Measuring Noise and Noise-Like Digital Communications Signals with Spectrum and Signal Analyzers
Noise. It is the classical limitation of electronics. In measurements, noise and distortion limit the dynamic range of test results.
In this four-part paper, the characteristics of noise and its direct measurement are discussed in Part I. Part II contains a discussion of the measurement of noise-like signals exemplified by digital CDMA and TDMA signals. Part III discusses using averaging techniques to reduce noise. Part IV is about compensating for the noise in instrumentation while measuring CW (sinusoidal) and noise-like signals.
Table of contents
Part I: Noise Measurements
Part II: Measurements of Noise-like Signals
Part III: Averaging and the Noisiness of Noise Measurements
Part IV: Compensation for Instrumentation Noise
Burst carriers, such as those used in TDMA mobile stations, are measured differently than continuous carriers. The power of the transmitter during the time it is on is called the "carrier power."
Carrier power is measured with the spectrum analyzer in zero span. In this mode, the LO of the analyzer does not sweep, thus the span swept is zero. The display then shows amplitude normally on the y axis, and time on the x-axis. If we set the RBW large compared to the bandwidth of the burst signal, then all of the display points include all of the power in the channel. The carrier power is computed simply by averaging the power of all the display points that represent the times when the burst is on. Depending on the modulation type, this is often considered to be any point within 20 dB of the highest registered amplitude. (A trigger and gated spectrum analysis may be used if the carrier power is to be measured over a specified portion of a burst-RF signal.)
Using a wide RBW for the carrier-power measurement means that the signal will not have noise-like statistics. It will not have CW-like statistics, either, so it is still wise to set the VBW as wide as possible. But let’s consider some examples to see if the sample-mode bandwidths of spectrum analyzers are a problem.
A zero-span measurement of carrier power is made with a wide RBW, so the independence of data points is determined by the symbol rate of the digital modulation. Data points spaced by a time greater than the symbol rate will be almost completely independent.
Zero span is sometimes used for other noise and noise-like measurements where the noise bandwidth is much greater than the RBW, such as in the measurement of power spectral density. For example, some companies specify IS-95 CDMA ACPR measurements that are spot-frequency power spectral density specifications; zero span can be used to speed this kind of measurement
Noise Floor Extension (NFE) for noise compensation
A very convenient form of noise compensation is now built-in to some advanced spectrum analyzers: NFE. It goes beyond the power subtraction we just discussed.
NFE creates a model of the noise of the spectrum analyzer as a function of its state. This model works for all RBWs, all attenuation settings, all paths (such as preamp on and off), even all detectors (such as peak and average), and VBW settings. The user does not need to characterize his analyzer before turning NFE on, because the models are already in the analyzer.
NFE is most effective with noise-like signals, but it also works on CW and impulsive signals. It greatly reduces the errors caused by noise, replacing them with a smaller uncertainty. See the reference “Using Noise Floor Extension in the PXA Signal Analyzer” for more information on this topic.
NFE can often allow the user to see signals below the theoretical noise floor of a room-temperature termination impedance. See the reference for discussion on that topic, too.
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