Transmitters and Receivers Dr Costas Constantinou School of Electronic, Electrical & Computer Engineering University of Birmingham W: E:

The Communication Process Every communication system has 3 basic elements (in blue): – Transmitter – Channel – Receiver Information Source Transmitter Channel Information Sink Receiver Message signal Estimated message signal Transmitted signal Received signal 2

The Source OSI reference model data A PS T N A PS T A PS A P A A PS T N D analogue Note accumulation of control data at each level. For small packets control information can be much more than the data itself 3

The source The message signal can be either analogue or digital The transmitted signal is always analogue – why? – Simultaneous communications: Multiplexing – Bandwidth limiting 4

Multiplexing No multiplexing = one physical transmission medium per user! Sharing transmission medium is central to communications 5

Why Multiplex Mobile phone has voice plus many control channels simultaneously Optical fibre very high capacity for many simultaneous channels Putting many telephone calls over one cable 6

Key feature of digital waveforms – limit bandwidth frequency A t F = 1/t time A Pass through raised cosine filter to remove frequency side- lobes Choose bit rate to match channel bandwidth 7

Key feature of digital waveforms – limit bandwidth All standardized common TTL circuits operate with a 5 V power supply. TTL signal is defined as: – "low" when between 0V and 0.8V with respect to the ground – "high" when between 2.2V and 5V CMOS works with a wider range of power supply voltage – usually anywhere from 3 to 15V Current ~ 1 mA or lower A time 8

Multiplexing Time multiplex several digital signals multiplexer t t t t 9

The Transmitter Transmitter power must be sufficient to achieve adequate signal strength at the receiver Received signal must be higher than noise to be intelligible power tx distance power distance receiver noise tx receiver signal to noise ratio signal to noise ratio good !bad ! 10

Transmitter power Need amplification to increase transmitted power to overcome loss in the channel Power level depends on channel loss Channel loss depends on distance. Typical order of magnitude figures – telephone cable ~ 20 dB – optical fibre~ 30 dB – wireless channel~ 80 dB 11

Transmitter bandwidth We want to get as many user channels into the transmitter bandwidth as possible Baseband voice bandwidth ~ 3kHz Percentage of user channel to centre frequency telephone kHz ~ 26 % multiplexed telephone1 – MHz~ 0.29 % mobile phone850 – MHz~ 3 x % optical fibre300 THz – 300 THz + 3 kHz~ % Conclusion – upconvert to higher frequencies 12

Other reasons to upconvert Fibre optic – cannot get electrical signals down an insulating glass fibre Wireless – for efficient operation antennas => λ/2 at 3 kHz λ = c/f = 3 x 10 8 /3 x 10 3 = 100 km at 3 GHz λ = 0.1 m 13

Upconverters Use a mixer – Assume input signal is digital 1,0,1,0,1….. – Apply carrier signal to other port, – Output is product (mixer is multiplier) VcVc VoVo t freq 14

Upconverters Simplify using trigonometric expansion gives Mixer produces difference and sum frequencies of all components in input waveform VcVc VoVo t 15

signal f 1 carrier f 2 sum (f 2 + f 1 ) and difference (f 2 - f 1 ) Assume input is digitised speech signal = kHz carrier = 6 kHz sum = kHz difference = kHz freq Upconverters 16

To reduce bandwidth remove sum frequency mixerfilter 0-3kHz 6kHz 6-9kHz 3-6kHz freq Upconverters 17

Non-linear devices such as diode have a current/voltage relationship which includes a square law characteristic 2 nd term is the product that we want for upconversion I V Upconverters – the mixer circuit 18

Using gives We need to filter the DC term as well as the much higher frequency 2ω c and much lower frequency 2ω o Upconverters – the mixer circuit 19

The Transmitter – so far Note – can change output frequency by tuning ω c ωcωc multiplexer source t freq local oscillator amplifier 1 amplifier 2 freq t t 20

ωcωc multiplexer source amplifier 1 amplifier 2 Amplifier 1: needed to get digital signal up to level needed by mixer circuit Amplifier 2: needed to get mixer output up to level required by channel e.g. mobile phone output power 1 watt max. mixer output – 1 mA at 5 V amp. output- 100 mA at 5 V The Transmitter – so far 21

The Channel Channel problems – A – channel attenuates signal (attenuation can be variable) – B – channel is dispersive (speed varies with frequency) CableA – moderate, B – limits upper frequency and data rate FibreA – low, B – limits upper frequency and data rate WirelessA – very high and variable, B – bad in urban and indoors Tx Rx Cable or fibre Tx Rx wireless 22

Pulse or packet waveform spectrum frequency time Dispersion Signals are usually many frequencies added together 23

