---
title: "Short note: Backward wave oscillator"
subject: "RF and Microwave Engineering"
chapter: 5
key: "semester_7/rf_and_microwave_engineering:backward-wave-oscillator-note"
source: https://www.bctengineer.com/q/semester_7/rf_and_microwave_engineering/backward-wave-oscillator-note
site: BCT Engineering
---

# Short note: Backward wave oscillator

RF and Microwave Engineering · Chapter 5: Microwave Generators

- Asked in: 2074 Bhadra, 6 marks; 2071 Bhadra, 5 marks (asked 2 times)
- Video: [Backward Wave Oscillator (Basics, Types, Parameters, Structure, Working & Applications) Explained](https://www.youtube.com/watch?v=DNSXJPTkEbY) — 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/backward-wave-oscillator-note
- Whole chapter: https://www.bctengineer.com/md/semester_7/rf_and_microwave_engineering/chapter-5

## Question

Write a short note on backward wave oscillator.

## Short answer

A backward wave oscillator (BWO) is an electronically tunable microwave tube where an electron beam interacts with a backward space harmonic on a slow-wave structure. The wave’s group velocity opposes beam direction, creating internal feedback; oscillation frequency is voltage-tuned over a wide range. It serves as sweep signal generators and local oscillators.

## Answer

A **backward wave oscillator (BWO)**, also called a **carcinotron**, is a linear-beam (O-type) slow-wave tube that works as an **electronically tunable microwave oscillator**. Its beam interacts with a **backward space harmonic**: the wave's group velocity (energy flow) is opposite to the beam direction, while the phase velocity of that harmonic is along the beam.

**Construction**
- An electron gun gives a beam focused by an axial magnetic field.
- A **slow-wave structure** (helix, interdigital line or folded line) runs along the beam.
- The RF output is taken from the **gun end** of the structure. The collector end is matched with an attenuator or termination. There is no RF input.

```text
 RF out                          matched load
   ^                                 |
 gun |>=== slow-wave structure ======| collector
       beam  ----->  (electrons)
       energy <----- (group velocity)
```

**Working**
1. Noise sets up a wave on the structure. The beam is velocity-modulated by the backward space harmonic, whose phase velocity is in step with the electrons.
2. Bunches form as the beam moves forward. The bunched beam gives energy to the wave, and that energy travels **backward** toward the gun.
3. Near the gun end this energy modulates the fresh beam again. This is built-in **feedback**, so oscillations start when the beam current exceeds a start-oscillation value.
4. Since the synchronous condition depends on beam velocity, the **frequency is tuned by the beam voltage**, since the beam velocity v₀ ∝ √V₀.

**Features:** a wide electronic tuning range (up to an octave), output of mW up to about 1 W (O-type), low efficiency (about 5–20%).

**Applications:** swept-frequency signal generators, local oscillators, broadband jammers, and mm-wave sources.

## Check yourself

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

1. In a BWO, the energy flow (group velocity) of the interacting harmonic is:
   A. Along the beam
   B. Opposite to the beam, toward the gun
   C. Zero
   D. Transverse
   Answer: B. The backward space harmonic carries energy toward the gun end. (Key point: Backward wave)

2. How does a BWO oscillate without an RF input?
   A. External feedback loop
   B. Backward-travelling energy re-modulates the fresh beam, an internal feedback
   C. A magnetron
   D. An amplifier stage
   Answer: B. Internal feedback starts oscillation when beam current exceeds the start-oscillation value. (Key point: Feedback)

3. How is a BWO frequency tuned?
   A. By changing the beam voltage
   B. By changing the load
   C. By moving the collector
   D. By temperature only
   Answer: A. Synchronism depends on beam velocity, roughly v0 ∝ √V0, giving wide electronic tuning. (Key point: Tuning)

4. Where is the RF output taken?
   A. The collector end
   B. The gun end, with the collector end terminated in a matched load
   C. Mid-way
   D. From the magnet
   Answer: B. Output is at the gun end because the energy flows backward. (Key point: Output location)

## Related in RF and Microwave Engineering

- [Bunching effect and backward wave oscillator](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Abunching-effect-bwo) — RF and Microwave Engineering, chapter 5
- [Slow backward wave structure and LNA](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aslow-wave-structure-lna) — RF and Microwave Engineering, chapter 5

## Related in other subjects

- [Feedback concept and square wave oscillator](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Afeedback-theory-square-wave) — Basic Electronics Engineering, chapter 4
- [Infinite line: surge impedance and forward/backward waves](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Ainfinite-line-surge-impedance-waves) — High Voltage Engineering, chapter 3
- [Barkhausen criteria and Wien bridge oscillator](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abarkhausen-wien-bridge) — Electronics Devices and Circuit, chapter 5, asked 2×
- [Barkhausen criteria and relaxation oscillator](https://www.bctengineer.com/question.md?key=semester_1%2Ffundamental_of_electrical_and_electronics_engineering%3Abarkhausen-relaxation-oscillator) — Fundamental of Electrical and Electronics Engineering, chapter 6