---
title: "Short note: Spectrum analyzer"
subject: "RF and Microwave Engineering"
chapter: 8
key: "semester_7/rf_and_microwave_engineering:spectrum-analyzer-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/spectrum-analyzer-note
site: BCT Engineering
---

# Short note: Spectrum analyzer

RF and Microwave Engineering · Chapter 8: RF and Microwave Measurements

- Asked in: 2080 Chaitra, 5 marks; 2080 Bhadra, 4 marks (asked 2 times)
- Video: [Spectrum Analyzer (Basics, Uses, Working, Types & Parameters) Explained](https://www.youtube.com/watch?v=rANoa_-QHSo) — Engineering Funda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/spectrum-analyzer-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-8

## Question

Write a short note on spectrum analyzer.

## Short answer

A spectrum analyzer displays amplitude versus frequency using a swept local oscillator that mixes the input to a fixed IF filter of resolution bandwidth RBW, followed by log amp and detector. Key specifications are RBW, VBW, sweep time and dynamic range. It measures harmonics, spurious signals and modulation spectra.

## Answer

A **spectrum analyzer** displays signal amplitude versus frequency (frequency domain), whereas an oscilloscope shows amplitude versus time.

**Superheterodyne (swept-tuned) type:**

```text
RF in-->[Atten]-->[Mixer]-->[IF filter]-->[Log amp]
                     ^         (RBW)          |
                     |                    [Detector]
                [Swept LO]                    |
                     ^                  [Video filter]
                     |                     (VBW)
              [Ramp generator]                |
                     |                        v
                     +---->[ Display: X=freq, Y=amp ]
```

Working:

1. The input attenuator sets a safe level for the mixer.
2. A **swept local oscillator**, driven by a ramp, mixes with the input; each input frequency fin passes the fixed IF filter when fLO − fin = fIF.
3. The **IF filter** bandwidth is the **resolution bandwidth (RBW)**: it decides how close two signals can be and still be seen apart.
4. A log amplifier and envelope detector give amplitude in dB; the **video filter (VBW)** smooths noise.
5. The same ramp drives the horizontal axis, so the screen shows amplitude vs frequency.

Key specifications: frequency range, RBW, sweep time, dynamic range, sensitivity (displayed average noise level), phase noise.

Uses: measuring harmonics, spurious signals, modulation spectra, intermodulation, noise, EMI testing and occupied bandwidth. Modern analyzers also use FFT and real-time processing.

## Check yourself

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

1. How does a spectrum analyzer differ from an oscilloscope?
   A. It shows amplitude versus frequency, not versus time
   B. It shows amplitude versus time
   C. It shows phase only
   D. It shows resistance versus voltage
   Answer: A. A spectrum analyzer is a frequency-domain instrument. (Key point: Frequency vs time domain)

2. When does an input signal pass the IF filter?
   A. When fLO - fin = fIF as the LO sweeps
   B. When fin = fIF always
   C. When fLO = 0
   D. When fin = 2 fLO
   Answer: A. The swept LO mixes with the input, and each input passes the fixed IF filter when the difference equals the IF. (Key point: Swept superheterodyne action)

3. What does the RBW determine?
   A. How close two signals can be and still be seen apart
   B. The maximum input power
   C. The sweep ramp voltage
   D. The display brightness
   Answer: A. RBW is the IF filter bandwidth and sets frequency resolution. (Key point: Resolution bandwidth)

4. What does the video filter (VBW) do?
   A. Smooths noise on the detected signal
   B. Sets the LO frequency
   C. Attenuates the input
   D. Defines the IF
   Answer: A. After the log amplifier and detector, the VBW averages out noise on the displayed trace. (Key point: Video filter)

## Related in other subjects

- [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×
- [Short note: Analog spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Ashort-note-spectrum-analyzer) — Communication Systems, chapter 1
- [Working principle of analog spectrum analyzer](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Aanalog-spectrum-analyzer) — Communication Systems, chapter 1