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TESTING THE PM10DC
To begin testing the PM10DC we will need the following items:
AC Power supply between 8 and 12VAC or..
DC power supply between 8 and 14VDC.
Or 9V DC battery
50 ohm terminator. (Not the one that rides a motorcycle).
A signal source to confirm measurement like an FM10, RF signal
generator (preferable) or simply a long antenna.
The very first thing you want to do is check your assembly. Make sure your
ICs are installed in the correct orientation, electrolytic capacitors are in the
correct direction, and that there are no solder bridges or cold solder joints. All
parts should be flush to the board, and not standing up in the air.
If you have worked with calibrating equipment before you have probably
heard about a little nasty called a transfer function. A transfer function is really
a curve-fitting program, allowing you to match the output of a sensor (in our
case the directional coupler and the AD8307s) with the physical display. We
have found that the AD8307 is amazingly accurate with slope, meaning you
will almost always have 25mV out per dB, but the offset can vary quite a bit
from unit to unit.
To further complicate matters, it is hard to get an accurate base-line for
noise to adjust to. We don't really have the luxury in most cases to have an
accurate signal source, so we are going to have to give this a best guess cali-
bration. Luckily without any strong fields like a nearby transmitter present, the
base line is easy to calibrate to. Just make sure you turn off any transmitters
that you may be operating, and don't run the microwave either. Yes, this can
pick up microwaves, but not very effectively.
Plug in the power to your PM10DC. We will adjust the display contrast so
we can actually see something. Using a small screwdriver adjust R4 until you
see something. Hopefully you will see readings. If you only see a bar across
the top, you have assembly problems that you will need to fix before you con-
tinue.
Press both mode switches to get to the display which shows the raw DAC
readings (they will be prefixed with f: and r: for forward sensor and reverse
sensor). The display will now show you the direct output of the analog to digi-
tal converter in the range of 0-4095.
Using a small screwdriver, adjust R13 so that the reverse reading shows
1.0000-5.0000 with nothing connected to either BNC jack, which means
PM10DC 27
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