HP 8720 schematic
1
ECE 435
Ω
INTRODUCTION TO THE MICROWAVE
NETWORK ANALYZER
Latest revision: October 1999
Introduction
A vector network analyzer (VNA) is a device capable of measuring both the magnitude and phase
of a sinusoidal voltage signal. This makes it very similar to a vector voltmeter. However, a
network analyzer is more advanced than a vector voltmeter in many ways. Complex RF circuitry is
used to make the analyzer much more accurate, with a typical dynamic range of 70-110 dB.
Microprocessor circuitry allows flexible processing of data including smoothing, averaging and a
variety of displays. Most importantly, a VNA is capable of acquiring measurements over a swept
band of frequencies, allowing the rapid wideband characterization of a wide range of microwave
networks.
To compensate for the frequency dependent phase and amplitude characteristics of the cables and
receiving system, it is necessary to calibrate using the known responses of standard loads (open,
short, and 50Ω terminations). The calibration procedure is an important part of network analysis,
and must be done repeatedly (ideally, each time a new measurement is taken).
The HP 8720B network analyzer incorporates in one package each of the three components of a
network analyzing system: a swept frequency source, a receiver, and an S-parameter test set. The
sweeper produces a sinusoidal signal with a frequency that can be swept between 130 MHz and 20
GHz with a 100 kHz resolution. The receiver allows the measurement of complex voltage. The Sparameter test set incorporates directional couplers to send the signal into the chosen port of a twoport network while measuring either the transmitted or reflected signal. More advanced network
analyzers (such as the HP 8510C) consist of separate components assembled into an analysis
system.
In this lab you will characterize the reflection and transmission characteristics of a 100
load, an open-ended coaxial cable, a cavity wavemeter and a 10 dB attenuator. You will be able to
make in a few minutes measurements which might take days using the slotted line or vector
voltmeter techniques studied in earlier labs.
NOTE: The HP 8720B network analyzer, cables and calibration kit cost about $70,000.
Please treat the equipment KINDLY so that students may use them for years to
come.
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IMPORTANT NOTE ON CONNECTORS ************************************
The connectors on the 8720B S-parameter test set cables and on the standard loads are
about the only things that can be easily damaged. Accurate measurements depend on the
careful handling of all connectors. (HP has a special publication just on the cleaning and
handling of connectors.)
PLEASE follow these rules when using connectors:
1. Do not touch the connector threads.
2. Always keep caps on unused connectors.
3. Do not force the connectors. If they don't seem to be meshing, you don't
have them aligned properly.
4. Don't adjust the APC connectors. Only the N-type connectors are used by
the students. (APC connectors require a torque wrench for proper adjustment.) If the APC connectors need adjusted, ask the lab instructor to do it
for you.
5. Don't turn the device being connected! Only turn the sleeve on the male Ntype connector.
6. Hand-tighten the N-type connectors snugly, but not so tight they can't be
easily loosened.
7. Don't bang the connectors on the table top.
8. Don't bend the cables past where they move freely.
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I. REFLECTION MEASUREMENTS
A. Power-up
1. Turn on 8720B and PC.
2.
3.
B. Set instrument state
1. Run all programs from the C:\USR directory on the PC. Type NAM3 (net-
2. Push LOCAL on VNA to return it to local control.
3. Push START and set starting sweep frequency to 0.2 GHz. Push STOP and
4. Push MENU and set no. pts to measure to 201.
5. Push AVG and set average to ON. The VNA will now take 16 sets of
6. Push MEAS and set the measurement to S11.
7. Push CAL and select CAL KIT. Set to N 50Ω.
Insert your formatted floppy into computer. Type SCAN A: (or B:). If there
are viruses on your disk REMOVE IT IMMEDIATELY and see your
instructor.
Note: all measured data should be stored with the .NDF extension (New
Data Format).
work analyzer measurements) to start the measurement software. Control is
now with the PC.
set ending sweep frequency to 2.2 GHz.
measurements and average the results. Set IF bandwidth to 300 Hz. This is
the IF filter width.
C. Save instrument state
1. From main menu of NAM3, enter instrument state menu.
2.
3.
3. Push any key. The VNA will now transfer the instrument state.
4. Enter a filename. Choose something you will remember (try S11.IS). The
5. Return to the main menu of NAM3.
6. The instrument state is now saved and can be recalled at any time.
D. Recall instrument state
1. From the main menu of NAM3, choose to exit the program.
2. Press LOCAL and PRESET on the VNA to return instrument state to its
3. Start NAM3.
2. From main menu of NAM3, enter instrument state menu.
3. Choose to upload the instrument state to the VNA from disk.
4. Enter your filename. The VNA should return to your instrument state.
Choose to save the instrument state.
Push LOCAL.
instrument state will now be saved to disk.
initial setting.
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