RF Spectrum Analyzers and its applications

Modern Spectrum analyzers are complex electronic devices used in various applications to measure and analyze characteristics of RF signals. Here we will see what are spectrum analyzers, block diagram, specifications, and applications of spectrum analyzers.

RF Spectrum Analyzers are electronic devices primarily used for measuring the RF (radio frequency) spectrum and the communications signals emitted by various types of electronic devices. This is usually done by using a wideband receiver and a calibrated transmitter. The RF spectrum analyzer is able to measure the frequency of the radio waves and their amplitude or power.

These tools can be used for regulatory purposes, from managing radio interference to identifying radio frequency hazards. With new technological advances, their usage is becoming more widespread. The spectrum analyzer displays the frequency of the signal on the X-axis and the Power (amplitude) of the signal on the Y-axis. Modern Signal and Spectrum analyzers are capable of analyzing complex signal behavior and signal patterns in various modulation schemes.

Block Diagram of Spectrum Analyzers

RF input attenuator

RF attenuators are the primary stage of every spectrum analyzer. It ensures the input power does not exceed certain levels which is safe for later stages. It also makes sure the power level will not cause any distortion at the mixer and no overload to the instrument. In modern Spectrum analyzers, the attenuation level is set automatically (manual selection is also possible), and it protects the instruments from high-power signal inputs.

Low Pass Filter (Pre-selector)

Low Pass Filter is the second stage of the Spectrum Analyzer after the input attenuator. As the name implies, this unit blocks the high-frequency components of the input signal. This is significant in the signal mixer to avoid harmonics generation.

Mixer Stages – Tuning

Tuning of the spectrum analyzer is a complex process with multiple frequencies mixed to get desired intermediate frequency.

A sweep generator is a component in various electronic equipment that produces a periodic sawtooth waveform designed to modify the output of a second signal generator, usually a radio frequency (RF) generator. A periodic sawtooth wave rises to a peak level, then quickly falls to a low level and repeats the process. You can use the swept generator’s output to control the signal generator’s frequency output to produce a swept frequency output. Sweep generators generally allow test setups to measure the response of devices within a frequency span or range almost simultaneously.

Tuning is a function of the IF filter center frequency, the LO frequency range, and the frequency range that the mixer can reach from the outside world (allowed) through a low-pass filter) of all the mixed products exiting the mixer. These two have the highest amplitude and are, therefore, the most desirable those generated from the sum of the LO and input signals and the difference of the LO and input signal.

If the signals we want to study can be rearranged to be either above or below the LO frequency through the IF, only one of the desired mixing products is produced. It falls within the passband of the IF filter and is detected to generate an amplitude response of the spectrum analyzer screen.

IF Gain

A variable gain amplifier is used to adjust the vertical position of the signal on the display. It does not affect the signal level of the input mixer. By changing the IF gain, the reference level value is changed accordingly to maintain the correct display value for displayed signals. In general, you don’t want to change the reference level while Changing the input attenuation; that is, the input attenuation and gain settings are related.

Changing the input attenuation automatically changes the IF gain to compensate for the effects of changing the input attenuation and keep the signal at a constant position on the display.

Logarithmic Amplifier

The most common way to view a signal on a spectrum analyzer is by using a logarithmic amplitude scale. Example: 10 dB per div or 1 dB per div. Therefore, the IF signal usually goes through a logarithmic amplifier. A logarithmic amplifier approximates the gain characteristics to a logarithmic curve. Deviations from the perfect logarithmic response, therefore, contribute to the amplitude uncertainty. Similarly, when the spectrum analyzer is in linear mode, the linear amplifier’s response is not perfectly linear.

Envelope Detector: A circuit element whose output follows the envelope but does not follow instantaneous changes in the input signal. In a superheterodyne spectrum analyzer, the input to the envelope detector comes from the final IF, and the output is the video signal. With the analyzer set to zero span, the envelope detector demodulates the input signal, and the modulated signal can be viewed on the display as a function of time.

Video Filter

A post-detection, lowpass filter determines the bandwidth of the video amplifier. It is used to average or smooth traces for display.

Specification of Spectrum Analyzers

Modern spectrum analyzers are used in various applications and the specifications vary based on the applications as well.

