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Agilent E2507B/E2508A
Noise Power Ratio (NPR) Measurements
Using the Agilent E2507B/E2508A Multi-
format Communications Signal Simulator
Product Note
can be used as a stimulus for making
NPR Stimulus
Device NPR measurements at frequencies
up to 26.5 GHz and above. It also dis-
Under cusses using the Agilent 71910A wide-
bandwidth receiver and 89410A vec-
Test tor signal analyzer (VSA) together as
the measurement receiver.
Traditionally, spectrum analyzers
have been used as measurement
receivers. However, the receiver/VSA
combination offers some key advan-
tages that include band power averag-
ing to calculate the power in preset
bands. This improves repeatability
NPR Response and removes the ambiguity of single-
point marker measurements.
Figure 1. The Agilent MCSS uses digital synthesis to create the noise stimulus for noise
power ratio measurements. This product note is divided into five
sections. Section 1 is the introduction.
Section 2 is an overview of the noise
1. Introduction power ratio measurement. Section 3
Noise power ratio (NPR) is a criterion The NPR measurement requires a describes the equipment required to
for evaluating the performance of sys- stimulus source to generate condi- make the measurement. Section 4
tems, subsystems, components, and tioned noise and a measurement explains how to create and modify
other circuits under conditions, that receiver to analyze the changes in waveforms. Section 5 discusses how
simulate a full spectrum of traffic and the noise after it passes through the to make the measurement as well as
interference signals. NPR is a distortion device under test. This product note some of the limitations of the system.
measurement that helps determine a shows how the Agilent Technologies The appendix provides a step-by-step
system's maximum spurious-free E2507B or E2508A multi-format com- procedure for generating an NPR
dynamic range. munications signal simulator (MCSS) stimulus.
2. NPR overview
Introduced over 30 years ago, NPR
has been and continues to be an
objective measure of the intermodula-
tion distortion (IMD) of active cir-
cuits in telecommunications. IMD is
the result of non-linearities in these
circuits. Intermodulation signals cre-
ated by additional traffic around the
frequency of interest can lead to inter-
ference and degradation of bit-error
rate for the channel of interest.
NPR provides an additional level of
information beyond the traditional
measurements such as gain, gain flat-
ness, and group delay. It allows the
manufacturer to evaluate the perform-
ance of an amplifier or system in the Figure 2. Multi-format communications
presence of realistic signal conditions. signal simulator
After analyzing the NPR results, the
manufacturer can choose the appro-
priate operating point of the device through the device under test (DUT), the filters. The signal generated by
under test to maximize efficiency and and the output is monitored. Non-lin- the diode noise source is non-repeat-
still have an acceptable NPR. Figure 9 earities in the DUT will lead to spec- able, resulting in long averaging
on page 7 shows the relationship be- tral components within the notch times. The ability to reproduce a sig-
tween NPR and output power for a (presumably created by intermodula- nal from unit to unit is dependent on
typical solid-state amplifier. The NPR tion distortion). NPR is the ratio of the filters and is generally poor.
drops to an unacceptable level well the power of the spectral components
before the device reaches the P-1dB outside of the notch to the power of The digital source creates a noise
compression point. those in the notch area. spectrum that is discrete, that is, it
repeats in time. To create noise, mul-
Although there is no recognized stan- The two predominate approaches for tiple CW tones are generated with
dard for NPR, measurement of this creating the NPR stimulus are analog, close spacing in frequency and ran-
parameter is often required in the using an analog noise diode condi- dom phase relationships, resulting in
mobile and satellite communications tioned with bandpass and band-reject a pseudo-random noise stimulus.
industries, and is being pushed down filters to create the spectral shape of Filtering is accomplished at the gen-
through the vendor chain to manufac- the NPR stimulus; and digital, creat- eration of the signal by turning off
turers of amplifiers and subsystems. ing a synthetic noise spectrum with the tones in a specific bandwidth.
The noise stimulus consists of white a digital source, then converting the This results in a spectral shape that
(Gaussian) noise from which a portion signal to analog with an ultralinear has high repeatability. More informa-
of the spectrum has been removed, digital-to-analog converter (DAC). In tion on the use of a digital stimulus
creating a notch in the pass band of the analog approach, the spectrum and a comparison between analog
interest. This stimulus is passed created is continuous and the shape and digital techniques can be found
of the notch is highly dependent on in the paper "Effective Evaluation of
Noise Power Ratio."1
2
3. The measurement system random phases. Generating a signal Traditionally, a spectrum analyzer has
The Agilent E2507B or E2508A multi- using the same phase seed will always been used for the receiver portion of
format communications signal simu- produce the same random phase set. the system. The spectrum analyzer has
lator (MCSS), shown in Figure 2, pro- This allows for repeatability in the limitations in its ability to measure
vides an accurate, precise, repeatable, peak-to-average ratio, ensuring that all the average power accurately across
and fast method for creating the noise of the devices under test receive the a fixed bandwidth. The Agilent 89440
stimulus required for the NPR meas- same stress from the source, and pro- series vector signal analyzers (VSAs)
urement. Using digital synthesis to vides repeatability from system to eliminate this measurement uncertainty
generate noise, the MCSS creates CW system. The AWS uses a DAC to con- by using band power markers to aver-
tones in the frequency spectrum at vert the signal from digital to analog. age the power in a fixed bandwidth.
