Updated March 14,
here to go to our main page on microwave measurements
here to go to our main page on S-parameters
here to learn how to extract effective dielectric from sample
transmission line measurements
here to go to our page on reference planes
here to learn how (not) to trash a cal kit!
to go to our page on RF probing
here to go to our page on "Smoothing is cheating!"
It offers an example of using smoothing to pass a VSWR specification.
Click here to see how smoothing
can be used to improve group delay measurements (this is NOT cheating!)
This site used to have some great
app notes on Agilent network analyzers. The site doesn't seem
to be maintained these days, but you might find a few nuggets of
information. Thanks to Ian for pointing out its slow demise!
Here's a link to Agilent's
official PNA page.
Network analyzers fall into two
categories. Vector analyzers are capable of measuring complex (magnitude
and phase) reflection and transmission; scalar analyzers only measure
One of the most essential pieces
of TE in the lab is the network analyzer. It can be used to measure
impedance, VSWR, loss, gain, isolation, and group delay of any two
ports of a multi-port network (don't make us draw a potato with
arrows here). The two big guys in network analyzers are Agilent,
the 800 pound gorilla once known as Hewlett Packard, and Anritsu
once known as Wiltron before they turned Japanese. Can anyone tell
us why Anritsu didn't add a Playstation option to their analyzers
A word about acronyms concerning
network analyzers... vector network analyzers (VNAs) are often called
"ANAs" by old farts, including the Unknown Editor. ANA
stands for "automated network analyzer". A long time ago
during the Carter administration, the original network analyzer
(H-P 8409) was not automated, in the sense that TE error correction
was done by hand. Return loss measurements could not exceed the
VSWR of the equipment, so you couldn't resolve beyond 20 dB return
loss in most cases. Gain and insertion loss and phase were calculated
from the subtraction of two measurements (first the through connection,
then the DUT connection). It was a bad time to be alive...
Then the first automated network
analyzers were introduced. A minicomputer (about equal to a 1000
watt, five dollar calculator) grabbed the vector data from the 8409,
and did some fancy manipulations that resulted in automatic error
correction and accurate magnitude and phase of the four S-parameters.
It was considered magic. The next step was to build the error correction
into the test equipment (no external computer) and display the error-corrected
measurements in nearly real time (the original HP 8510, circa 1982).
Today vector network analyzers are all automated (error correction
is built in). And the acronym ANA has stuck.
This type of network analyzer
consists of a sweep oscillator (almost always a synthesizer so that
measurements will be repeatable), a test set which includes two
ports, a control panel, an information display, and an RF cable
or two to hook up your DUT. Each port of the test set includes dual
directional couplers and a complex ratio measuring device. Other
options include a means for bias voltage/current injection, and
a computer controller to manipulate and store data. The "classic"
vector network analyzer is the Agilent (HP) 8510, shown below. Depending
on how much you spend, this analyzer can make measurements from
45 MHz to 110 GHz.
Before you jump into vector network
analyzer measurements, you will have to calibrate the network analyzer.
There are many types of calibration techniques, and even more types
of calibration standards. A typical calibration will move the measurement
reference planes to the very ends of the test cables. You will have
the choice of calibrating for reflection or transmission only, using
either of the two ports or both of them together. For most tasks
you will probably calibrate both test ports for reflection and transmission,
which will allow you to measure full two-port scattering matrices
(S-parameters for your device under test (DUT). This is referred
to as "twelve-term error correction". I don't know why
there are twelve terms for two ports, I will get back to you on
Before you perform a calibration,
you should do a little "preflight" check-out of the TE
and DUT. Ask yourself:
- What frequency range do you
need to measure?
- Does the cal kit, cables and
any adapters you need operate over the desired band?
- Are the cables in good condition?
(Connect them together and see what the effects of gently bending
them have on uncalibrated transmission and reflection parameters).
- Will the cables reach the
DUT? (This seems obvious, but I have seen people waste time calibrating
only to discover that the test cables are too short to reach both
ports of the DUT).
a vector network analyzer
The reflection calibration for
each port requires three standards, typically: an open circuit,
a short circuit, and a matched 50-ohm load (for waveguide calibration,
a pair of offset shorts and a load are used. An open in waveguide
usually acs closer to a load due to radiation). The matched load
can be a "broadband load", meaning that it has very low
reflection coefficient over a lot of bandwidth, or a sliding load.