Frequency

The frequency range of a spectrum analyzer is usually given as the range of frequencies it can measure in Hertz (Hz). In modern spectrum analyzers, measurable frequency range from 2Hz up to 110GHz is available.

Power

Power rating is yet another significant specification of spectrum analyzers. Standard analyzers have a maximum input power of 26 to 30 dBm. This limitation can be overcome with additional attenuators at the input port of spectrum analyzers for high-power measurements.

Digital Signal Processing Capabilities

In wireless communications and other applications, the complexity of the signals is increasing with the introduction of new modulation schemes and multiplexing methods. Modern spectrum analyzers are capable of a higher level of signal processing with the help of computing power (either using an inbuilt PC or using a PC equipped with software to process the signal).

For example, 5G signals have many complex characteristics from previous generations. With sophisticated software, complex 5G signals can be analyzed in modern spectrum analyzers.

Connector Types

The connector type of spectrum analyzer is determined by the range of frequency in which it operates. Low-frequency spectrum analyzers will have N-type RF connectors (with SMA adapters below 18GHz) or high-frequency RF connectors for millimeter-range applications.

Form Factor

Spectrum analyzers are available in various sizes and shapes. The most common types are the tabletop models used in laboratory applications. Handheld spectrum analyzers are used in the characterization of base stations and range mapping etc.

Applications of Spectrum Analyzers

• RF spectrum analyzers are electronic devices that allow us to see RF (radio frequency) signals in their environments and measure the frequency, power, and quality of those signals. They are widely used in radio engineering, radar systems, wireless communications, and scientific applications.
• RF Spectrum Analyzers are also used to analyze how radio waves interact with objects, as well as to measure the efficiency of a receiver. The device uses an antenna to capture the radio waves and then processes them using a digital signal processor (DSP) to display the spectrum on a computer screen.
• Spectrum Analyzer can be used in hospitals or airports to detect different frequencies of electromagnetic waves that patients or passengers may be exposed to. It is also used by law enforcement to detect signals that might be related to illegal activities.

RF Test Laboratories

RF Spectrum Analyzers are a powerful tool for engineers, scientists, and anyone who needs to measure the performance of a radio system. The spectrum analyzer measures the strength of a radio signal, including noise, along with its frequency and phase. When you are trying to determine the cause of a problem, such as interference or poor performance, an RF Spectrum Analyzer can help you pinpoint the source of the issue.

These devices can be used for troubleshooting, analysis of signal quality, generation of a reference frequency for calibrating a signal generator, and determining the number of channels in a system. Spectrum Analyzers are commonly used to test the performance of wireless systems, such as Wi-Fi networks. They also help determine the interference and noise level of an RF signal, which can be problematic in some RF applications.

Radar System Characterization – Ensuring Accurate Performance Testing.

Radar systems can be very complex and require careful design, testing, and verification. Spectrum analyzers provide an important function in ensuring that a radar system is performing according to its specifications. By comparing signals captured in the operational waveform to expected or ideal responses, technicians can quickly identify issues and observe how changes in configuration impact the results. Consequently, systematic analysis of waveforms using spectrum analyzers can shorten product development times, improve the accuracy of measuring systems, and lead to more efficient operations all around.

EMC Compliance Measurement – Monitor Signals to Detect Interference Sources

Spectrum analyzers also allow engineers to look for signals which indicate interference is present in the environment. This type of measurement is important for EMC compliance, as it’s not always obvious what will trigger a malfunction in an electronic system. It could be as small as an unexpected frequency working its way into the system or a single harmonic wavelength that triggers an issue. Through spectrum analysis, it’s possible to detect and analyze those rogue frequencies with greater accuracy.

Cellular Network Analysis – Finding Optimal Network Performance Parameters

As mobile networks and cellular technologies grow, so does the importance of validating network parameters. With a spectrum analyzer, engineers can take measurements of each sector or base station to make sure that the frequencies are within regulations and that there is no interference between neighboring networks or systems. By measuring noise floor, transmitter power levels, channel bandwidths, and more; engineers can fine-tune the metrics for optimal performance of the entire system.

Reference: Keysight