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Agilent PN 89400-5
Measuring Transmitter Transients with the
89400 Series Vector Signal Analyzers
Product Note
Understanding and controlling the types of analysis can be performed on delay. The IF trigger is the most con-
transient behavior of transmitters the same transient event. These include venient for transmitter measurements
has become increasingly important calculating the spectrum, instantaneous because it allows the measurement to
with the introduction of TDMA and power, phase, amplitude, or frequency be triggered on a change in transmit-
spread spectrum systems. In these deviation versus time, to name a few. ter power level. The trigger delay is
systems and others, the carrier The time-capture buffer can also be typically set to a negative number so
power is continuously changing in stored to disk, allowing the analysis the measurement is pre-triggered. The
order to maximize spectrum utiliza- to be performed later, at a more con- use of pre-trigger delay allows events
tion and conserve battery power. The venient time and location. leading up to the transient to be
transients that occur because of included in the analysis, and provides
changes in carrier power and fre- There are three steps to capturing for settling in the overlapped meas-
quency can cause interference to data. The first is to select a center urements described later.
other stations. Unsettled carriers frequency and span. The span must
can also cause high bit-error rates. be wide enough to allow all of the Power Transients
transmitter's power to be measured, Instantaneous power measurements
The Agilent Technologies 89400 yet narrow enough to prevent inter- are one of the most common meas-
Series vector signal analyzers (VSAs) ference from other signals that might urements made today. The measure-
can capture a single transient event be present (such as another station). ment is made to find the peak power
and repeatedly analyze it in the The VSA's information bandwidth in a digitally modulated signal, to set
time, frequency, and modulation (span) can be adjusted with nearly signal levels into the power amplifier,
domains. Phase, frequency, and infinite resolution. Setting the span or to test a transmitter pulse against
amplitude transients, or transitions, automatically band limits the analysis. an amplitude mask found in a com-
can be viewed directly. Also, the munication standard.
spectrum can be displayed simulta- The second step is to select the
neously with the transient displays, amount of data to be captured. The Traditionally, instantaneous power
providing a unique look at the rela- length of the time-capture buffer is a measurements were made with a
tionships between transients and function of span, the number of input swept spectrum analyzer in zero-span
spectrum occupancy. One of the ana- channels enabled, and the amount of mode or with a peak power meter.
lyzer's unique features, overlap pro- capture memory installed in the
cessing, allows captured transients instrument. The standard time-capture Measurements made by the Agilent
to be played back in "slow motion" buffer can hold up to 65K samples. 89400 Series VSAs closely resemble a
for easier analysis. On a 5-MHz span with one channel zero-span measurement because the
enabled, this allows roughly 10 msec power measurement is made over a
Capturing the Transient of data to be captured. Decreasing the finite bandwidth. In the case of the
Before a transient can be analyzed, it span by a factor of two doubles the swept analyzer, the bandwidth is set
must be captured into the analyzer's length of the capture buffer in sec- by the selected RBW filter. In the VSA
time-capture buffer. Only the raw time- onds. Option AY9 increases the cap- the span determines the information
data is held in memory; the final pro- ture memory length to 1M sample. bandwidth. By combining excellent
cessing of the data into a measured level accuracy, true RMS power detec-
result is performed after the capture The final step in capturing the tran- tion, and precise noise bandwidths,
process is complete. Because the raw sient is to select the type of triggering the VSA produces exceptionally accu-
data is always available, many different and an appropriate amount of trigger rate power measurements.
To display instantaneous power as the measurement decreases allowing greater than 7 MHz directly. The sec-
a function of time, simply select the improved frequency resolution. The ond major difference between time
measurement data type of main time RF VSA provides band-limited meas- interval analyzers and VSAs is sensi-
(in vector mode) and set the data for- urements with a maximum span of tivity. The VSAs' increased sensitivity
mat to display the trace in log magni- 7 MHz. Frequency transitions greater makes measurements easy without
tude coordinates. A variety of engi- than 7 MHz cannot be completely physically connecting the instrument
neering units are available, the most analyzed from start to finish, but the to the transmitter. Good sensitivity
common being dBm or watts. Once final setting can be observed with the and excellent frequency selectivity
displayed, markers measure instanta- VSA. Time interval analyzers can make off-the-air transient measure-
neous and peak power. Offset mark- characterize frequency transitions ments very easy to perform.
ers can determine rise-time, over-
shoot, and settling. In addition, the
RMS power over a selected portion
of the time record can be computed
using band-power markers.
