5091-7194E.pdf

Agilent 89440A-1

Frequency and Time-Selective

Power Measurements with the

Agilent 89410A and 89440A

Product Note

When you need to measure the

power of signals or noise, the Agilent

Technologies 89410A and 89440A

vector signal analyzers (VSAs) offer

several unique advantages over other

types of instruments. Excellent level

accuracy, true RMS power detection,

and precise noise bandwidths com-

bine to produce exceptionally accu-

rate power measurements. Accuracy

is coupled with advanced features

such as time-gating, arbitrary reso-

lution bandwidth, and band-power

markers to create an instrument

that performs complex power meas-

urements with unprecedented versa-

tility and ease. Such versatility is

essential for measuring the power of

time-varying signals found in com-

munication and video applications.

Limitations of traditional

instruments

You may have used power meters,

voltmeters, noise figure meters, or

oscilloscopes to measure power.

These instruments are adequate for

many types of signals but they also

have several limitations including

inadequate dynamic range, accuracy,

or both. In addition, these instruments

are not frequency selective—they only

provide a reading of the total power

across the instrument's entire band-

width. A vector signal analyzer's fre-

quency selectivity not only allows you

to measure the power at one frequen-

cy, but by measuring the noise over a

frequency band, it allows you to deter-

mine the "shape" of the power in the

frequency domain. For example, fre-

quency selectivity and dynamic range

allow you to measure noise power

independently from signals that may

accompany the noise.

have distinct disadvantages. First, the

analog detectors found in most swept

spectrum analyzers are designed to

measure the spectral components of

deterministic signals, not random

noise. When measuring random noise,

a correction factor must be applied to

the analyzer's displayed noise level.

Second, traditional swept spectrum

analyzers have analog resolution

bandwidth (RBW) filters that typically

have a bandwidth accuracy of ±20%.

When you make noise-power meas-

urements with these analyzers and

calculate the noise-power bandwidth

using the nominal value of the RBW

(or use the built-in noise level func-

tion), errors of up to 1 dB will result.

Third, after the signal is detected, a

swept spectrum analyzer normally

implements some type of peak detec-

tion to ensure that the peak of a sig-

nal will always be displayed. The

average value of peak-detected noise

is biased, so most swept spectrum

analyzers will allow you to turn peak

detection off by selecting a "sample"

detector mode. However, with a sam-

ple detector the level of a single tone

cannot be measured accurately.

Traditional swept spectrum analyzers

are frequency-selective and have

excellent dynamic range, but, when it

comes to measuring noise power, they

VSAs overcome traditional

constraints

Vector signal analyzers do not have

these constraints. In vector mode, the

Agilent 89410A and 89440A vector

signal analyzers accurately digitize

the signal and then calculate the fre-

quency spectrum using the fast

Fourier transform (FFT). You can

measure the power of the signal in

either the time domain or the fre-

quency domain. The FFT calculation

results in the true RMS power of the

signal whether it is a single tone,

noise, or any complex signal. In addi-

tion, the noise-power bandwidth at

each frequency is the same as the

RBW which is precisely known and

repeatable. Finally, there is no need

for peak detection and the signal can

be averaged without biasing the

results. In scalar mode, the VSA

implements very narrow RBWs by

performing several stepped FFTs and

there may be more information than

can be displayed and stored. In this

case a data reduction or detection

must occur (several detector schemes

are provided). For noise measurements

in scalar mode, use the sample detector.

Basic noise measurements

made simple

The noise level measured by a signal

analyzer is directly proportional to

the analyzer’s RBW setting. Often,

you will want to normalize the meas-

urement to a 1-Hz bandwidth by

dividing the measured noise level by

the noise-power bandwidth of the

RBW setting. The 89410A and 89440A

vector signal analyzers make this nor-

malization easy by having precisely

known, repeatable noise bandwidths.

Moreover, when you select the power

spectral density (PSD) measurement

data, the entire trace is normalized

for you. Figure 1 depicts a PSD meas-

urement (with 1000 averages)

performed using the 89410A.

Rotating the knob moves the marker

along the trace and displays the nor-

malized power at each frequency

point.

The FFT algorithm also gives the VSA

an advantage when it comes to meas-

urement speed. In vector mode, the

89410A and 89440A are typically tens

to hundreds of times faster than tra-

ditional swept spectrum analyzers at

a given frequency span and RBW

setting. This speed advantage is sig-

nificant if you want to reduce the

variance in your noise-power meas-

urement by averaging hundreds or

even thousands of measurements.

Figure 1. Select the PSD measurement data to display the power density of

the signal as a function of frequency. The trace data and marker readout

are automatically normalized to 1 Hz.

You can normalize the noise power to

a bandwidth other than 1 Hz, or inte-

grate power over a range of frequen-

cies, by using the band power markers

to select the frequency band of inter-

est. The 89410A and 89440A calculate

the total power in the selected fre-

quency band and display the result

in the lower, left-hand corner of the

display. In Figure 2, the Agilent 89440A

was used to measure noise power in a

frequency band near a carrier.

To perform carrier-to-noise or signal-

to-noise measurements, use the main

marker to measure the signal power,

and use the C/N or C/No band power

markers to select the frequency band

of interest. The C/N marker function

calculates and displays the total power

in the selected frequency band rela-

tive to the signal power. The C/No

marker function normalizes the power

in the band to a 1-Hz bandwidth.

Figure 2. Use the band power markers to calculate the total power in a user-selected

frequency band.

Time-variant measurements

made easy

In many of today’s applications, the

signals that need to be measured are

not stationary but are time-varying,

burst, or transient. In these applica-

tions the most significant attribute of

the Agilent 89410A and 89440A vec-

tor signal analyzers is the ease with

which they make time-selective meas-

urements. They capture your burst or

transient signal, let you examine the

entire time- record, and then let you

select the portion of the signal you

are interested in for further analysis.

All the features mentioned previously

can be used to measure the power of

the selected portion of the time-record.

If the signal is recurring, then a vari-

ety of triggering modes allow you to

make averaged power measurements.

You may want to use the time-record

of the signal to determine the peak

instantaneous power or to calculate

the RMS power of the signal over a

specific period of time. You should

enable the VSA’s time-domain cali-

bration when you are making time-

domain measurements. The displayed

time-record is a filtered version of

the input signal that only includes

the spectral energy within and near

the VSA’s measurement span. For

time-domain measurements, the 3-dB

bandwidth of the VSA is 12 to 17

percent greater than the VSA’s fre-

quency span. In figure 4, the 89440A

was used to capture a time division

multiple access (TDMA) signal as

the transmitter was turned on. The

selected data format is linear magni-

tude. This measurement is similar

to a zerospan measurement with a

swept spectrum analyzer or to a peak

power meter measurement. Band

power markers select the portion of

the time-record over which the power

is calculated. Moving the band power

markers along the trace will reveal

how the power changes as a function

of time.

Figure 3. The C/N and C/No band power markers display the power in the band relative

to a signal.

Figure 4. Use time-domain band power markers to calculate the power over a selected

portion of the time-record.

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