[AMPS] RF Applications Wattmeters

[Bruce R. Knox]

Hello everyone:

I saw the discussion about RF power meters and thought I would offer some information in answer to a couple of questions I saw about how the RF Applications line of wattmeters work.  This information applies to all of our wattmeters including our new VFD.

The Remote Sensor:

We use large cores that do not saturate at legal power levels.  We have commercial users that regularly exceed 3 kW with this sensor.  The design is a two-transformer style that is quite flat over 1.8 MHz to 30 MHz.  The detectors are indeed 1N5711s and there are small caps used to filter the dc output.  These caps are not used for any type of peak detecting (short time constant.)

The voltage that comes out  is dc and is typically about 1/30th of the rf voltage on the line.  The high impedance outputs (forward and reflected voltages proportional to power)  from the coupler are sent up to the display unit.

The Display Unit:

The outputs from the sensor are fed through a calibration pot, impedance transformed and connected directly to a/d inputs.  Again, no r/c time constants are used for "peak detecting."  The inputs are sampled at a multi-KHz rate.

The rest of the operation of the unit is software based.  Software peak detectors are used to "grab" the highest instantaneous voltage from the detector.  That value is squared and divided by a constant (e*e/r=power) to display the RF power.   Software determines when and how to "dump" the peak detectors.  A digital filter of sorts is used to eliminate errors from the initial power spike often discussed on this board.

The P-3000 has a "true" power display mode.  This is the arithmetic difference between Forward and Reflected power.  Some people like to follow this value while operating.

VSWR is computed by dividing the sum of the forward and reflected voltages by the difference of the forward and reflected voltages (along with some scaling).

As pointed out on the board, the detectors go to square law operation at low power levels.  These units are not spec'd below 15 watts for this reason.  There are values displayed all the way down, but they can only be used as relative indicators.

These meters all use 8 bit a/d converters.  That is why, as power increases, there are "holes" in the displayed powers (the math: values from 0-255 squared divided by a constant like 20).  E.g., (25*25)/20 = 31 and (26*26)/20 = 33 (integer math, thank you).


Thanks for reading,

Bruce R. Knox W8GN
RFA

w8gn@contesting.com
bknox@rfapps.com




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