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you've adjusted the base line of the detector to the base line of the Analog to
digital converter. Then adjust R19 for the same on the other side for the for-
ward power. These numbers will vary up and down a bit due to noise, but you
should try and average these values visually.
Now we are going to match forward and reflected power sensitivity. Connect
your arbitrary signal source (Like an FM25B) to the RF IN jack. Connect a 50-
ohm termination to the RF out jack. Use the up/down key to get the VSWR
display where the prefix is "V:" and "d:" The value after "V:" is VSWR and after
"d:" is directivity. If your frequency source is in the 100MHz range, you should
see a VSWR of less than 1.5 and a directivity of at least 20dB. This indicates
that the directional coupler transformer you wound is working (as well as the
rest of the kit).
Cycle through using the mode switch to the display with the prefix "F:" and
"R:" displayed. You should be able to read the power output of your source
and any power reflected from the termination. Since we are using a good 50
ohm termination, you should only see a very tiny amount of reflected power—
typically a hundred times less than the forward power! Note the reading of the
forward power at this point, and write it down. We will be adjusting R13 to
match this power after we swap connections.
Now swap the connections on the jacks, connecting the signal source to RF
out, and no load or termination to RF in. Adjust R13 so that reverse power on
the "F:" and "R:" display matches what the forward power reading was. You
shouldn't have to adjust it very far as we should be close already.
If you have a calibrated source or the next best thing, a top quality signal
generator, you can calibrate your unit for even more accuracy. Since the base
line does vary slightly from unit to unit, it will give your PM10DC a +-2dB accu-
racy. If you can calibrate with a known signal level, you will achieve a better
than +-1dB accuracy.
To perform a calibration on this meter, you will need the following in addition
to the first setup:
A calibrated signal source. We use an HP 8647A Signal generator
with a 50 ohm output.
A calibrated spectrum analyzer to verify signal generator output.
50 Ohm RF termination.
1. Connect the signal generator to the PM10's RF input jack.
2. Connect the 50 ohm terminator to the PM10's RF output jack.
PM10DC 28
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