5091-7195E.pdf
(
118 KB
)
Pobierz
Agilent PN 89440A-4
Characterization of Digital Communications
Channels with the Agilent 89410A and 89440A
Product Note
Accurate measurements of channel
imperfections are important to the
designers and operators of digital
communication systems. These meas-
urements are especially critical
because of our desire for greater
information density. Conventional
network analyzers cannot measure
systems that span several miles.
The Agilent Technologies 89410A
(dc–10 MHz) and 89440A (dc–1.8 GHz)
vector signal analyzers (VSA) define
a new class of analyzers that can
easily measure amplitude and group
delay variations of existing commu-
nication channels.
Introduction
Digital communication schemes are
rapidly replacing older analog
schemes in many applications. The
desire for greater information density
has increased the importance of
understanding the imperfections of
the communication channel. Devia-
tions from constant amplitude or lin-
ear phase limit the complexity of the
modulation scheme that a channel
can support.
While simulations may provide some
valuable information, they can be
costly and may not accurately repre-
sent the real system. For example, it
is not practical to construct a simu-
lated CATV network that exhibits the
myriad of micro-reflections that
plague real systems.
Advent of time-selective vector
signal analyzers
The 89410A (dc-10 MHz) and 89440A
(dc-1.8 GHz) vector signal analyzers
represent a new class of measure-
ment instrument. These analyzers cal-
culate both frequency and modulation
domain characteristics from a time-
record. The time-record of the desired
frequency span is produced by accu-
rately digitizing the input waveform,
mixing with a digital quadrature local
oscillator, and band-limiting with dig-
ital filters.
Limitations of traditional
measurements
In older analog systems, precise chan-
nel characterization was not neces-
sary. Crude methods were sufficient
to “measure” amplitude response and
group delay, such as observing the
envelope of a frequency sweep or the
base line of a two-tone burst on an
oscilloscope display. These methods
are clearly inadequate as digital mod-
ulation schemes are added to these
older systems.
This product note describes the VSA
channel characterization measure-
ment method and specific cable tele-
vision (CATV) applications. Although
examples from the CATV industry
are used, the VSA method may be
applied to other communication
channels.
Selectable trigger delay and time-record
length control the portion of the time-
domain waveform that is captured. In
addition, time-gating allows a subset
of the time-record to be selected for
subsequent calculations.
A conventional network analyzer is
capable of measuring the characteris-
tics of a communication channel if
both ends are in the same location.
However, in a typical cable television
(CATV) system, the receiving location
may be twenty miles or more from the
cable head-end. With the exception
of only a few systems, this spatial
restriction limits the use of a network
analyzer to laboratory simulations.
This ability to easily measure time-
variant signals is essential for meas-
uring communication systems. With a
VSA, it is easy to make accurate
measurements of the amplitude and
group delay variations of existing
communication channels.
Channel characterization with a
VSA
To understand channel characteriza-
tion with a VSA, a baseband measure-
ment is discussed first. Later, the
technique is extended to RF.
This captured time record is then
converted to a vector spectrum in the
frequency domain. The spectrum con-
tains both amplitude and phase infor-
mation for every frequency compo-
nent of the test signal. Dividing this
vector spectrum by the previously
measured reference yields the trans-
fer function of the network with an
arbitrary delay term (phase ramp).
The actual propagation delay cannot
be measured with this technique;
however, in most applications, the
delay is irrelevant and only the devia-
tion from linear phase is of interest.
Amplitude and group delay variations
across the frequency band are dis-
played as shown in Figure 3.
If necessary, improved signal to noise
ratio is achieved by time averaging
several measurements. Because the
noise is not correlated with the repet-
itive test signal, the noise averages to
zero over time.
The 89410A automates the entire
process, from capturing the test sig-
nal to displaying the amplitude and
group delay variations of the network
under test.
Baseband measurement
Figure 1 shows a baseband measure-
ment block diagram. A reference
measurement of the test signal is
taken without the network under test
in the circuit. This measurement is
made once, then stored in nonvolatile
RAM or on disk. The subsequent
measurements with the network
under test will be normalized to this
reference measurement.
89410A
89410A
The difficulty in measuring the net-
work with a traditional network ana-
lyzer is that there is no common
phase reference available at the ends
of the network under test. The VSA
overcomes this difficulty by using a
test signal with known phase rela-
tionships among all its frequency
components. The signal must also be
frequency band-limited to avoid inter-
ference with adjacent channels.
Although many signals, including a
frequency chirp and a sin(x)/x pulse,
have these attributes, the chirp is
referred to in this paper because of
its superior signal-to-noise ratio
across the frequency band of interest.
