1 Transmitters A transmitter must generate a signal with the right type of modulation, with sufficient power, at the right carrier frequency, and with reasonable efficiency The modulation should be done accurately enough to ensure the demodulated signal is a faithful copy of the original signal

2 Frequency accuracy and stability Determined by the carrier oscillator In SONET/SDH standards, the frequency deviation is typically less than 2E-5 (200kHz at 10Gb/s)

3 Frequency agility Frequency agility refers to the ability to change operating frequency rapidly

4 Spectral purity Spurious signals are often harmonics of the operating frequency or of the carrier oscillator Class C amplifiers typically produce a large amount of harmonic energy

5 Power output Full-carrier AM: carrier power Suppressed-carrier AM: peak-envelope power (PEP) FM: total power output

6 Efficiency Overall efficiency is the ratio of output power to power input from the primary power source

7 Modulation fidelity Dynamic range In analog systems, it is often defined as SFDR

8 Transmitter architectures We will discuss: AM FM Heterodyne systems

9 Full carrier AM transmitter Frequency multiplier Matching circuit Baseband signal processing Baseband signal input BufferFrequency synthesizer Driver Power amplifier/ modulator When modulation is accomplished at the output of the last stage in the transmitter, it is referred to as high-level modulation

10 Power amplifier/modulator Collector Emitter Base RL Modulation in Carrier in Vout L Vcc L Pt = Pc (1 + m 2 /2) 100% modulation results in 50% increase in the power

11 Audio circuitry Audio circuitry is required to amplify small signal from a microphone to a sufficient level to modulate the transmitter Studio equipment amplifies the tiny signals from microphones, phonograph cartridges, and tape heads

12 Output impedance matching The transmitter output must be designed to transform the standard load resistance at the output terminal to whatever is required by the active devices For class C amplifiers, Vpeak = Vcc, Vrms = Vcc/sqrt(2), P = V 2 /R, therefore R L = Vcc 2 /2Pc

13 Two matching networks Collector Emitter Base Collector Emitter Base Pi network T network

14 Frequency multiplier Matching circuit ALC Audio signal input BufferFrequency synthesizer Driver Power amplifier/ modulator A CB transmitter

15 A CB transmitter Frequency multiplier RF combiner Audio signal input BufferFrequency synthesizer Driver Power amplifier/ modulator

16 Class D (switching) amplification Pulse Duration Modulation (PDM) can be employed to increase the efficiency Signal Modulated Demodulation is simply based on low pass filter

17 Single sideband AM transmitter Audio Crystal filter BPF Synthesizer Carrier DSB->SSB Up-conversion to operating band

18 DSBSC generation DSBSC can be generated by multiplying the baseband signal (without bias) with the carrier Bandpass filter Carrier oscillator Signal Balanced modulator SSB out DSBSC

19 SSB generation by filter method The obvious way to eliminate the unwanted sideband, once the carrier has been suppressed, is to use a band pass filter It is more convenient to null the carrier with a balanced modulator and then remove the sidebands by filtering However the filter method has to handle very low frequency baseband components

20 SSB generation by filter method To transmit a USB signal at a carrier frequency of 21.5MHz, what should be the frequency of the LO? Audio Crystal filter, center 9MHz, 3kHz BW BPF 22.5MH, 5M BW Synthesizer MHz Carrier M (USB) or M (LSB)

21 Spectra illustration MHz MHz USB LSB f LO = f 0 – f CO = 21.5MHz – MHz = MHz

22 FM transmitters FM modulation can probably be applied at the oscillator stage Class C amplifiers can be applied Frequency multiplication is often used as a way of increasing deviation Audio Driver FM modulator Pre- emphasis Frequency multiplier Buffer Power amp

23 Direct-FM modulators Clapp oscillator

24 Direct-FM modulator using a transistor Collector Emitter Base Rb1 Rb2 RL Re Ce Bypass FM out Modulation In Common base configuration LC oscillator

25 FM transmitter with multiplication and mixing Multiplication to increase the maximum frequency deviation FM modulation center 5MHz Multipli cation Oscillator 95 MHz BPF center 5MHz X 18 Multipli cation The maximum frequency deviation is increased by 18x18 = 324 However, the VCO output frequency error has to be small

26 PLL FM generator Phase detector LPF Modulating signal VCO Output Crystal oscillator divider The loop filter allows the system to ignore the rapid variations of frequency associated with modulation, while preventing drift of the center frequency of the VCO away from its nominal value

27 In-direct FM transmitter Phase modulator Multi plier Crystal oscillator Integrator Pre- emphasis Based on phase modulator Signal is integrated before driving the modulator