Sliding loads are expensive and fragile standard which should only
be used if your measurement requires great accuracy (perhaps you
want to be able to tell the difference between a 1.01:1 VSWR and
a 1.02:1 VSWR). The sliding load recognizes that a "perfectly
matched" 50 ohm calibration standard can never exist, but a
series of loads with equal mismatch but varying phase can be used
to draw a circle around the center of the Smith chart, thereby solving
for the perfect load. My advice to you: unless someone takes the
time to show you how to use the sliding load properly, NEVER TAKE
IT OUT OF THE BOX.
The particular set of cal standards
(and test cables) that you use will depend on what frequency band
you need to cover. Coaxial calibration kits come in type N, 7 mm,
3.5 mm, 2.92 mm, 2.4 mm, 1.87 mm and 1.0 mm. There are waveguide
calibration kits for every waveguide band. Be sure not to exceed
the frequency capability of the test set, cables, adapters and calibration
kit (see our section on connectors for the frequency limits
of different connector types).
Sage Advice: remember
that cal kits are expensive, and pieces of the cal kit should NEVER
be used as adapters loads in any test set. And always put the little
plastic covers onto the calibration pieces, you want to prevent
dirt, skin, grease, etc. from degrading the accuracy of future calibrations.
We have a separate page on how (not)
to trash a cal kit!
To check the validity of your
calibration, as well as the general health of the test equipment,
you need to look at a few things after you calibrate. If you are
doing transmission measurements, check the residual error in a "through"
connection (connect the test cables to each other). You should see
0 dB plus or minus 0.05 dB or better. The phase should be very close
to 0.0 degrees as well. The return loss of both ports should be
at least 40 dB but can be better than 60 dB if you are using good
equipment. The transmission and reflection parameters should not
vary significantly when you gently bend the test cables, or you
have a bad connection. If you see a issue with the calibration you
just did, figure out the problem before you perform another calibration,
or you will be wasting your time and adding needles wear and tear
to the cal kit and test cables.
Some of the options you will
be asked by the network analyzer need to be discussed so you will
know how to answer them.
Omit isolation: during
the calibration, you can "omit isolation". If you are
measuring the loss of some test cables and don't expect to see transmission
data under -20 dB, go ahead and omit the isolation cal standard.
But if you want to see the steep skirts of a filter or the reverse
isolation of a multi-stage amplifier, you should perform the isolation
averaging will improve the accuracy of your data, so long as you
do it during the calibration as well as the actual measurement.
But it will slow down the measurement process noticeably, a consideration
if you have a lot of data to collect in limited time.
IF bandwidth: an option
on most new network analyzers, reducing IF bandwidth also improves
measurement accuracy. Try reducing from 35 kHz down to 500 Hertz.
is cheating, no ifs ands or buts. Smoothing reduces the "bumpiness"
of a frequency response by averaging data across a couple of frequency
points and using the result at one frequency. But if you need to
cheat to get some data for the boss who is standing behind you,
go for it. The only time that smoothing may actually improve measurement
error is in group delay mode (note: this is referred to as the "aperture"
setting when you are using Anritsu (Wiltron) equipment. The group
delay is actually calculated from the slope of the phase angle versus
frequency, and the "aperture" allows the user to define
how much frequency band top take the slope over.
Auto scale: go ahead and
use the auto scale to quickly display visual information on the
parameter you are investigating. But when you actually plot the
data on a pen plotter or printer or using an Excel spreadsheet,
use a scale that makes sense. Like 2 or 5 or 10 dB per division.
NOT 3 or 6 dB per division. This is because we live in a base-ten
world, not a base-three world. Also, when you are plotting the same
type of data for units of the same type that you are measuring,
use the SAME scale for all of them so they are easy to compare.
this type of cal offers a shortcut to data, but offers a reduced
amount of error-corrected data. A transmission response cal will
merely measure the magnitude and phase of the through connection,
which will be subtracted from all subsequently measured data. But
you won't know anything about the magnitude and phase of the DUTs
reflection coefficients. A reflection response cal will provide
magnitude-only information for reflection coefficients. If you find
yourself using the response cal feature often, you should consider
using a scalar analyzer and stop tying up the expensive equipment.
Scalar network analyzers are
so lowly that they don't have an acronym. So you have to say at
least "scalar analyzer" or no one will know what you are
The obvious difference between
a vector network analyzer and a scalar analyzer is that the scalar
analyzer doesn't give you any phase information. It only detects
We are still working on this
section, so check back later!!!