Well, I can tell you how I do it. I usually work
at 900 MHz, so I don't know how well this method
will scale with frequency, though it should if you
use the right cables.
First of all, you need
a vector network analyzer. The 8753D or later model
from HP/Agilent is a nice choice. The important
thing when using a VNA is to calibrate the ports.
More on that later.
On your board, you need
a way to disconnect the DUT (device under test)
from the rest of the circuitry. Normally, there
is a DC blocking cap in line with most parts of
an RF circuit, so the easiest thing to do is to
remove the DC blocking capacitor. Next, take a short
section of RG174 cable with a SMA connector on one
end and strip back the other end to expose the shield
and center conductor. You will also need a complete
SMA RG-174 cable with SMA connectors on both ends
Solder the shield of the
test cable to a ground point very close to the antenna,
filter, or whatever you are testing. Solder the
center conductor to the pad of the DC block you
just removed so that the cable is now electrically
connected to the DUT.
Now, connect the other
cable (the one we are using for calibration) to
one of the ports of the VNA; Lets use port 1 for
this example. Using the instructions for the VNA,
perform a 1-port calibration on port 1. THis will
involved you connected calibration standard loads
to the end of the cable; there will be a 50-ohm,
a short, and an open standard. Once complete, the
VNA is now calibrated, including the length of cable
you are using with the DUT.
You should now
be able to connect the cable soldered to the DUT
to port 1 of the VNA and perform your measurements.
You can do the same trick with 2 cables
if you need to do 2 port analysis for filters.
Please post any questions you have if this isn't
- Steve Montgomery