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Using Noise Floor Extension in an X-Series Signal Analyzer

Application Notes

Introduction

Keysight’s Noise Floor Extension technology can provide up to 12 dB improvement in analyzer noise floor, revealing some previously hidden signals and allowing others to be more accurately measured.

Overview

Dynamic range is a core measure of spectrum analyzer performance and can significantly affect other core measures such as accuracy and measurement speed. Spectrum analyzers have many different measures of dynamic range, and most of these include noise, specifically the analyzer’s own internally generated noise. Reducing the effective analyzer noise floor, therefore, improves dynamic range and the quality of many measurements.

Most spectrum analyzer users are aware that any measurements made near an analyzer’s displayed noise level (within about 20 dB) will be affected by noise. The analyzer’s noise adds to the apparent power of the signal to be measured, producing a result that is somewhat higher than the true figure (on average).

The analyzer’s noise also often increases the variance or “noisiness” of the result. Some users may consider the errors from the analyzer’s noise contribution to be negligible unless they are measuring to within 5 to 10 dB of the analyzer noise floor, but the high accuracy of today’s spectrum analyzers can make the added-noise error significant even for measurements with better signal/noise ratios.

The typical approach to handle these problems is to reduce the RBW of the analyzer to reduce its noise contribution, and to consider some type of averaging such as VBW reduction, trace averaging, or the use of an average detector to reduce measurement variance. RBW reduction is effective but can slow the measurement significantly and does not solve the problem if the goal is to measure the signal’s noise level. This is because reducing RBW will reduce the apparent noise level of the signal and the analyzer DANL together.

Reducing attenuation can improve the SNR of the measurement, but attenuation at or near 0 dB can result in a poorer source match (the input signal is connected more directly to the analyzer’s first mixer, not a perfect 50Ω device) and reduced amplitude accuracy. It can also endanger the analyzer’s input mixer due to the possibility of damage from excess signal power, including transients.

Adding a preamp can improve SNR but may increase distortion products from the analyzer if large signals are present along with the small signal(s) to be measured. Distortion products from the preamp can be difficult to separate from those of the signal under test.

While lowering an analyzer’s inherent noise floor through hardware design and component choices is obviously beneficial for dynamic range, there are practical limits, and another approach offers significant improvement. With sufficient processing and other technical innovations, the noise power in a signal analyzer can be modeled and subtracted from measurement results to reduce the effective noise level. In the Keysight’s X-Series Signal Analyzers1 operation is called Noise Floor Extension (NFE). The technique, its benefits, and practical use considerations are described in this application note.

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