5091-8686E.pdf

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

When measuring a burst communica-

tions transmission or other nonsta-

tionary signal, it is often necessary to

observe or analyze the signal for a sig-

nificant period of time to adequately

characterize it. The Agilent Technologies

89400 Series vector signal analyzers

(VSAs) feature a time-capture capabil-

ity that enables the user to collect up

to 1 Msample of data at a time and

later choose what type of analysis to

perform on the data.

FFT, demodulation, or averaging.

When using time capture, the instru-

ment collects a (possibly) much larger

number of samples and stores them

to memory, but does not perform the

analysis immediately. Instead, the

user can choose the type of analysis

to be performed at a later time.

The ability to perform signal analysis

separately from signal acquisition gives

the user some significant advantages.

Foremost is the ability to analyze the

signal in many different ways after

the data has been collected. Another

advantage is that the data record has

no gaps in it. The instrument collects

the data in one time-continuous block.

(When using measure from input, there

is a short period of time when the ana-

lysis is being performed on the time

record that has just been collected.

While this period of time is small,

there still may be some data missed.)

To understand how time-capture

works in the 89400 Series VSAs, refer

to the instrument block diagram in

Figure 1. The process involves two

principal events: data acquisition and

analysis. During data acquisition the

signal is conditioned, sampled, and

converted into a stream of digital val-

ues that are stored in the sample

RAM. During analysis, a coordinate

transform is applied to the digital val-

ues and the data is scaled for display.

Measurements can be done in one of

two ways: measure from input and

time capture . When using measure

from input, the instrument collects a

block or record of samples and per-

forms the analysis immediately. This

analysis may involve performing an

The remainder of this paper describes

setting up time-capture measurements,

and looks at how to exploit this flexi-

ble analysis capability to its fullest. In

addition, there’s an overview on how

to save and recall captured data from

a mass storage device, and how to

transfer data to an external computer

for additional analysis.

Set the center frequency and span

The frequency span should be as nar-

row as possible while still including

all the important components of your

signal. This prevents unwanted signals

from corrupting the measurement

and also allows the collection of the

longest possible time record.

This is often referred to as a cardinal

span. By using the up/ down keys the

instrument will usually select the near-

est “nice” value (often divisible by a

power of ten), or the nearest cardinal

value. If you set the frequency span to

a value that is not a cardinal span, the

instrument will use the sample rate

appropriate for the next larger cardi-

nal span, consuming a larger amount

of sample RAM. For example, if you

chose a 5.1 MHz span, the instrument

will collect data at the same rate as if

you had set a 10 MHz span.

The 89400 Series VSAs offer unique

versatility in their ability to implement

arbitrary frequency spans and resolu-

tion bandwidths, which has implica-

tions when making time-capture meas-

urements. To obtain the longest time

capture for the range of frequencies

you wish to capture, set the frequency

span to a value that can be represented

as 10 MHz/2 N , where N is an integer.

Acquiring time-capture data

Time capture is critical when you wish

to measure a signal that will only be

present once and/or whose exact char-

acteristics aren’t known in advance;

however, you do need some basic

knowledge of the signal’s characteris-

tics before you start the time-capture

procedure.

If you plan on performing digital

demodulation analysis (Option AYA)

on the time-capture data, use the fol-

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

The choice of instrument mode affects

the way the instrument collects the

data, and how it can be viewed or

analyzed later. It is not possible to

perform time capture in Scalar mode.

After the data collection is complete,

you can choose between Vector and

Demodulation modes depending on

how you would like to view the data.

where:

k = 2.56 if the receiver is [ch1 = j*ch2], k = 1.28 for all other receivers.

Table 1. 89400 Series VSAs Standard Configuration with 64k of Time-Capture Memory

One Channel Instrument

Two Channel Instrument

Baseband

Zoomed

Baseband

Zoomed

Measurement

Set the correct receiver

Make note of your choice of receiver

(RF, IF, external, or ch1+j*ch2) when

you save time-capture data. You must

set the instrument to the same receiver

to analyze the time-capture data as it

was when the data was originally col-

lected. You can examine the data and

the header information by using the

Structured Data Format (SDF) utilities

provided with the instrument.

Maximum

65,536

32,768

Number of

Sample Points*

Maximum Time

2.56 ms

5.12 ms

1.28 ms

2.56 ms

Length

89400 Series VSAs With 1-Meg Extended Time-Capture Memory (Option AY9)

One Channel Instrument

Two Channel Instrument

Baseband

Zoomed

Baseband

Zoomed

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

obtained with the number of frequency points set to 401.

which the capture took place. The

length is simply the number of sam-

ples collected multiplied by the time

between samples.

