Download Transmitters-3

Chelmsford Amateur Radio Society
Advanced Course
Transmitters
Part-3 - Power Amplifiers & Interference
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
1
Amplifier Class & Bias
• Class-A, B, AB and C are defined by the
bias and operating region of the
transistor
IC
IC
Distorted
Output
Output
VBE
Input signal
normal bias voltage
Chelmsford Amateur Radio Society
Advanced Licence Course
VBE
Input signal
low bias voltage
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
2
Amplifier Classes
• Class-A
Biased well on for high fidelity but also results in low efficiency and
high heat dissipation on poweramps
• Class-B
Gives only only half the waveform, so usually used in Push-Pull
configurations. Fairly efficient, but can get crossover distortion
• Class-AB
A variation of Class-B with but biased on each transistor to conduct
for slightly more than half cycle for better fidelity
• Class-C
Nonlinear 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
3
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
4
Poweramp Circuits
• Modulation schemes which carry information in their amplitude require
good linearity in all stages, or else distortion will occur
• Any form of Amplitude Modulation will require that amplifiers need
filtering to avoid harmonics or bandwidth spread
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
5
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 in License is 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
6
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
• The electric field on the Valve Grid controls electron flow
– similar to the Gate in FET
• The equivalence to a FET is that in a Triode…
Drain=Anode, Grid=Gate, Source=Cathode
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
7
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
8
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
9
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 rechannelised 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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
10
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
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
11
CW Modulation
• Morse, also called CW, is the simplest form of digital mode.
Keyer /Data
CW Signal
• Fast Edges can give key clicks or cause
overshoot/ringing in the Poweramp
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
12
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
Keying
stage
Power
amplifier
Low-pass
filter
Key
Chelmsford Amateur Radio Society
Advanced Licence Course
Murray Niman G6JYB
Slide Set 8: v1.2, 3-Dec-2012
(4) Transmitters - Poweramps & Interference
13