigating anomalies at the higher frequencies.

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.

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