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