baseband (0 to 50 MHz) with the arbi- The baseband signal is then upcoverted
trary waveform synthesizer (AWS). to the frequency of interest and sent Two possible configurations using the
These tones have controllable spacing into the DUT. By turning the tones MCSS as an NPR stimulus and the VSA
and amplitude, and the phase relation- "On" in areas simulating noise and as a measurement system are shown
ships between the tones are random. "Off" in notch areas, the MCSS func- in Figure 3. Figure 3a uses the down-
The random phase relationships result tions as a noise source and filter to converter supplied with the 89441A
in a largely varying signal amplitude in create the noise spectrum and a notch for frequencies up to 2.65 GHz with an
the time domain. Due to the repeating in the band of interest. By using a digi- IF measurement bandwidth of 10 MHz.
nature of the digitally generated pseu- tal stimulus, this system provides both Figure 3b replaces the downconverter
do-random noise, the peak-to-average flexibility and repeatability when cre- portion of the 89441A with the 71910A
ratio is predictable and repeatable in ating NPR signals. modular measurement system wide-
time. The random phase relationship bandwidth receiver.
is set by selecting a specific phase seed. The MCSS can be used with standard
The phase seed identifies a starting or custom upconverters for frequency
point in digital synthesis for generating coverage up to 26.5 GHz.
E2508A
MCSS
E2508A
MCSS Device
Controller under
test
Device
Controller under
test I
Vector
signal Wideband
analyzer receiver
Vector signal analyzer Q
71910A
89441A 89410A
Figure 3a. RF system Figure 3b. Microwave system
3
FRONT REAR
70001A Display
MSIB
89440A VSA 89440A
Ext Ref In
Ext
Source Trigger Ch 1 Ch 2 Ext Ref Out
GPIB Serial 2
Amp
70902A 70310A 7092A 70900B 70910A A B
IF Prec IF Section Local RF
Section Ref Osc Section GPIB 1
Device To
Out In Amp In
Cal RF In Under Computer
Test GPIB
Video FM MSIB
86794B Ext Data
86794B Upconverter 10 MHz RF Output 1
In In
RF Out LO In
Ext Data In
GPIO To Computer
8770A AWS
8770A ECL System
AWS GPIB Clock
Out
Ext Data In In 1 2
RF Out
10 MHz Ref Sampling Clks
Sys Start Out AC coupled clk/8
Input 1 2 In Input Out
8648C Signal Generator
Packet Advance Marker Outputs
Ready Trigger Scan Packet Seq Equa
Output Input Start Start Start Address
RF Out
8648C
GPIB 2
10 MHz 10 MHz To
Figure 4. System connections for a Ref In Ref Out GPIB Computer
microwave NPR measurement system
4
This increases the frequency capability 4. Generating an NPR stimulus MCSS. Figure 6 shows the default
to 26.5 GHz and the IF measurement The MCSS software makes it easy to screen for the NPR application. You
bandwidth to 20 MHz. By using the create an NPR waveform. Several set the center frequency for the NPR
71910A, we are able to generate the degrees of flexibility are built into spectrum and the initial power level
I and Q signals and send them to the the application. You have complete in the MCSS Control window. The
VSA separately. Using the second control of the five key parameters upper frequency limit is 2.0 GHz for
10 MHz input option and the I +jQ required for creating an NPR stimulus. the E2507B, and 2.5 GHz for the
feature on the VSA, the effective band- Center frequency, spectral bandwidth, E2508A. Once you generate an NPR
width is doubled. This combination of notch depth, notch width, and total signal, you can change its center fre-
receiver and analyzer is described in power level are all easily controlled quency and power level in real time,
more detail in Agilent product note from the Windows-like interface. that is, without generating a new NPR
89400-13.2 Table 1 is a list of equip- Figure 5 shows an example of an signal. The RF frequency and the
ment required to set up this configu- NPR stimulus and its key compo- power are set in the upconverter and
ration. The 71910A comprises a variety nents. MCSS allows you to control do not require a new digital waveform
of modular equipment and is shipped each of these parameters using the from the DAC.
as a unit. NPR application software within the
Table 1. Required Equipment
Model
Number Option Description
E2508A Multi-format
or communications
E2507B signal
simulator
89410A 1C2 Vector
AY7 signal
AY9 analyzer
AYA
AYB
AYH
UFG
UG7
71910A Wideband
receiver
Center Frequency
Spectral Bandwidth
Notch
Depth
Notch Width
Figure 5. Critical features of an NPR stimulus Figure 6. NPR application display
5
By using the Noise Power Ratio Spectral spacing is the space between Figure 8 shows the Notches window.
