1 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference Chelmsford Amateur Radio Society Advanced Course Transmitters Part-3 - Power Amplifiers & Interference

2 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference Amplifier Class & Bias Class-A, B, AB and C are defined by the bias and operating region of the transistor Input signal normal bias voltage V BE ICIC ICIC Input signal low bias voltage Distorted Output Output

3 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

4 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

5 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

6 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

7 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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 Drain=Anode, Grid=Gate, Source=Cathode

8 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

9 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

10 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

11 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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

12 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference CW Modulation Fast Edges can give key clicks or cause overshoot/ringing in the Poweramp Morse, also called CW, is the simplest form of digital mode. CW Signal Keyer /Data

13 Chelmsford Amateur Radio Society Advanced Licence Course Murray Niman G6JYB Slide Set 8: v1.2, 20-Apr-2009 (4) Transmitters - Poweramps & Interference 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 Power amplifier Low-pass filter Keying stage Key