Performing the capture

To perform time capture, simply

press the [fill buffer] softkey. This key

is reached by pressing [Instrument

mode] and then [capture setup]. The

instrument display will not update

during the capture; data is only being

collected at this point. When it is fin-

ished, a message will appear on the

display indicating the length of time

over which the capture took place.

Set the trigger and input configuration

To capture a transient event, IF trig-

gering often provides the best results,

since the instrument will only start

collecting data once a signal appears

in your frequency span. You can also

set a negative trigger delay to capture

time before the trigger event, much as

you might do with an oscilloscope.

The number of samples to be collected

is under the user’s control and can

range from 512 to 64k (standard

instrument) or 1 Meg (Option AY9

extended time capture). The time

between samples is inversely related

to the frequency span and can take on

a wide range of values, from as short

as about 40 ns to almost 1 second. The

resulting range of time-capture lengths

extends from a few microseconds to

several thousand seconds. This capa-

bility allows measurement of events

such as the first few milliseconds dur-

ing the turn-on of a mobile radio to the

long-term drift of a relatively stable

VCO. Figure 2 illustrates the inverse

relationship between the frequency

span, the sample rate, and the meas-

urement record length. A large extent

or length in the time data yields a

measurement with a narrower fre-

quency span while finer time resolu-

tion and a shorter time length yields

a wider frequency span.

Set the source type and level

If your measurement requires the

89400 Series VSAs internal source as a

stimulus, set the source type and level.

Once the capture is completed

you can view the number of bytes of

data collected, block size, and other

parameters the instrument generates

to describe the organization of the

data in the capture buffer. To get to

this display press [Instrument Mode],

[capture setup], [buffer info on]. It’s a

good idea to do a trial time-capture

measurement to ensure the triggering

is set up correctly and that the sam-

ple rate and number of samples were

quantized by the instrument to the

values expected.

Set the input channel(s)

For the signal you are capturing set

the input channel(s) to the proper

range and impedance. Turn off the

second input channel if you don’t

need it (see the Input hardkey). This

allows the instrument to use all of the

capture RAM for channel 1.

Specify the amount of data to be captured

The amount of data captured can be

specified in units of time, number of

records, or number of sample points.

Often you will want to know the

longest time or largest number of

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

capacity of the sample RAM. In this

product note we will often refer to

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

Figure 2. Some measurements have both time-domain and frequency-domain aspects.

Because of the nature of the fast Fourier transform, these two aspects are not independent.

The extent and resolution of the time and frequency data are interrelated as shown here.

Analyzing the captured data

After the capture is complete, you

can view the data by pressing the

[Meas Restart] hardkey. You can con-

trol the analysis to be performed on

the data by changing the following

parameters:

Saving and recalling

time-capture data

You can store time-capture data

to any available mass storage unit,

(internal or external disk, volatile or

nonvolatile RAM disks). Before stor-

ing a time-capture file, make sure the

mass storage unit you’ve chosen has

enough free space to save the entire

file. Although the instrument allows

you to split a file across multiple

disks if there is not enough room, it is

more convenient and faster if the file

fits completely on one disk. You can

estimate the file size before you try to

store it from the number of sample

points chosen. For a baseband meas-

urement, the file size in bytes will be

about 4 times the number of sample

points, while a zoomed measurement

will take about 8 times the number of

sample points.

ments can be specified.) When over-

lap processing is enabled, the display

represents data calculated partly

from the current time record and

partly from the next time record.

The percent overlap parameter sets

the amount of the current time record

used. For example, if 90% overlap is

specified, 90% of the samples come

from the time record used to calculate

the current display and 10% of the

samples contain new data from the

next time record in the time capture

buffer. If you set the overlap percent-

age to zero, the data is played back

from the capture buffer as fast as

possible and each display update

reflects entirely new data.

• Instrument mode (demodulate the

data, or view it in vector mode)

• Analysis region (the portion of the

capture buffer to analyze)

• Resolution bandwidth

• Averaging

• Windowing

• Measurement data type (time,

power spectrum ... )

• Data format (magnitude, phase,

group delay ... )

• Gating

This capability provides a way to

view rapidly changing, complex sig-

nals. You can use overlap processing

in combination with single sweep

mode, or with the spectrogram or

waterfall displays (Option AYB). To

view the contents of the buffer in

reverse, change the direction of the

capture playback under the [analysis

region] softkey.

