Chelmsford Amateur Radio Society Advanced Course Transmitters Part-3 - Power Amplifiers

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Chelmsford Amateur Radio Society Advanced
CourseTransmittersPart-3 - Power Amplifiers
Interference
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Amplifier Class Bias

• Class-A, B, AB and C are defined by the bias and
  operating region of the transistor

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Amplifier Classes

• Class-ABiased well on for high fidelity but also
  results in low efficiency and high heat
  dissipation on poweramps
• Class-BGives only only half the waveform, so
  usually used in Push-Pull configurations. Fairly
  efficient, but can get crossover distortion
• Class-ABA variation of Class-B with but biased
  on each transistor to conduct for slightly more
  than half cycle for better fidelity
• Class-CNonlinear but efficient - high distortion
  needs filtering - Used for FM and in GSM mobile
  phones
• Other Classes exist but are out of scope D, E,
  F, G, H, S etc

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Transmitter Amplifiers

• Modulation schemes which carry information in
  their amplitude require good linearity in all
  stages, or else distortion will occur
• AM SSB require linear amplification eg Class-A
• An FM-only transmitter does not need to be
  linear, so a Class-C amplifier can be used which
  is more efficient
• CW is only on or off, so Class-C is also fine for
  this.
• Data Modulation Frequency or Phase-shift keyed
  schemes are like FM and could use Class-C. If
  Amplitude changes then a linear amplifier is
  needed
• Non-linear amps need filtering to avoid harmonics
  or bandwidth spread

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Poweramp Circuits

• Modulation schemes which carry information in
  their amplitude require good linearity in all
  stages, or else distortion will occur
• AM amps need filtering to avoid harmonics or
  bandwidth spread

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Automatic Level Control

• Splatter, distortion and damage can occur if a
  Poweramp is overdriven
• Heat dissipation and output power varies with
  nature of drive and modulation. For example, a
  long SSTV transmission has a higher duty cycle
  than SSB - check if rating is for continuous
  operation
• Power Limits in Licence are PEP so may need to
  back-off transmissions on SSB unless speech
  processors are used to average out voice peaks
• Automatic Level Control, ALC, can display the
  need to reduce the drive level, or do so
  automatically.
• External PAs can link ALC back to the
  transceiver. ALC is easier to integrate on
  internal Poweramps
• Excess SWR detection is often also built in as a
  protection measure

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Valve Poweramps

• Older amplifiers and some very high power
  amplifiers still use Valves
• Valve Operation is similar to a FET, except heat
  causes electrons to be initially emitted -
  Thermionic Emission
• In a Triode DrainAnode, GridGate, SourceCathode

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Valve Poweramp Benefits

• Advances in technology are encroaching on Valves,
  although they continue to be developed for
  specialist purposes
• Advantages
• Readily handle high powers at higher frequencies
  - VHF, UHF upwards
• Robust in the face of mismatches compared to
  transistors
• Disadvantages
• Need high voltages in amplifier and PSU - SAFETY
  ISSUE
• Physically fragile and prone to vibration
  (including from cooling fans)
• Deteriorate with age

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Transmitter Interference

• Interference can be in band, adjacent channel or
  out of band
• In band/Adjacent can come from key clicks, drift
  chirp
• Spurii from synthesisers, mixers, and multipliers
  can also be causes
• Harmonics and Intermods etc can cause
  interference on other bands

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Modulation Terms

• FM Deviation refers to the max shift away from
  the nominal carrier
• Narrow vs Wideband FM. The FM section 2m band has
  been re-channelised to 12.5kHz spacing compared
  to 25kHz on most of 70cms
• Peak Deviation is 2.5kHz for 12.5kHz channel
  spacing and 4.8-5kHz for a 25kHz spacing.
• Wide deviation on narrowband receivers will
  interfere in adjacent channels. It will also get
  clipped by IF filters/discriminators and result
  in choppy received audio

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Synthesiser Spurii

• Phase comparator time constant and frequency has
  a degree of uncertainty which manifests itself as
  phase noise
• Situation is not helped if small frequency step
  resolution, but rapid tuning are both desired
• Synthesisers must detect out of lock and
  inhibit transmission
• Modern synthesisers use dual loops to get small
  step sizes
• DDS steps would also show up as sidebands/jitter
  unless filtered out

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CW Modulation

• Morse, also called CW, is the simplest form of
  digital mode.

• Fast Edges can give key clicks or cause
  overshoot/ringing in the Poweramp

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CW Key Click Filter

• Block diagram of CW transmitter
• Modified Keying stage switches RF
• Slower rise and fall time of RF envelope will
  avoid excess bandwidth

RF oscillator