Technical Insights > RF + Microwave

IS YOUR SIGNAL ANALYZER UP TO TASK? PART 2: Three Advanced Noise Measurements to Expect from Your Modern Signal Analyzer

2020-12-17  |  7 min read 

Last week, in the first part of this blog, we discussed three top measurements to characterize the power of your device. Hopefully that got you excited to follow the second part, to discuss how to take advantage of your signal analyzer to characterize the noise. The main three noise-relevant measurements we will discuss here are: Noise figure,Pphase noise, and Real-time spectrum analysis.

   1. Noise Figure (NF):

NF measures the degradation of the signal-to-noise ratio, caused by components in a signal chain. It is a number by which the performance of an amplifier or a radio receiver can be specified, with lower values indicating better performance. RF designers are often concerned with the NF of their devices, as noise figure directly affects the sensitivity of receivers and other systems. Some signal analyzers, such as the X-Series, have optional NF measurement capabilities available. This option provides control for the noise source needed to drive the input of the device under test as well as firmware to automate the measurement process and display the results. Figure 1 shows a typical measurement result, with DUT noise figure (upper trace) and gain (lower trace) displayed as a function of frequency.

Figure 1. Noise figure measurement

   2. Phase Noise Measurement:

Phase noise is the frequency-domain representation of random fluctuations in the phase of a waveform, corresponding to time-domain deviations from perfect periodicity. It can adversely impact the Bit Error Rate (BER). Also, in Doppler radar systems, it can decrease the ability to receive the return pulses from stellar objects of interest. The phase noise measurement feature in X-series signal analyzers can track the phase noise as a function of frequency (Figure 2). Recent modulation techniques often include both phase and amplitude information resulting in a more efficient data transfer. Quadrature Phase-Shift Keying (QPSK), is a digital modulation technique, in which two bits are modulated at once; this allows the signal to carry twice as much information using the same bandwidth. This two bits per symbol performance is possible because the carrier variations are not limited to two states. Instead of being limited together amplitude option A or B, in QPSK, the carrier varies in terms of phase and not frequency, resulting in four possible phase shifts. QPSK is used in various applications in modern digital communication systems and provides a high performance on bandwidth efficiency and bit error rate. Some of the main applications for QPSK modulation schemes include wireless, mobile, and satellite communications.

Figure 2. Phase noise measurement

​   3. Real-Time Spectrum Analysis (RTSA) for Agile Signals:

As signals continue to become increasingly complex and agile, gap-free measurement techniques such as real-time spectrum analysis and time domain analysis become more important. While non-real time spectrum analyzers take a series of samples to process and display, the real time spectrum analysis provides the capability to continuously monitor the spectrum. This ensures that transient effects, which may not be visible on other forms of spectrum analyzers, are captured and highlighted, as it takes successive overlapping FFT samples. Design and troubleshooting tasks are much more difficult when dealing with agile signals, and design challenges often grow in complexity when these signals are in an environment of other agile signals. To keep up with evolving analysis needs, new types of signal analyzers and application software have emerged in recent years. High-performance X-Series signal analyzers with real-time spectrum analysis capability can capture the behavior of dynamic and elusive signals with true gap-free spectrum analysis.

Figure 3 shows the real-time spectrum analyzer, which in contrast with the swept spectrum screen, readily shows the main characteristics of the signal using the density or histogram display. The density or histogram display collects a large amount of real-time spectrum data into a single display that shows both rare and frequent events, with an indication of relative frequency of occurrence. The X-Series signal analyzer RTSA mode and density display provide a fast and insightful representation of this wideband, dynamic and agile signal. The blue color of all but the noise floor indicates that the pulses, while prominent, have a very low frequency-of-occurrence. This is the principal characteristic that makes it difficult to measure this signal with only a swept spectrum analyzer.

Figure 3. Real-time spectrum analysis

   Summary:

Modern signal analyzers are continuously improving with capabilities and features to enable more specific measurements and tasks, which are not supported by basic generation of analyzers. These advanced features provide application-specific measurements, such as ACP, NF, and phase noise. They provide real time gap-free capability to effectively deal with interference and digital modulation analysis measurements defined by industry or regulatory standards, such as 5G, LTE, GSM, cdma2000®, 802.11, or Bluetooth®. If you own a Keysight Signal analyzer access our 30-Day Free software Trial program and gain immediate insight into your signal.