THEORY EXAM DATE!!!!

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Wednesday 7th July 2010 @ 2PM

• 12 or more for the exam to be held in

Limerick

• IF NOT YOU HAVE TO TRAVEL TO

COMREG IN DUBLIN

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SEND YOUR FEE NOW!!

• Sean Nolan EI7CD, 12 Little Meadow,

Pottery Road, Dunlaoghaire, Co. Dublin

• 50 Euro or 25 Euro for full-time registered

students, repeat candidates and those who

are retired, unemployed or have a disability.

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PAYMENT

• Cheques and Money Orders should be made

payable to the IRTS. When forwarding the

exam fee intending candidates should

enclose their postal address as well as phone

and e-mail contact details.

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Session 9

TRANSMITTERS

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Module 9 Transmitters

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Modulation Methods

Transmitter Design

Transmitter Characteristics

Spurious Transmissions

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Review of Modulation Methods

• CW

• AM

• SSB

• FM

• Digital

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Emission Codes

Common modulations use codes:

A1A - Hand Sent(?) On/Off keying of the carrier - Morse

A3E - Amplitude Modulated Voice Telephony - AM

F3E - Frequency Modulated Audio - FM

J3E - Single Sideband

Data modulation

F1B - Direct Frequency shift keying

F2B - FSK Audio on an FM Transmitter

J2B - FSK Audio on an SSB Transmitter

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CW (A1A)

• Continuous Wave

• On-Off keying of transmitter output by Morse Key

• Narrow Bandwidth

• Simple CW-only transmitter realisable

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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

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AM (A3E)

• Amplitude Modulation

• Output amplitude varies in proportion to amplitude of modulating

signal

• Original carrier plus two sidebands transmitted

• Occupies a bandwith equal to twice the modulating frequency

• Lot of power in the carrier

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AM Depth

• AM Depth refers to the extent of AM modulation.

If 100% depth is exceeded, clipping/distortion occurs

AM Depth, m = b/a, and is often expressed as a percentage

AM - at nearly 100% Depth

a

b

a = level of unmodulated carrier, b = modulation peak level

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SSB (J3E)

• Amplitiude Modulation Single Side Band

• Carrier is suppressed, generally by a balanced modulator

• One sideband is suppressed, generally by a filter

• Occupies half the bandwidth of AM, i.e., highest modulating frequency

• No carrier, all power in sideband so more efficient

• No carrier so no “beats” or heterodyne whistles

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AM Waveform

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F M (F3E)

• Frequency Modulation

• Deviation of carrier output frequency is proportional to amplitude of modulating signal. Amplitude of carrier constant

• Theoretically infinite number of sidebands

• Amateurs use narrow band FM (NBFM) which restricts deviation and power in sidebands

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FM Waveform

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Digital

• Text is encoded on a computer depending on mode being

used

• Modem (software or hardware) generates audio tones

which modulate an SSB or FM transmitter

• Can be very narrow bandwidth, e.g., PSK31 or wider, e.g.,

RTTY

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Transmitter Design

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Tx Design

• Signal is generated at output frequency (simple

CW Tx) or generated and modulated at sub-

multiple and then multiplied up to output

frequency (simple FM Tx)

• Signal is generated and modulated at low

frequency and then translated up (by mixing) to

output frequency (modern multi-band Tx)

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CW Transmitter

• Carrier keyed on-off by Morse key

• Connected to Driver/Buffer or PA stage

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Master Oscillator

• Generates carrier at required frequency

• Crystal controlled or variable frequency (VFO) (LC, Frequency Synthesiser, DDS)

• Provide stable signal with low noise

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Buffer/Driver

• Buffer isolates oscillator from PA to prevent “pulling” due to varying load

• Buffer can be keyed for CW

• Driver amplifies output to provide sufficient power for PA

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Power Amplifier

• Class C amplifier (non-linear, but most efficient) may be used for CW

• Tuned network to match output impedance to 50 ohm of feeder/antenna

• Provides harmonic filtering

• May have Pi tank Circuit with tune/load controls

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SSB Transmitter

• SSB signal generated at fixed low frequency and then translated to output frequency

• The Variable Frequency Oscillator (VFO) tunes to the desired frequency

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SSB Generation

• Speech amplifier will process/tailor audio

• Balanced modulator “mixes” oscilator and audio to produce upper and lower sidebands with little carrier at fixed frequency (DSB)

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SSB Generation

• Bandpass filter (crystal or mechanical) removes unwanted sideband

• Filter characteristic determines bandwidth of signal

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Mixer

• Mixer mixes SSB and VFO signals up to final

frequency

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Mixers in Transmitters

• In general VFOs, Crystal Oscillators and Synthesisers do not directly generate the final RF Output frequency.

• Mixers are used to combine two or more frequency sources as part of the modulation and up-conversion scheme.

• Spurious outputs can also occur, as well as deviation issues on FM.

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Balanced Mixers • A Balanced Mixer is used to create SSB as it will nullify the

carrier component to leave the two sidebands

• RF Is applied to centre taps which results in null net carrier

• AM can be generated by deliberately unbalancing it

Audio

Input

Double

Sideband

Output

RF Carrier Input

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Linear Amplifier

• SSB signal passes through buffer/driver stage to Linear Power Amplifier

• Must be linear (Class A or, more efficiently, AB1, AB2)

• Must not be over driven as non-linearity and thus splatter will occur

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FM Transmitter

• Typically FM signal generated at a low frequency

and multiplied up to required frequency

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FM Generation

• In this diagram FM signal is generated at a sub-multiple of required frequency

• Modulator causes the frequency of the oscillator to vary in proportion to the amplitude of the audio

• May be a variable capacitance diode (varicap)

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Frequency Multiplier

• Frequency Multiplier is an amplifier with its

output tuned to a harmonic (often 3rd) of input

signal

• Modern Tx uses frequency conversion rather than

multiplication

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Multipliers

• Multipliers use a severely non-linear stage to deliberately generate harmonics - eg a Class-C amplifier or a diode

• The desired multiples of the input frequency can be selected by a bandpass filter.

