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Agilent PN 89400-10
Time-Capture Capabilities of the Agilent
89400 Series Vector Signal Analyzers
Product Note
Figure 1. Simplified block diagram showing basic signal flow in the Agilent 89400 Series VSAs
Introduction FFT, demodulation, or averaging. The ability to perform signal analysis
When measuring a burst communica- When using time capture, the instru- separately from signal acquisition gives
tions transmission or other nonsta- ment collects a (possibly) much larger the user some significant advantages.
tionary signal, it is often necessary to number of samples and stores them Foremost is the ability to analyze the
observe or analyze the signal for a sig- to memory, but does not perform the signal in many different ways after
nificant period of time to adequately analysis immediately. Instead, the the data has been collected. Another
characterize it. The Agilent Technologies user can choose the type of analysis advantage is that the data record has
89400 Series vector signal analyzers to be performed at a later time. no gaps in it. The instrument collects
(VSAs) feature a time-capture capabil- the data in one time-continuous block.
ity that enables the user to collect up To understand how time-capture (When using measure from input, there
to 1 Msample of data at a time and works in the 89400 Series VSAs, refer is a short period of time when the ana-
later choose what type of analysis to to the instrument block diagram in lysis is being performed on the time
perform on the data. Figure 1. The process involves two record that has just been collected.
principal events: data acquisition and While this period of time is small,
Measurements can be done in one of analysis. During data acquisition the there still may be some data missed.)
two ways: measure from input and signal is conditioned, sampled, and
time capture. When using measure converted into a stream of digital val-
from input, the instrument collects a ues that are stored in the sample
block or record of samples and per- RAM. During analysis, a coordinate
forms the analysis immediately. This transform is applied to the digital val-
analysis may involve performing an ues and the data is scaled for display.
The remainder of this paper describes Set the center frequency and span This is often referred to as a cardinal
setting up time-capture measurements, The frequency span should be as nar- span. By using the up/ down keys the
and looks at how to exploit this flexi- row as possible while still including instrument will usually select the near-
ble analysis capability to its fullest. In all the important components of your est "nice" value (often divisible by a
addition, there's an overview on how signal. This prevents unwanted signals power of ten), or the nearest cardinal
to save and recall captured data from from corrupting the measurement value. If you set the frequency span to
a mass storage device, and how to and also allows the collection of the a value that is not a cardinal span, the
transfer data to an external computer longest possible time record. instrument will use the sample rate
for additional analysis. appropriate for the next larger cardi-
The 89400 Series VSAs offer unique nal span, consuming a larger amount
Acquiring time-capture data versatility in their ability to implement of sample RAM. For example, if you
Time capture is critical when you wish arbitrary frequency spans and resolu- chose a 5.1 MHz span, the instrument
to measure a signal that will only be tion bandwidths, which has implica- will collect data at the same rate as if
present once and/or whose exact char- tions when making time-capture meas- you had set a 10 MHz span.
acteristics aren't known in advance; urements. To obtain the longest time
however, you do need some basic capture for the range of frequencies If you plan on performing digital
knowledge of the signal's characteris- you wish to capture, set the frequency demodulation analysis (Option AYA)
tics before you start the time-capture span to a value that can be represented on the time-capture data, use the fol-
procedure. as 10 MHz/2N, where N is an integer. lowing formula to determine the max-
imum span:
To initiate the instrument for time capture:
Maximum span (digital demod) = 20 * [symbol rate]
Set the instrument mode to Vector k
The choice of instrument mode affects where: k = 2.56 if the receiver is [ch1 = j*ch2], k = 1.28 for all other receivers.
the way the instrument collects the
data, and how it can be viewed or
analyzed later. It is not possible to
Table 1. 89400 Series VSAs Standard Configuration with 64k of Time-Capture Memory
perform time capture in Scalar mode.
After the data collection is complete, One Channel Instrument Two Channel Instrument
you can choose between Vector and
Demodulation modes depending on Baseband Zoomed Baseband Zoomed
how you would like to view the data. Measurement Measurement Measurement Measurement
Set the correct receiver Maximum 65,536 65,536 32,768 32,768
Make note of your choice of receiver Number of
(RF, IF, external, or ch1+j*ch2) when Sample Points*
you save time-capture data. You must
Maximum Time 2.56 ms 5.12 ms 1.28 ms 2.56 ms
set the instrument to the same receiver
Length
to analyze the time-capture data as it
was when the data was originally col-
lected. You can examine the data and 89400 Series VSAs With 1-Meg Extended Time-Capture Memory (Option AY9)
the header information by using the
Structured Data Format (SDF) utilities One Channel Instrument Two Channel Instrument
provided with the instrument.