Figure 1 shows an example of a power
measurement made on a small hand
held FM transmitter. The marker is
displaying the peak power in watts.
The band-power markers show the
RMS power over the latter part of the
transient (annotated at the bottom of
the screen). The top traces in Figures
2 and 3 also show power transients.
Frequency Transients
When a transmitter changes power
levels, the carrier frequency should
remain stable. If a frequency change Figure 1. Power during a transmitter's turn-on transient. The marker indicates the peak
is intentional, as in a change of chan- power level. The RMS power during the time interval between the two cursors is shown
nels, the carrier should not overshoot in the lower left-hand corner.
its intended frequency, and should
settle quickly to its final value. Tra-
ditionally, measuring instantaneous
frequency versus time has been per-
formed using time interval analyzers.
There are three major differences
between time interval analyzers and
the 89400 Series VSAs. First, time
interval analyzers are broadband
instruments. Unless external filters
are added, the time interval analyzer
is subject to more noise power as well
as contamination from other signals.
Since the information bandwidth of
the vector signal analyzer is deter-
mined by the span, other signals
(such as an adjacent carrier) can be
easily removed from the measure-
ment. Also, as the span is narrowed, Figure 2. The lower trace shows frequency deviation of the carrier over time. Note that
the total amount of noise power in the carrier power (upper trace) is stable long before the carrier frequency.
2
The third major difference is the served observed by an oscilloscope. Figure 3 shows the phase settling of a
manner in which the information is This technique has the limitation of carrier undergoing a change in power
extracted from the signal. The time requiring that the reference signal level. The upper trace shows the car-
interval analyzer uses counter tech- and measured signal a common fre- rier power; the lower trace, instanta-
nology which is based on the detection quency reference. Also, if the meas- neous phase. Notice how the change
of zero crossings in the measured sig- urement is not properly set up, the in power level causes a 13-degree
nal. In the VSA, the FM demodulator display on the oscilloscope is difficult shift in phase.
is a true demodulator, implemented to interpret.
as a DSP algorithm. The algorithm is As with the other modulation meas-
insensitive to any AM which may be The VSA measures instantaneous urements, markers can be used to
present on the signal. phase over time through the use of measure peak-phase error, phase set-
a phase demodulator implemented as tling time, or the RMS phase over a
To measure frequency transients a DSP algorithm. For transient meas- specified time interval. The band-
with the VSA, the instrument mode is urements, it's best for the instrument power markers show the RMS phase
changed to demodulation and an FM and the measured signal to share a noise after the carrier-power level has
demodulator selected. The measure- common frequency reference, though settled at 0.136 degrees.
ment data is the main time record it's not absolutely necessary. If a fre-
with real trace coordinates. The auto- quency reference is shared, then the Overlap Processing for Transient Analysis
carrier detector should be disabled so measurement center frequency should It is often important for a designer to
that the displayed frequency is rela- exactly match the carrier frequency understand how a specific transmit-
tive to the center frequency of the and the auto-carrier detect mode set ter transient affects the occupied
measurement. The time record will to phase-only. bandwidth of a carrier. In other
show carrier frequency deviation ver- words, which transients are most
sus time. The phase measurement is performed likely to cause interference on adja-
by setting the instrument mode to cent channels? For example, is spec-
The VSA makes it possible to observe demodulation and selecting a PM tral splatter caused by the turn-on
the interaction between instantaneous demodulator. The measurement data power transient or a phase transient
power and instantaneous frequency-- is in main time, and can be viewed that occurs at the start of data trans-
something no other instrument can do. with the data format set to real or mission? Using the overlap processing
Figure 2 shows how data registers phase (wrap or unwrap). The results capability of the VSA, it is possible to
make it possible to simultaneously are displayed in units of degrees or observe the effects of various tran-
view results from previous measure- radians versus time. sients in the signal to determine how
ments, such as the power measure- the spectrum changes with time.
ment described earlier, with the cur- As with the FM demodulator, the
rent result which is based on the phase resolution is a function of The best way to understand this
same raw data. span. The narrower the span, the measurement is to imagine the same
better the resolution. signal being applied to a parallel bank
As with the power measurement,
markers can be used to determine
peak and RMS frequency, as well as
frequency overshoot and settling.
