--- title: "Working principle of analog spectrum analyzer" subject: "Communication Systems" chapter: 1 key: "semester_6/communication_systems:analog-spectrum-analyzer" source: https://www.bctengineer.com/q/semester_6/communication_systems/analog-spectrum-analyzer site: BCT Engineering --- # Working principle of analog spectrum analyzer Communication Systems · Chapter 1: Introduction - Asked in: 2065 Kartik (CS I), 8 marks - Video: [Understanding Basic Spectrum Analyzer Operation](https://www.youtube.com/watch?v=P5gxNGckjLc) — Rohde & Schwarz - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/communication_systems/analog-spectrum-analyzer - Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1 ## Question Explain the working principle of Analog spectrum analyzer with the help of block diagrams. ## Short answer An analog spectrum analyzer uses a swept-tuned superheterodyne principle: a sawtooth ramp sweeps a VCO and horizontal display. The input mixes with the VCO; components at f_in = f_LO - f_IF pass the narrow IF filter (RBW). The IF signal is log-amplified, envelope-detected, video-filtered, and displayed vertically. Narrower RBW improves resolution but requires slower sweep. Filter-bank analyzers use parallel fixed filters for real-time display. ## Answer An **analog spectrum analyzer** displays the magnitude of the frequency components of an input signal versus frequency, i.e. it is a frequency-domain oscilloscope. Two analog forms exist: the **filter-bank (real-time) analyzer** and the **swept-tuned superheterodyne analyzer**, which is the common one. #### Swept superheterodyne spectrum analyzer ```text RF in | [Attenuator]->[LPF]->( X )->[IF BPF]->[Log IF amp] Mixer (RBW) | ^ [Envelope det] | | [VCO] [Video filter] ^ | | v [Sawtooth gen]---------+-----> X CRT <- Y ``` **Working principle:** 1. **Input attenuator and LPF:** the attenuator keeps the mixer within its linear range; the low-pass filter removes image frequencies above the analyzer's range. 2. **Sweep generator:** a sawtooth (ramp) voltage is produced. It does two jobs: tunes the **VCO** linearly across a frequency range, and drives the **horizontal (X) deflection** of the CRT. Hence the horizontal position of the spot always corresponds to the frequency being analyzed. 3. **Mixer:** multiplies the input with the VCO output, producing sum and difference frequencies. When $f_{LO}-f_{in}=f_{IF}$ for an input component, that component falls in the IF passband. 4. **IF filter:** a narrow band-pass filter at fixed IF. Its bandwidth is the **resolution bandwidth (RBW)** and decides how close two components can be and still be seen as separate. 5. **IF amplifier (often logarithmic):** amplifies the IF signal; the log characteristic gives a dB display with wide dynamic range. 6. **Envelope detector:** converts the IF signal amplitude into a DC level proportional to the strength of that input component. 7. **Video filter:** a low-pass filter that smooths the detected output and reduces displayed noise. 8. **Display:** the detected voltage drives **vertical (Y) deflection**. As the ramp sweeps, each component produces a vertical line at its frequency with height equal to its amplitude. **Example:** an AM signal at 1 MHz with 5 kHz tone shows three lines: the carrier at 1 MHz and sidebands at 0.995 and 1.005 MHz. #### Filter-bank (real-time) analyzer The input is applied to many fixed, adjacent narrow band-pass filters in parallel, each followed by a detector. An electronic switch scans the detector outputs onto the display. It shows all components at the same time (good for transients) but has limited resolution and range. #### Important trade-offs and uses - Narrow RBW gives fine resolution and a lower noise floor but requires a slower sweep (sweep time $\propto$ span/RBW$^2$). - Uses: measuring harmonic distortion, modulation index and sidebands, occupied bandwidth, spurious emissions, noise and interference, and filter responses. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. An analog spectrum analyzer displays A. Voltage versus time B. Amplitude of frequency components versus frequency C. Phase versus time D. Noise only Answer: B. It is a frequency-domain display of a signal's components. (Key point: Purpose of the analyzer) 2. Why does horizontal position on the screen indicate frequency? A. The ramp that sweeps the VCO also drives X deflection B. The envelope detector drives X C. The attenuator sets X D. The video filter sets X Answer: A. Both the VCO frequency and spot position follow the same sawtooth. (Key point: Sweep and display) 3. The envelope detector and video filter together A. Convert IF amplitude to a smoothed DC level for the vertical axis B. Generate the sweep C. Shift frequency D. Attenuate the input Answer: A. The detector gives a level proportional to component strength; the video filter smooths it and reduces displayed noise. (Key point: Envelope detector and video filter) 4. Compared with the swept analyzer, a filter-bank analyzer A. Shows all components simultaneously (good for transients) but has limited resolution B. Has unlimited resolution C. Needs a sweep generator D. Cannot show transients Answer: A. It uses many parallel fixed band-pass filters, so it is real-time but limited in resolution and range. 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