Baseband Zoomed Baseband Zoomed
Measurement Measurement Measurement Measurement
Maximum 1,047,552 1,048,064 523,264 523,776
Number of
Sample Points*
Maximum Time 40.92 ms 81.88 ms 20.44 ms 40.92 ms
Length
* The maximum number of points will be less if you use an arbitrary span. For Option AY9, the maximum number of
points varies slightly with changes in the number of frequency points selected. The values in this table were
2 obtained with the number of frequency points set to 401.
Set the trigger and input configuration which the capture took place. The Performing the capture
To capture a transient event, IF trig- length is simply the number of sam- To perform time capture, simply
gering often provides the best results, ples collected multiplied by the time press the [fill buffer] softkey. This key
since the instrument will only start between samples. is reached by pressing [Instrument
collecting data once a signal appears mode] and then [capture setup]. The
in your frequency span. You can also The number of samples to be collected instrument display will not update
set a negative trigger delay to capture is under the user's control and can during the capture; data is only being
time before the trigger event, much as range from 512 to 64k (standard collected at this point. When it is fin-
you might do with an oscilloscope. instrument) or 1 Meg (Option AY9 ished, a message will appear on the
extended time capture). The time display indicating the length of time
Set the source type and level between samples is inversely related over which the capture took place.
If your measurement requires the to the frequency span and can take on
89400 Series VSAs internal source as a a wide range of values, from as short Once the capture is completed
stimulus, set the source type and level. as about 40 ns to almost 1 second. The you can view the number of bytes of
resulting range of time-capture lengths data collected, block size, and other
Set the input channel(s) extends from a few microseconds to parameters the instrument generates
For the signal you are capturing set several thousand seconds. This capa- to describe the organization of the
the input channel(s) to the proper bility allows measurement of events data in the capture buffer. To get to
range and impedance. Turn off the such as the first few milliseconds dur- this display press [Instrument Mode],
second input channel if you don't ing the turn-on of a mobile radio to the [capture setup], [buffer info on]. It's a
need it (see the Input hardkey). This long-term drift of a relatively stable good idea to do a trial time-capture
allows the instrument to use all of the VCO. Figure 2 illustrates the inverse measurement to ensure the triggering
capture RAM for channel 1. relationship between the frequency is set up correctly and that the sam-
span, the sample rate, and the meas- ple rate and number of samples were
Specify the amount of data to be captured urement record length. A large extent quantized by the instrument to the
The amount of data captured can be or length in the time data yields a values expected.
specified in units of time, number of measurement with a narrower fre-
records, or number of sample points. quency span while finer time resolu-
Often you will want to know the tion and a shorter time length yields
longest time or largest number of a wider frequency span.
sample points that can be collected.
This depends on instrument setup, as
mentioned above, and on the size of
the sample RAM available. The capac-
ity of the time-capture buffer also
depends on whether a baseband or
zoomed measurement is being made.
For a baseband measurement, the
selected span starts at exactly zero Hz,
while in general, a zoomed measure-
ment covers a frequency span which
does not include zero Hz. Table 1
shows the maximum amount of data
which can be captured for several
instrument configurations.
There are two ways to think about the Figure 2. Some measurements have both time-domain and frequency-domain aspects.
capacity of the sample RAM. In this Because of the nature of the fast Fourier transform, these two aspects are not independent.
product note we will often refer to The extent and resolution of the time and frequency data are interrelated as shown here.
the size of the sample RAM, and the
length of the time capture. The size
refers to the number of samples the
instrument will collect, while the
length is the amount of time over
3
Analyzing the captured data ments can be specified.) When over- Saving and recalling
After the capture is complete, you lap processing is enabled, the display time-capture data
can view the data by pressing the represents data calculated partly You can store time-capture data
[Meas Restart] hardkey. You can con- from the current time record and to any available mass storage unit,
trol the analysis to be performed on partly from the next time record. (internal or external disk, volatile or
the data by changing the following nonvolatile RAM disks). Before stor-
parameters: The percent overlap parameter sets ing a time-capture file, make sure the
the amount of the current time record mass storage unit you've chosen has