Stimulus window (also shown in the tones generated by the MCSS. This window, usually found minimized
Figure 6), you can specify all the spe- These tones, generated with the and below or behind the stimulus
cial signal parameters for the noise phase parameters set in the Phase window, allows control of up to ten
signal. These parameters include Distribution panel, create the noise notches. Maximize this window by
those mentioned at the beginning of stimulus for the NPR measurement. either clicking on it or using the win-
this section plus the spectral density, The default phase distribution is ran- dow's pull-down menu. You can set
the phase information, and any neces- dom. This most accurately represents width, depth, and offset from the cen-
sary amplitude shaping. Gaussian noise. (The options for phase ter frequency for each notch. The sys-
distribution are random, parabolic, tem has a 1 MHz notch at the center
You set the bandwidth and the spectral constant, and custom.) frequency as a default. Examples of
spacing of the tones for the noise uses for additional notches include
spectrum in the Noise Distribution The closer the tones are spaced, the looking at the IMD in special bands
panel of the Noise Power Ratio Stimulus more closely this stimulus will repre- and creating CW tones in the primary
window (Figure 7). You can choose sent Gaussian noise. The disadvan- notch for use as a calibration tool.
bandwidths from 10 kHz to 35 MHz for tage of smaller tone spaces is that it The on-line help menu shows how to
the noise spectrum. Only bandwidths will increase the time required to create multiple notches.
up to 25 MHz are recommended, how- generate a signal the first time.
ever, due to possible band-edge limi-
tations of the MCSS upconverter. For Table 2 shows typical times required Table 2. Time Requirements for Signal
a listing of the specific bands generat- to generate a 20 MHz spectrum with Generation
ed by the MCSS, please refer to the various spectral spacings.
technical specification for the MCSS.3 Spacing Time
(Hz) (seconds)
7,630 5
3,820 7
1,910 7
954 21
470 32
238 105
Figure 7. Noise Distribution panel Figure 8. Notches window
6
Clear definition of notch edges makes It is desirable to have the input sig- 5. Making the measurement
repeatable and accurate measure- nal's NPR at least 10 dB lower than After signal generation, perform the
ments inside the notch possible. the NPR of the device under test. This measurement analysis with the receiver
Unlike the analog method, which can ensures that the effect of the input and analyzer listed in Section 3 of
have rounded notch edges as a result signal is negligible when measuring this document. The most important
of filter shaping, the digital techniques the DUT's NPR.4 advantage of using the Agilent 89410A
used in the MCSS create the notch by vector signal analyzer is the band
turning off the tones in the notch area. A consideration for setting notch power marker feature, which averages
This creates the "brick wall" effect width is the number of IMD products the power over a specific bandwidth.
seen in the NPR stimulus. Figure 10 present in the notch. IMD products, This improvement over a typical spec-
shows the sharpness of the notch wall or tones, in the notch are a result of trum analyzer measurement removes
created by the MCSS. The maximum intermodulation distortion from the the guesswork associated with a single
achievable notch depths vary depending spectrum being generated outside marker and its relative placement.
on the width of the noise spectrum and of the notch. As the notch width
the width of the notch. For a 30 MHz decreases, the number of IMD tones The graph shown in Figure 9 shows
spectral bandwidth with a 1 MHz in the notch decreases to a point NPR vs average input power. This data,
notch, typical notch depth is 38 dB. where there are too few IMD products taken with the system setup shown in
With narrower spectrums, you can in the notch to accurately calculate Figure 3b (page 3), shows the relation-
obtain notch depths as great as 50 dB. the average power. If narrow notch ship between NPR and input power.
This depth creates a baseline for the widths are required, compensate by The signal was set up with a 30 MHz
measurement. Figure 9 shows the decreasing the spacing of the generat- bandwidth and a 1 MHz notch. The
baseline of the MCSS for the condi- ed tones and increasing the width of band power marker feature, with a
tions given by flattening out the NPR the notch that is used to calculate the spacing of 300 kHz, was used to cap-
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