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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