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PM10DC THEORY OF OPERATION
If we've done our job you will come away from this theory with a greater un-
derstanding of electronics, dBm, and how power is measured from transmit-
ters. You may also learn why this power meter has such fantastic range, and
you'll find out why our engineer (me) has such a bad headache after writing
routines to convert an analog to digital value to dBm and then to watts, and
then to VSWR using nothing but assembly language on an 8-bit processor!
The C language equivalent just wouldn't fit in such a small microcontroller.
The heart of the power meter is the broadband directional coupler. There are
several possible directional coupler designs, but the one we chose is the most
broadband, but also rather difficult to hand assemble. We figured you could
tackle this because it's worth the trouble in the end.
Power
In
Reverse
Out
GND
figure 1
power going in both the forward and reverse directions there should be less
power at D and C. The power is sampled on D and C, and this is done with
the AD8307 log detectors. VSWR is calculated from the ratio of the power be-
tween C and D.
The AD8307 IC used on each side of the directional coupler is pretty fantas-
tic. This IC has a range of roughly –80dBm to +12dBm. This translates to a
range of 92dB! This is simply amazing since it converts this range to a linear
dB to volts output, which can then be sampled by an analog to digital con-
verter. Since 10 Watts translates to +40dBm, we see that this little part would
be overwhelmed by such a high signal. So to remedy this, we make sure we
have 30dB of attenuation between the directional coupler and the AD8307.
Part of the attenuation is performed by the turns ratio, the other part is from a
small "pad" consisting of R12, R8, and R1 on the forward power side. This
means +40dBm becomes +10dBm, and –40dBm becomes –70dBm.
The AD8307 outputs 25mV per dB of signal so if we run this signal into an A/
D converter we can convert this 25mV/dB into a usable number for further cal-
culations. In our case we are using a 12-bit A/D converter to maximize resolu-
Refer to figure 1: A directional coupler works
Power
by having two cross-coupled transformers
Out
that either cancel each other's signal out, or
not, depending on the direction of power. It is
tricky to describe this in easy to understand
Forward
terms, but I will summarize by saying that
Out
when power is going from port B to port A,
some small portion of power will be also be
on port C. The amount of power seen on port
C is determined by the ratio of the number of
windings, shown here as 7:1. Power coming
back flows from A to B, which causes a small
portion to be seen on D. If there is equal
PM10DC 5
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