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Keysight Technologies
Using Fast-Sweep Techniques
to Accelerate Spur Searches
Application Note
Introduction
Measurement speed is a major issue for a wide variety of RF and microwave products,
and this influences production costs in industries ranging from commercial wireless to
aerospace and defense. As a result, manufacturers in these areas are looking for ways to
shorten design cycles, reduce manufacturing costs and increase yield.
One important opportunity for improvement is the search for spurious emissions. These
tests can be especially difficult and time-consuming because measurements must be
made over wide frequency ranges and with high sensitivity. Unlike measurements of har-
monics, the locations of spurious signals are not accurately known beforehand and there
is often no choice but to sweep across wide frequency spans using narrow resolution
bandwidths. In addition, the amplitudes of spurious signals are often near the measured
noise floor, creating challenges for measurement accuracy and repeatability.
The most commonly used tool for such measurements is an RF/microwave spectrum
or signal analyzer. In recent years, the technological evolution of these analyzers has
enabled faster spurious measurements. For example, high-speed analog-to-digital con-
version and digital signal processing (DSP) has supplanted analog technology with digital
intermediate frequency (IF) sections and therefore digital resolution bandwidth (RBW)
filters.
Digital filtering can provide some special benefits for narrow-RBW, wide-span operations
such as spurious measurements. For example, existing digital filters have a better shape
factor and can maintain full accuracy while being swept several times faster than equiva-
lent analog filters. This technique is called oversweep.
The latest advances in signal processing provide remarkable improvements in sweep
speeds by implementing a new type of digital RBW filter in Keysight Technologies, Inc.
PXA, MXA and EXA X-Series signal analyzers.1 This new filter allows sweep speeds
to be up to 50 times faster without compromising accuracy in terms of amplitude and
frequency. This application note focuses on this new technology and its use for spurious
measurements.
1. The fast-sweep filtering technique is
available in all PXA models and in MXA
and EXA models configured with options
MPB, DP2 or B40.
Reviewing current RF performance requirements are steadily increasing, driven by demand for greater
problems and practices communications throughput and wider bandwidths within the realities of a crowded
spectral environment that places tight limits on undesirable emissions. The densely
in spur measurements populated shared spectrum increases the likelihood that excessive spurious and
harmonic signals will be both troublesome and noticed.
One way to understand the optimization of RF testing across the life cycle of a
typical product is to separate the process into three stages: R&D, design validation
and manufacturing. In R&D, RF testing is typically done manually on one to several
prototypes and speed is rarely a major consideration.
In design validation, a pilot run of tens to hundreds of units may be produced.
Characterization of these units includes spur measurements (frequency and ampli-
tude) using a narrow RBW and autocoupled sweep-time rules over a span of inter-
est, often from the fundamental to the tenth harmonic. This wide frequency survey
provides accurate information about all spurs generated by the product.
With current practice, this "spur characterization" process is performed in a single
pass over the frequency range of interest and the use of standard sweep rates
ensures that the measurements are accurate per the specifications of the spectrum
analyzer. However, the use of these slower sweep rates results in measurement
times that are not acceptable when the product is transferred to manufacturing.
In an efficient manufacturing process, thousands to millions of products are built
and tested as quickly as possible across a wide span versus a pass/fail limit deter-
mined by a spurious-free dynamic range (SFDR) specification that is often derived
from a published standard. The limits are sometimes described in terms of a spectral
mask or spectrum emission mask (SEM). Highly stringent SFDR specifications
demand clear separation of spurious signals from noise: this requires the use of
narrower RBWs, which result in slower tests. Because sweep rates for RBW filters
generally vary with the square of the RBW, finer resolution results in dramatically
slower sweep rates and longer measurement times.
3
Accelerating spurious In the last 20 years, spectrum analyzers have made increasing use of digital
measurements technologies, especially DSP, to improve measurement speed and performance. In
addition to the digital IF filters and oversweep mentioned previously, some have
implemented techniques that subtract noise power, thereby reducing the effect of
broadband noise on measurements of low-level signals. This subtraction can be
performed manually or automatically as in the "noise floor extension" (NFE) feature
of the Keysight PXA X-Series signal analyzer.1
These signal-processing technologies can be used to enhance high-performance
and midrange signal analyzers without requiring expensive improvements in analog
circuits. With either class of analyzer, the advanced technology expands the perfor-
mance envelope, offering new levels of productivity through the optimization of the
tradeoffs between measurement speed, accuracy, repeatability, and so on.
The latest advances in signal processing provide considerable improvements in
sweep speeds by implementing a new type of digital RBW filter. The new filter allows
sweep speeds to be up to 50 times faster without compromising accuracy in terms of
amplitude and frequency with the new filter (see sweep times at lower-right beneath
each trace).
Figure 1a. On the left, sweeping the full 26.5-GHz frequency range of a spectrum analyzer using a 30-kHz RBW filter requires 35.49
seconds when using existing digital filter technology.
Figure 1b. On the right, the same sweep performed with new digital filter technology takes 717 ms, a speed improvement of more than
49 times.
Applications such as spur searches that require sweeps over wide spans using
resolution bandwidths in the range of several to hundreds of kilohertz will benefit
greatly from this new technology. Although spurious measurements are perhaps the
most likely to benefit from this speed advantage, any measurements that require
wide frequency spans and narrow RBWs--whether to reduce noise floor or resolve
closely-spaced signals--will see substantial speed improvements.
1. See application note Using Noise Floor
Extension in the Keysight PXA Signal
Analyzer, Keysight literature number
5990-5340EN
4
Using oversweep and a One way to accelerate spurious testing in existing analyzers that have digital
two-pass approach: A filters--but not the fast-sweep technology--is to utilize the speed of a large amount
of oversweeping and compensate for the resulting errors by measuring the desired
partial solution frequency range in two passes, using different measurement configurations and test
limits. Even with two passes, the total measurement time is significantly shorter
than when sweeping at a normal autocoupled rate that provides accurate results in
one pass.
First, the frequency range in question is measured using oversweeping and a wide
RBW. In these measurements, the oversweeping involves a sweep rate several
times faster than the autocoupled value over wide frequency spans. This produces
at least three types of errors:
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