• Multipliers are not very efficient, needing up to Watts of input power for milliwatt outputs

• Used in simple crystal based PMR VHF radios, before synths.

• Main role now is in microwave multiplier chains eg. for x2, x3, x5

– 432MHz x 3 = 1296MHz (23cms)

– 3.4GHz x 3=10GHz

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Power Amplifier

• Does not need to be linear for FM

• Has matching and filtering network

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FM using Phase Modulator

• For a sinusoid, frequency is the rate of change of phase

• Varying phase varies frequency

• Phase modulator after oscillator changes phase, hence frequency

• Can be simple RC phase shift network, with variable capacity diode

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Transmitters Characteristics

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Tx Characteristics

• Frequency stability – ability to maintain

same frequency over a period of time, i.e.,

not to drift. Affected by heat and

mechanical considerations

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RF Bandwidth

• RF Bandwidth – the bandwidth of a signal

is determined by mode and by audio

response. FM is widest, then AM, SSB and

CW. Digital modes depend on system used

• In an SSB Tx the filter will limt the

sideband to 1.8-2.4kHz

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FM Bandwidth

• Unlike AM, FM has a whole series of continuous sidebands which extend beyond the nominal deviation

• A good guide is Carsons Rule:-

FM Bandwidth = 2 x (Maximum Audio Freq + Peak Deviation)

or

BW = 2 ( Fmax + f)

• Examples:-

• For 70cms: BW=2x (3kHz + 5kHz) = 16kHz (need a 25kHz FM Channel)

• At 2m: BW=2 x (2.8kHz+ 2.5kHz) = 10.6kHz (for a 12.5kHz Channel)

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AF Range

• The audio frequency range is the range of

frequencies used to modulate the transmitter.

Typically 100Hz – 2.7kHz.

• It is determined by the microphone/speech

amplifier and may be conditioned further by the

filter in an SSB transmitter

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Non-linearity

• Non-linearity is where an amplifier introduces

distortion, i.e., the output is not an exact magnified

copy of the input.

• The signal becomes clipped leading to generation

of harmonics/sidebands

• Overdriving of amplifiers is a major cause

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Output Impedance

• Maximum power is delivered to a load (antenna)

when the output impedance of the generator

(transmitter) is equal to the load impedance.

• Standard is 50 ohm

• Output network (Pi tank) matches the output

impedance of the amplifier final device to 50 ohm

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Output Power

• Output power is the power from the transmitter.

• For CW and FM it is the d.c. (steady state) power.

• For SSB it is the Peak Envelope Power

• Measured in dBW, power relative to 1 watt

• 20 dBW=100W 26dBW=400W

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Efficiency

• Efficiency, as applied to an amplifier, is the ratio of output power to d.c. input power expressed as a %

• A Class C amplifier will give an output of 100W for a d.c. input of 150 W and thus has an efficiency of 66%

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Deviation and Modulation Index

• When a signal is frequency modulated the

frequency varies in proportion to the amplitude of

the modulating signal. The maximum size of this

variation is known as the peak frequency

deviation

• The modulation index is the ratio of the peak

deviation to the highest modulating frequency

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Amplifier Classes

• 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-A Biased well on for high fidelity but also results in low efficiency and high heat dissipation on poweramps

• Class-C Nonlinear but efficient - high distortion needs filtering - Used for FM and in GSM mobile phones

• Other Classes exist : D, E, F, G, H, S etc

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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

VBE VBE

IC IC

Input signal

low bias voltage

Distorted

Output Output

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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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Spurious Transmissions

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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.

• SSB Power is measured in 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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Key Clicks & Chirps

• When a carrier is interrupted, as in CW, a sharp

interruption will cause sidebands which manifest

as Key Clicks. The rise time must be conditioned

with a key click filter.

• If the frequency of the transmission varies

instantaneously as the key is depressed this give a

chirp-like sound, which occupies more bandwidth

than necessary. It is caused by poor power supply

or poor oscillator/buffer isolation/design

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Spurious/Unwanted radiation

• Even a “linear” amplifier has some residual non-linearity leading to harmonic generation

• Over-driving an amplifier makes it non-linear

• Unwanted mixer and intermodualtion products

• Self oscillation where an amplifier oscillates near the working frequency

• Spurious (parasitic) oscillations where internal feedback causes an amplifier to oscillate at a frequency not necessarily related to the working frequency

• Spurious signals from frequency synthesisers

• Excessive audio bandwidth and over modulation or over deviation

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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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Direct Radiation

• Cabinet radiation directly from the

transmitter enclosure due to poor screening

• Radiation from power and control or

microphone cables

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Phase Noise

• All oscillators have some noise, phase noise, as a result of noise voltages in the circuit causing phase variations, frequency jitter

• This noise is a lack of purity of the oscillator signal and manifest itself as noise sidebands, (mainly) adjacent to the oscillator frequency which will increase Tx bandwidth and band noise

• It is least in crystal oscillators; PLL synthesiser circuits tend to be plagued with it

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Amateur Station

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HF Station

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Multimode Transmitter

• Modern radios often have a multimode architecture

• The modulator may be switchable for AM, SSB and FM

• Mixer changes modulated signal to final output RF frequency

Mic

Audio

Amplifier

Lowpass

Filter

Frequency

Synthesiser

Crystal

Oscillator

Crystal

Oscillator

Modulator

& Filter

Filter &

RF Driver

RF Power

Amplifier Mixer