Phase Transients
After a change in carrier power,
the carrier phase must settle before
digital data transmission can begin.
The traditional way of measuring
phase transients, although relatively
straightforward, usually provides un-
calibrated results that can be difficult
to interpret. The measurement tech-
nique involves using a mixer and a
reference source to mix the measured
signal to dc, so the phase difference Figure 3. Phase settling (lower trace) of a carrier undergoing
between the two signals can be ob- a change in power level (upper trace). 3
of several hundred (or thousand) time menu), or indirectly by setting determined by the amplitude of the
bandpass filters. Each filter has the the RBW. The percent of measure- spectrum at the frequency indicated
same bandwidth, but a slightly differ- ment overlap corresponds roughly to by the horizontal axis.
ent center frequency. The output of the frequency at which the output of
each filter is detected and then sam- the filters is sampled. As the percent- Summary
pled. At each point in time, the collec- age of overlap increases, more spec- Until now, transient measurements
tion of output samples is used to cre- trums are computed for a given have been difficult to make or have
ate an estimate of the power spec- amount of time data in the time- required several different instru-
trum. Each filter is designed with a capture buffer. ments. The 89400 Series VSAs make
narrow bandwidth to provide the nec- all of these measurements with one
essary frequency resolution, and a The overlapped measurements on a instrument--easily and with a high
short, well behaved impulse response time-capture buffer can produce hun- degree of accuracy.
to provide adequate time resolution. dred or thousands of spectrum meas-
urements. The Agilent 89400 Series The Agilent 89400 Series VSAs' time-
This measurement could be made VSAs can display these measurements capture capability can capture a sin-
with a swept spectrum analyzer con- in several different ways. The simplest gle transient and analyze it in a vari-
figured for zero-span operation. As approach is to show the measurements ety of ways. It is no longer necessary
described, the measurement requires one after the other using a standard to put the transmitter in a special
compiling the results of 400 zero-span spectrum display. Each measurement test mode to provide the instrumenta-
measurements, each at a slightly dif- is like a frame in a movie, with the tion with repeated signal access.
ferent frequency, into a single result. overall effect of a slow motion picture
Obviously, the device being measured of the rapidly changing spectrum. With Excellent sensitivity and selectivity
needs to generate the same transient option AYB, multiple spectrum meas- along with frequency coverage to dc
400 times--once per measurement. The urements can be viewed on a single allow measurements at any point in
VSA generates the same result, in one grid in a waterfall or spectogram dis- the transmitter's block diagram--from
measurement of a single transient. play. Each row of pixels in the spectro- the baseband modulation data, to
gram shown in Figure 4 corresponds local oscillators, to off-the-air meas-
To make the measurement using the to a spectrum measurement. The urements of the transmitted signal.
VSA, the analyzer is configured for intensity (or color) of the pixel is
vector mode operation with overlap
processing. Overlap processing is a
term that describes the analyzer's
ability to process, overlapping blocks
of time-domain data. Normally, the
percentage of overlap used for this
measurement is close to the 99%
limit. This means each display update
uses 99% old data plus 1% new data,
allowing more spectra to be generated
from the transient, which improves
resolution.
Compared to the parallel filter exam-
ple, here's how the various measure-
ment setup parameters affect the
result. The impulse response is deter-
mined by the selected FFT window.
The best window to use is the
Gaussian-top window. The length of
the impulse response is simply the
length of the time record used in the
measurement. Set this directly (in the Figure 4. A spectrogram can be used to simultaneously display several hundred over-
lapped spectrum measurements. Note the frequency instability and sidelobe structure
when the transmitter is first turned on (top of the spectrogram). Also note the mid-burst
4 transient which is not apparent in the time-capture buffer (lower trace).
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