---
title: "Short note: Analog spectrum analyzer"
subject: "Communication Systems"
chapter: 1
key: "semester_6/communication_systems:short-note-spectrum-analyzer"
source: https://www.bctengineer.com/q/semester_6/communication_systems/short-note-spectrum-analyzer
site: BCT Engineering
---

# Short note: Analog spectrum analyzer

Communication Systems · Chapter 1: Introduction

- Asked in: 2067 Mangsir (CS I), 5 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/short-note-spectrum-analyzer
- Whole chapter: https://www.bctengineer.com/md/semester_6/communication_systems/chapter-1

## Question

Write a short note on analog spectrum analyzer.

## Short answer

An analog spectrum analyzer displays signal amplitude versus frequency using a swept-tuned superheterodyne architecture. A sawtooth generator sweeps a VCO and horizontal CRT deflection simultaneously. The mixer downconverts input components to a fixed IF; only the component matching f_LO - f_in = f_IF passes the narrow IF filter (RBW). The envelope-detected output drives vertical deflection, showing each frequency component as a vertical line.

## Answer

An **analog spectrum analyzer** is an instrument that displays the amplitude (or power) of the frequency components of a signal against frequency on a screen, i.e. it shows the signal in the frequency domain. The usual type is the **swept-tuned superheterodyne analyzer**.

```text
 Input->[Atten]->(X)->[IF filter]->[IF amp]
         Mixer    ^      (RBW)         |
                  |               [Env. det]
                [VCO]                  |
                  ^              [Video filt]
                  |                    |
 [Sawtooth gen]---+--> X (horiz)   CRT Y (vert)
```

**Working:**
1. The input is attenuated to a safe level and low-pass filtered.
2. A **sawtooth generator** sweeps the frequency of a **voltage-controlled oscillator (VCO)** over a range; the same ramp drives the horizontal deflection, so the x-axis represents frequency.
3. The **mixer** shifts each input component to the fixed **IF**. Only the component for which $f_{LO}-f_{in}=f_{IF}$ passes the narrow **IF filter**; its bandwidth is the **resolution bandwidth (RBW)**.
4. The IF output is amplified (often logarithmically, for a dB scale), **envelope detected** and smoothed by a **video filter**, and applied to vertical deflection.
5. As the sweep progresses, each frequency component appears as a vertical line whose height is its amplitude.

**Key points:** narrower RBW gives better resolution but needs a slower sweep. Analyzers are used to measure harmonics, modulation sidebands, bandwidth, noise and interference. A **filter-bank (real-time)** analyzer uses many parallel fixed band-pass filters instead of sweeping.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In a swept superheterodyne analyzer, the sawtooth generator
   A. Only drives the vertical deflection
   B. Tunes the VCO and also drives horizontal deflection
   C. Filters the IF
   D. Detects the envelope
   Answer: B. The same ramp sweeps the VCO and the X-axis, so horizontal position corresponds to frequency. (Key point: Horizontal axis generation)

2. A component passes the IF filter when
   A. f_LO = f_in
   B. f_LO - f_in = f_IF
   C. f_LO + f_in = 0
   D. f_in = f_IF/2
   Answer: B. The mixer translates the matching component to the fixed IF where the narrow filter passes it. (Key point: Mixing condition)

3. A narrower resolution bandwidth gives
   A. Better resolution but slower sweep
   B. Worse resolution and faster sweep
   C. Faster sweep and better resolution
   D. No effect on sweep
   Answer: A. The IF filter bandwidth sets resolution; narrower filters need longer settling, so sweep must be slower. (Key point: RBW trade-off)

## Related in Communication Systems

- [Working principle of analog spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-spectrum-analyzer) — Communication Systems, chapter 1
- [Energy and power spectral density; spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aenergy-power-spectrum-analyzer) — Communication Systems, chapter 1, asked 2×

## Related in other subjects

- [Short note: Spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aspectrum-analyzer-note) — RF and Microwave Engineering, chapter 8, asked 2×
- [Digital vs analog spread spectrum](https://www.bctengineer.com/question.md?key=semester_5%2Fdata_communication%3Aspread-spectrum-digital-vs-analog) — Data Communication, chapter 6