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Exploring the Architectures
of Network Analyzers
Application Note 1287-2
Table of Contents
Page
Introduction 2
Network Analyzer Architecture 2
Coupling Signals 3
Directional Bridges 6
Network Analyzer Detection 6
Comparing Dynamic Range 8
The Spectrum
Analyzer Alternative 9
The Test Set 9
Other Considerations 10
2
Introduction Network analyzers have become one of the most important measurement
tools for characterizing the performance of high-frequency components and
devices. A modern vector network analyzer can measure a component's
magnitude, phase, and group delay, show port impedances on a Smith
chart, and, with time-domain capability, show the distance from a test
port to an impedance mismatch or circuit fault. Understanding a network
analyzer's capabilities and operation can help an operator derive optimum
performance from the instrument.
Hewlett-Packard Company offers an extensive line of RF and microwave
network analyzers for applications from DC to 110 GHz. These analyzers
are available with a variety of test sets and calibration kits and can
be equipped with such options as time-domain capability for making
distance-to-fault evaluations in transmission lines. The company also
supplies linear and nonlinear computer-aided-engineering (CAE) software
tools such as the HP EEsof Microwave Design System (MDS) and
Series IV Suite, useful in creating device and component models based
on vector network analyzer measurements.
Network Analyzer Network analyzers differ in form and function from another tool
Architecture commonly used to characterize communications systems and components,
the spectrum analyzer (Figure 1). Spectrum analyzers measure unknown
external signals. In contrast, network analyzers utilize synthesized-
frequency sources to provide a known test stimulus that can sweep across
a range of frequencies or power levels. Network analyzers also can perform
ratioed measurements (including phase), which require multiple receivers.
These measurements cannot be performed with a spectrum analyzer,
even when it is complemented by a tracking generator.
Spectrum analyzers are generally employed to measure signal
characteristics such as carrier level, sidebands, harmonics, and phase
noise. They are usually configured as a single-channel receiver without a
source. These instruments have a wide range of IF bandwidths available
in order to analyze diverse types of signals and are often used with
external sources for nonlinear stimulus/response testing. When combined
with a tracking generator, spectrum analyzers can be used for scalar
component testing to show magnitude versus frequency information
but not phase information.
Figure 1.
Differences
between Network
Amplitude Ratio
8563A
and Spectrum
Power
Analyzers
Measures Measures
known unknown
signal signals
Frequency Frequency
Network analyzers: Spectrum analyzers:
measure components, devices, measure signal amplitude characteristics
circuits, sub-assemblies (carrier level, sidebands, harmonics, etc.)
contain source and receiver can demodulate (& measure) complex signals
display ratioed amplitude and phase are receivers only (single channel)
(frequency or power sweeps) can be used for scalar component test (no
offer advanced error correction phase) with tracking gen. or ext. source(s)
3
Network analyzers can provide a wealth of knowledge about a device
under test (DUT), including its magnitude, phase, and group-delay
response. To accomplish this, a network analyzer must provide a source
for stimulus, signal-separation devices, receivers for signal detection, and
display/processing circuitry for reviewing results (Figure 2). The source is
usually a built-in phase-locked (synthesized) voltage-controlled oscillator.
Signal-separation hardware allows measurements of a portion of the
incident signal to provide a reference for ratio measurements, and it
separates the incident (forward) and reflected (reverse) signals present
at the input of the DUT. Hardware for this purpose includes power
dividers (which are resistive and broadband, but have high insertion
loss), directional couplers (which have low loss but are usually limited
in bandwidth), and directional bridges (which are useful for measuring
reflected signals over a broad bandwidth, but may also have
significant loss).
Figure 2.
Generalized Incident Transmitted
Network DUT
Analyzer Block
Diagram SOURCE Reflected
SIGNAL
SEPARATION
INCIDENT REFLECTED TRANSMITTED
(R) (A) (B)
RECEIVER / DETECTOR
PROCESSOR / DISPLAY
Coupling Signals Directional couplers are useful for measuring both the incident and
reflected signals present at the input of the DUT. Directional couplers
consist of a "through" path, and a "coupled" path that diverts a small
amount of the power traveling along the through path (Figure 3). The
amount of coupled power is determined by the coupling factor:
Coupling factor (in dB) =