Spectrum Analyzer and Noise Figure Principle

Spectrum Analyzer and Noise Figure Principle

The ubiquitous noise is the enemy of RF and microwave designers and should not be surprised. Noise limits the ability of the communications receiver to detect weak signals, thereby preventing designers from achieving optimal receiver performance. The noise in the transmitted signal deteriorates performance not only for the transmitted signal but also for the surrounding spectrum. Since noise is ubiquitous, years ago, the RF and microwave industry established a measurement parameter called the noise figure to quantify how much noise a component or system adds to the signal passing through it.

Although the noise figure is a parameter used to describe the radio and microwave system noise and receiver sensitivity, it is also the most important and widely used parameter. For each measurement and measurement using different instruments, the noise figure measurement always requires high accuracy and repeatability. Accuracy and repeatability ensure that component and subsystem manufacturers and their customers perform consistent performance measurements.

Noise figure basis

The noise figure as a measurement parameter was used as early as the 1400s. Engineer Harold Friis defined it as the signal-to-noise ratio (SNR) at the input of the RF or microwave device expressed in decibels (dB) divided by the SNR at the output. . From its name, SNR is the ratio of signal level to noise level in a given transmission environment. The higher the SNR, the more signals exceed the noise, making the signal easier to detect. Therefore, the lower the noise figure, the better, because in an ideal case, microwave components, subsystems, or systems should have no noise applied to the passed signal. In reality, all electronic devices will add some noise, and the lowest noise is the best device. These devices have the lowest noise figure.

How important is the noise figure?

No matter how the noise figure is estimated, the importance of the overall system performance and cost will not be too high. For example, reducing the noise figure of a direct satellite by half, that is, from 2 dB to 1 dB, has the same effect as increasing the power of the satellite transponder by 25%. Obviously, manufacturers will find that the cost of increasing the space transmitter power is much higher than the improved ground station receiver low noise amplifier (LNA) performance.

In a satellite receiver production line, the noise figure can be reduced by 1 dB by simply adjusting the impedance level or selecting the appropriate transistor. The 1dB noise figure reduction has the same effect as increasing the antenna 25% of the area. Increasing the size of the antenna also increases the cost and increases the size and weight of the steering and support mechanisms. For applications such as DBS with aesthetic considerations, such antennas are too large.

In a wireless communication system, a base station with a low noise figure can reduce the mobile station transmit power with which it communicates, which has a positive effect on battery life, size, and weight.

Noise is also very important in transmitter design. For example, excessive noise in a wireless base station linear power amplifier can degrade the adjacent channel reception quality, that is, fail to meet regulatory requirements for interference.

Perform noise figure measurements

There are several techniques and instruments that can be used to measure noise figure, from dedicated noise figure analyzers to spectrum analyzers, network analyzers, and true rms power meters. As expected, a dedicated noise figure analyzer provides the lowest measurement uncertainty, followed by a spectrum analyzer (if equipped with a preamplifier).

The Agilent ESA-E Series Economy Spectrum Analyzers feature an optional integrated preamplifier (Option 1DS) that provides noise figure measurements from 10MHz to 1.5GHz or 3GHz depending on the analyzer's frequency range. The Agilent ESA-E Series spectrum analyzer complements the PSA Series High Performance Spectrum Analyzer and the Agilent NFA Series Noise Figure Analyzer. If your application requires only moderate performance spectral analysis tools, it is the most affordable solution. In the past, using a spectrum analyzer to measure the noise figure required many steps and several mathematical calculations, which was a complicated and error-prone process. The new ESA-E series noise figure measurement application now automates the entire process, including calculations. This is a very accurate and easy to use solution. The new measurement application is an integrated part of the spectrum analyzer's rich universal capability environment, including single-key power measurements, and phase and modulation analysis linked to the 89601A VSA software.

For higher spectrum analysis capabilities and excellent instrument uncertainty, the user can select the PSA Series spectrum analyzer. PSA has all the features you would expect from a high-performance spectrum analyzer, as well as a noise figure measurement application with the same user interface as the ESA-E series. As a result, customers can seamlessly move from one instrument to the next without worrying about familiarizing themselves with the nuances of the instrument.

Users of the ESA-E Series and PSA Series spectrum analyzers may consider that they no longer need a dedicated noise figure analyzer. However, all three instruments have their own adaptive environment.

The spectrum analyzer is the most commonly used and most versatile measurement tool in the designer's hands and can be found on almost every test bench. For example, the spurious signal can be located first, and then the noise figure of the device at the measurement frequency without interference noise can be measured. In this way, the ESA-E series with a noise figure measurement application becomes an ideal solution for designers who want to obtain numerous measurement capabilities at an economical price. This is the most flexible spectrum analyzer in the industry. It has a card box structure that fully meets the requirements for custom capabilities. The PSA Series is an excellent combination of flexibility, speed, accuracy, and dynamic range, providing state-of-the-art spectrum analysis capabilities. The noise figure analyzer is an application-specific instrument that measures noise figure, gain, and correlation quantities only. Compared to spectrum analyzers and other instruments, noise figure analyzers are faster, easier to use, more accurate, and have a wider frequency range. It is therefore the highest-end choice to get the best possible uncertainty, especially for frequencies above 3 GHz. The fastest and most accurate instrument that gives the full performance of 26.5 GHz is the Agilent NFA Series Noise Figure Analyzer.