Examining your signal in more

detail with Overlap Processing

Occasionally when playing back cap-

tured data of short time duration (i.e.,

one made with a high sample rate), the

buffer is displayed very quickly. There

are two ways you can slow the “play-

back” of captured data to better view

the details of your signal: by individu-

ally stepping through each record, or by

using a feature of the Agilent 89400

Series VSAs called overlap processing.

There is additional information stored

in a header with the time-capture data,

which helps the instrument interpret

and analyze the data. Frequency lim-

its, correction data, and other instru-

ment parameters are stored in this

header.

Other time-capture considerations

While you cannot playback captured

data through the baseband source,

(you may want to do this to stimulate

a circuit with a signal identical to one

you have measured previously), the

source can playback data from one of

the data registers, and these data reg-

isters can contain data saved from

one of the time records in the time-

capture buffer.

Captured data can also be transferred

from the instrument to an external

computer via GPIB or the LAN inter-

face if the instrument has option UFG

(4-MBytes extended RAM and addi-

tional 10) installed. The data format

for time-capture data is the same as

for trace data, but again the amount of

data transferred can be much larger.

You may have to take special steps to

insure your program has enough mem-

ory available to store all the data.

To inspect the time-capture buffer

a record at a time, switch the sweep

mode to single and press the [Pause/

Single] key. Each keypress allows

the next record in the buffer to be

processed and displayed. By setting

the [analysis region] (under the [cap-

ture setup] softkey), you can view a

selected portion of the buffer. (By

default, the analysis region includes

the entire capture buffer.)

One important point to keep in mind:

switching the instrument from time

capture back to measure from input

erases all previously collected time-

capture data. You will also lose the

time-capture data if you preset the

instrument, change receivers, or

recall an instrument state file from

mass storage, since this performs an

implicit instrument preset.

With overlap processing, the instru-

ment pulls less than a full time record

from the time-capture buffer for pro-

cessing and display. (Under the [Time]

hardkey the amount of overlap for

averaged and non-averaged measure-

At this scale, the sweep has covered

about 200 kHz and the transient

seems rather innocuous, it is just a

narrow line across the display, indi-

cating the signal went through a large

but very quick frequency excursion.

To see the full extent of this frequency

transient, and the amplitude pertur-

bation that may have occurred at the

same time, the overlap was increased

to 80 percent, the result is shown in

Figure 4. You will find, as we did here,

that you need to experiment with the

amount of overlap, using the value

which produces the most intuitive

picture on the display.

Once in an external computer, use the

SDF utilities to examine or manipulate

the data. The data can also be trans-

lated into the formats used by a num-

ber of popular analysis programs such

as MATLAB from The Math Works and

MATRIX X from Integrated Systems.

the measurement, and except for the

transient in question, remains at a

relatively constant amplitude (constant

intensity on the display) throughout.

To make this measurement, we con-

figured the 89440A (dc to 1.8 GHz) in

Vector mode with a center frequency

of about 24.5 MHz, and a frequency

span of 625 kHz. We set a capture

buffer size of 65,536 points, and since

this is a zoomed measurement, the

resulting time length is 81.92 ms. This

is the maximum number of time points

available when the instrument is con-

figured to measure only one channel.

(See Table 1.) IF trigger and negative

trigger delay were also used.

Time-capture data is recalled from

a storage device as you would recall

other stored trace data. Note that

when you recall time-capture data the

instrument state is modified to match

the state of the instrument when the

time capture took place.

A time-capture measurement

To illustrate the insight provided by

the time-capture capability of the

Agilent 89400 Series VSAs, a measure-

ment example is presented that uses

many of the techniques discussed here.

The signal of interest is the output of

a synthesized signal generator during

a frequency sweep. Signal sources

like this are often used as a stimulus

in network-analysis measurements.

Typically, this type of signal generator

employs a synthesis scheme involving

multiple phase-locked loops and often

generates significant frequency tran-

sients as the loop(s) divider values

change while sweeping. To investigate

these transients, we captured a portion

of the generator’s sweep and then dis-

played it with two different scales and

overlap processing on a spectrogram

display.

The spectrogram display (Option AYB)

is used to simultaneously show the

signal’s amplitude and frequency versus

time. The signal’s amplitude is repre-

sented by intensity or color, while the

changes in the spectrum over time are

represented by scrolling information up

the display from the bottom. The x-axis

represents frequency, as in other spec-

trum displays. As shown in Figure 3,

the signal’s frequency increases during

Figure 3. Spectrogram display showing a transient during the frequency sweep produced

by a synthesized signal generator

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: