In this note, we are going to learn a topic called “What is a Spectrum Analyzer?“. Also about its specifications, block diagram, working principle, advantages, and applications. Welcome to Poly Notes Hub, a leading destination for engineering notes.
Author Name: Arun Paul.
What is a Spectrum Analyzer?
A Spectrum Analyzer is an electronic tool that measures and analyzes the frequency spectrum of electrical signals. It graphs signal amplitude (y-axis) versus frequency (x-axis), allowing engineers and academics to investigate signal characteristics such as bandwidth, frequency components, harmonics, noise, and distortion.
A Spectrum Analyzer is a valuable tool for evaluating and measuring signal frequencies in a variety of industries, including telecommunications, electronics, and aerospace. Its capacity to detect and show frequency components is critical for signal quality, troubleshooting, and research.
Specification of Spectrum Analyzer
- The range of frequency of this device is from 9 kHz to 50 GHz.
- The input impedance is typically 50Ω or 75Ω, matching most RF systems.
- This devices takes to time to scan through the selected frequency range.
- The display provides graphical representation of amplitude vs. frequency.
- Filters noise in the displayed signal.
Spectrum Analyzer Block Diagram
Here is the block diagram of spectrum analyzer –
- RF Input Attenuator: The RF Input Attenuator is the spectrum analyzer’s first step, which adjusts the strength of the input signal before processing. When dealing with high-power signals, attenuation is essential to avoid the mixer and other internal components from becoming overloaded. It guarantees that the input signal remains within the analyzer’s dynamic range, reducing distortion and measurement mistakes. The attenuator is typically made up of resistive elements that can be adjusted manually or automatically based on signal strength.
- Mixer: The Mixer is a critical component that converts the incoming RF signal to an intermediate frequency (IF) for subsequent processing. It accomplishes this by combining the incoming signal with the signal produced by the Local Oscillator (LO). This process, known as heterodyning, changes the frequency of the input signal to a set IF, making it easier to analyze. The mixer produces sum and difference frequencies, and the desired IF component is chosen for further filtering. The mixer’s performance determines the spectrum analyzer’s accuracy and sensitivity.
- Local Oscillator or LO: The Local Oscillator (LO) produces an adjustable frequency, which is mixed with the input signal in the mixer. The frequency of the LO is changed over the necessary range, allowing the analyzer to scan various frequency components of the input signal. Accurate frequency measurements require a robust and precise local oscillator. Modern spectrum analyzers include phase-locked loop (PLL) oscillators to achieve high frequency stability and minimal phase noise, which is critical for resolving weak signals near strong ones.
- Intermediate Frequency (IF) Filter: After mixing, the signal passes via an Intermediate Frequency (IF) Filter, which determines the spectrum analyzer’s Resolution Bandwidth (RBW). The RBW filter isolates a small piece of the IF signal for analysis, allowing the spectrum analyzer to discern between closely spaced frequencies. A smaller RBW improves frequency resolution but slows the measurement procedure. Modern analyzers use digital signal processing (DSP) filters to improve performance and provide customizable RBW settings for various applications.
- Detector & Envelope Detector: The Detector extracts the amplitude information from the filtered IF signal. In most cases, an Envelope Detector is used to track the peak amplitude of the signal over time. The detected signal is then converted to a DC voltage proportional to the signal’s amplitude. There are different detection modes available in spectrum analyzers, such as peak detection, average detection, and sample detection, depending on the nature of the signal being analyzed.
- Sweep Generation: The Sweep Generator adjusts the frequency range under analysis by tuning the Local Oscillator (LO) throughout the appropriate spectrum. It guarantees that the spectrum analyzer scans a defined range of frequencies in a systematic manner. The sweep speed must be carefully adjusted; if it is too rapid, signals may be missed, while if it is too sluggish, measurements may take longer. Advanced spectrum analyzers use real-time processing to better capture transient signals.
- Display Unit: The Display Unit is the final stage, where the processed signal is displayed graphically as Amplitude vs. Frequency on a screen. The X-axis denotes frequency, while the Y-axis represents signal amplitude, which is commonly expressed in decibels (dB). Modern spectrum analyzers have LCD or OLED screens with digital processing, which allows for extra capabilities like markers, zoom functions, and automated peak recognition to improve signal analysis. Some high-end models additionally offer real-time 3D spectrum visualization to monitor dynamic signals.
Spectrum Analyzer Working Principle
The operation of a spectrum analyzer begins with the input RF signal being supplied to the analyzer via a coaxial connector. If necessary, the signal is attenuated before being blended with a local oscillator (LO) signal to generate an intermediate frequency (IF). This IF signal is then processed through a resolution bandwidth (RBW) filter, which aids in differentiating closely spaced frequency components.
After filtering, the signal is detected, transformed to a logarithmic scale, and amplified. Finally, the processed signal is shown as a frequency spectrum, with peaks representing the various frequency components found in the original signal.
Application of Spectrum Analyzer
Here are the list of applications of spectrum analyzer –
- Used for testing antennas, transmitters, and receivers.
- Helps in signal modulation analysis in telecom networks.
- Useful for debugging and designing electronic circuits.
- Identifies unwanted electromagnetic interference in devices.
- Used in ultrasound and medical signal analysis.