Source
Input
Channel Under Test
Figure 1. Baseband network characterization
The 89410A source is configured to
send the test signal repetitively. The
receiving 89410A is set to trigger on
the test signal with negative trigger
delay and a time-record length suffi-
cient to capture the entire transmit-
ted signal, as shown in Figure 2.
Figure 2. Frequency chirp test signal
2
Cable Television Association (NCTA)
convention in June 1993.
RF Measurement
The baseband measurement may be
extended to RF by providing external
modulation and demodulation. You
can insert the test signal between
other information on an active com-
munication channel, then use the
time-gating feature to easily select
just the test signal for analysis.
Example 2: 6-MHz digital CATV
channel
Direct RF Measurements
The 89440A 1.8-GHz vector signal ana-
lyzer allows you to directly measure
the RF-channel characteristics within
its 7-MHz information bandwidth
without using external modulation
and demodulation. An Instrument
BASIC program is required to remove
the phase of the RF downconverter.
To characterize the full 6-MHz band-
width of a CATV channel, the modu-
lator and demodulator must have a
6-MHz bandwidth and the channel
must not be occupied. This measure-
ment is discussed in more detail in
a paper entitled “CATV Channel
Characterization for Digital Data
Transmission Applications” by Dehart
et. al., presented at the National
Two examples of RF measurements
on a CATV network are described
next.
Example 1: 4.5-MHz Active NTSC
CATV Channel
Figure 4 shows the block diagram of a
CATV channel characterization meas-
urement.
With the 89410A, it is possible to
characterize the 4.5-MHz video por-
tion of any given NTSC channel on a
CATV network, without taking the
channel off the air. The test signal is
inserted into a single video line in the
vertical blanking interval, as shown
in Figure 5.
The recently developed Philips Ghost
Cancellation System uses a reference
chirp on line 19 that may be used as
the test signal for this measurement,
simplifying the block diagram.
Figure 3. Network characteristics
Video Source
Video Out
Channel Under Test
Demodulator
VTS
Modulator
Frame Trigger
89410A
89410A
Trigger
Trigger
Source
Input
Figure 4. Measuring an active NTSC channel
3
Conclusion
Understanding communication chan-
nel amplitude and group delay varia-
tions is critical to the designers and
operators of digital communications
systems. While these imperfections
have been historically difficult to
measure, a new class of measurement
device defined by the 89410A and
89440A vector signal analyzers make
it easy.
Although examples from the CATV
industry were described in this article,
the VSA characterization method may
be applied to other communication
channels that are within the 10-MHz
information bandwidth of the 89410A.
Agilent Technologies’ Test and Measurement
Support, Services, and Assistance
Agilent Technologies aims to maximize
the value you receive, while minimizing
your risk and problems. We strive to
ensure that you get the test and measure-
ment capabilities you paid for and obtain
the support you need. Our extensive sup-
port resources and services can help you
choose the right Agilent products for your
applications and apply them successfully.
Every instrument and system we sell has
a global warranty. Support is available
for at least five years beyond the produc-
tion life of the product. Two concepts
underlie Agilent’s overall support policy:
“Our Promise” and “Your Advantage.”
Our Promise
“Our Promise” means your Agilent test
and measurement equipment will meet its
advertised performance and functionality.
When you are choosing new equipment,
we will help you with product informa-
tion, including realistic performance spec-
ifications and practical recommendations
from experienced test engineers. When
you use Agilent equipment, we can verify
that it works properly, help with product
operation, and provide basic measurement
assistance for the use of specified capabil-
ities, at no extra cost upon request. Many
self-help tools are available.
Your Advantage
“Your Advantage” means that Agilent
offers a wide range of additional expert
test and measurement services, which you
can purchase according to your unique
technical and business needs. Solve prob-
lems efficiently and gain a competitive edge
by contracting with us for calibration, extra-
cost upgrades, out-of-warranty repairs, and
on-site education and training, as well
as design, system integration, project man-
agement, and other professional services.
Experienced Agilent engineers and techni-
cians worldwide can help you maximize
your productivity, optimize the return on
investment of your Agilent instruments and
systems, and obtain dependable measure-
ment accuracy for the life of those products.
Get assistance with all your
test and measurement needs at:
www.agilent.com/find/assist
Figure 5. Test signal is inserted on a video line
Product specifications and descriptions in this
document subject to change without notice.
Copyright © 1993, 2000 Agilent Technologies
Printed in U.S.A. 11/00
5091-7195E
Plik z chomika:
ciasteczkowypotwornie
Inne pliki z tego folderu:
5951-0064.pdf
(21620 KB)
11_Dec76.pdf
(10620 KB)
08970-99000.pdf
(7830 KB)
1973-09.pdf
(5248 KB)
5951-0062.pdf
(9328 KB)
Inne foldery tego chomika:
140
ASC
Zgłoś jeśli
naruszono regulamin