Slow backward wave structure and LNA

Slow (backward) wave structure

A slow-wave structure is a periodic circuit, such as a helix, folded line, interdigital line, coupled cavities or a corrugated guide. It makes the RF wave travel along the tube axis at a phase velocity much lower than c (about c/10), close to the velocity of an electron beam, so that the beam and wave can interact over a long length.

Because the structure is periodic (period p), its field is a sum of space harmonics with phase constants βₙ = β₀ + 2πn/p. Some harmonics (n < 0) have a phase velocity along the beam but a group velocity (energy flow) opposite to it. These are backward waves. A slow-wave structure designed to interact with such a harmonic is a slow backward-wave structure, as used in the BWO (carcinotron), where the energy flows back toward the gun and gives internal feedback.

 helix:  /\/\/\/\/\   pitch p, diameter d
 vp ≈ c·p/(πd)   (axial phase velocity)
 forward wave : energy ->  with beam  (TWT)
 backward wave: energy <-  against beam (BWO)

Low-noise amplifier (LNA) – low-noise TWT

Here LNA means a low-noise amplifier at the front end of a receiver. Among microwave tubes, the low-noise TWT is the standard LNA. (Solid-state LNAs using GaAs FET/HEMT follow the same aim: high gain with minimum noise figure.)

Construction

     RF in (weak)                 RF out
        |                           |
 gun |>=[  helix slow-wave line  ]=====| collector
  low-noise gun   [atten]
     PPM / solenoid focusing

• A low-noise electron gun: the beam is accelerated in several stages with special potential profiles near the cathode. This reduces velocity and current fluctuations (shot noise) in the beam.
• A helix slow-wave structure with input and output couplers and a lossy attenuator in the middle.
• Focusing magnet and collector, as in a power TWT, but with low beam current.

Working principle

1. The weak input signal travels along the helix at a phase velocity v_p ≈ c·p/(πd), about equal to the beam velocity v₀ = √(2eV₀/m). The beam is slightly faster.
2. The axial RF field velocity-modulates the beam, and bunches form continuously along the helix.
3. The bunches give energy to the wave, which grows exponentially, giving a gain of 25–40 dB.
4. The noise figure is kept low (about 3–8 dB in tubes) by low beam current, careful gun design that gives a smooth, quiet beam, and good matching at the input. The attenuator stops oscillations caused by reflections.
5. The amplified, low-noise signal is taken from the output end and passed to the mixer.

Uses: front ends of radar, satellite earth-station and microwave-link receivers. Today GaAs FET/HEMT LNAs (noise figure below 1 dB) and cooled parametric amplifiers or masers have largely replaced the low-noise TWT.