Wave groups – Ripples in pond from a dropped stone – Pulse on a transmission line Non dispersive – All frequencies travel at same speed. Packet shape not changed. Dispersive – Frequencies travel at different speeds. Packet shape widens. Dispersion 24

Channel Variability The effect of noise – Decision making in a 2 level signal time

Channel Variability Decision making in a 4 level signal time smaller amounts of noise are more significant as number of levels increases

Channel Variability Can we find a waveform that is less affected by amplitude noise? Fundamental properties of a signal – Amplitude – Frequency – Phase Amplitude modulation used so far 27

Amplitude modulation Frequency or phase modulation Possible Modulation Schemes 28

Noise mainly in peaks Frequency/Phase Modulation To remove amplitude noise from frequency or phase modulation – Amplify Clip – count zero crossings to determine instantaneous frequency Called a limiter – see Signal Processing module 29

Frequency Modulator Concept Amplifier Feedback - becomes oscillator (C determines frequency) Vary capacitance using varactor diode (frequency depends on signal voltage) VsVs f(V s ) 30

Final Transmitter? multiplexer source ωcωc modulator 31

Final Transmitter? Output waveform spectrum must meet template laid down by international agreement (ITU), especially for wireless systems Typical template (GSM) Freq (MHz) Power (dBm) Allowed out of band radiation channel Conclusion – must use band pass filter at output 32

Final transmitter multiplexer source ωcωc modulator 33

The Receiver Assume amplitude modulation of digital signal Single modulated pulse looks like freq time 34

Received signal Rectify pulse to remove lower half Low pass filter to get envelope Amplify 35

The detector The first radio sets used a rectifier and a tuned circuit. The rectifier was made from a wire touching a piece of crystal material, called a cats whisker 36

The detector Advantages – simple construction – suitable for cable systems – optical fibres use laser diode as transmitter and detector diode as receiver Disadvantages – not very sensitive to small signals – cannot be used in wireless systems Wireless systems – low signal strength – use low noise amplifier (LNA) – external noise – use filter Tx Rx external noise 37

External Noise Power power Freq (MHz) Filter for FM broadcast band Filter for GSM mobile phones Note – filters cover whole band – channel filters discussed later 38

Improved receiver The loss of the BPF and the detection process in the rectifier both contribute noise. Low noise amplifier (LNA) also adds noise, but at lower level. Gain of the LNA should be high enough so that LNA noise dominates. More details of noise calculations in the link budget lectures. low noise amp band-pass filter 39

Thermal Noise Power Since spectrum is flat with frequency (white noise) – then noise power must be proportional to bandwidth P n = k T B Watts – where P n = available noise power, in Watts – k = Boltzmann’s constant = 1.38 x Joule/Kelvin – T = absolute temperature of noise source, in Kelvin – B = bandwidth, in Hz 40

For a bandwidth of 1 MHz the available noise power from a source at temperature 300 K is P n = 1.38 x x 300 x 1 x 10 6 ~ 4 x W Compare this with a signal power generated by a 1.0µV source driving a 50 Ohm load which results in an available signal power of P s = (1.0 x ) 2 / 4 x 50 = 5 x W If the noise is comparable to the signal then subsequent amplification will not improve matters Thermal Noise Power 41

Improved Receiver - A Filter design V out /V in freq f0f0 ΔfΔf Quality factor = f 0 / Δf Max. Q factor for typical filter is few thousand low noise amp band-pass filter 42

Filters For GSM mobile phone band – complete band = 890 – 960 MHz = 70 MHz – channel bandwidth = 25 kHz Q factor needed – complete band = 925 / 70 = 11.8 – channel bandwidth = 925x10 6 / 25x10 3 = 33,000 Conclusion – can filter whole band, but not user channel – but downconversion may help…… Bandpass filter 890 freq (MHz) 960 band channel Tx Rx 43

Improved Receiver - B Downconverter is same as upconverter f out = f c – f s = f intermediate = IF low noise amp band filter ωcωc downconverter detector 44

Improved Receiver - B Additions – normal to include low-pass filter as part of mixer – as LNA may have only low gain, put in another amplifier – put in channel filter – IF signal retains phase/frequency as well as amplitude information – represent detector as block which could also detect PM/AM low noise amp band filter ωcωc detector channel filter IF amp downconverter 45

Improved Receiver - C IF amp may have gain ~ 40 dB Channel filter (assume IF = 100 MHz – Q factor = 100x 10 6 / 25x10 3 = 4000 – realised with a surface acoustic wave filter low noise amp band filter ωcωc detector channel filter IF amp downconverter 46

Improved Receiver - C Can tune local oscillator to choose receive frequency – f IF = f c – f s IF is fixed and f c is changed to select wanted channel Easier to tune oscillator than make tuned filter Example – f IF = 100 MHz, f c = 998, f s = 898 – f IF = 100 MHz, f c = 1040, f s = freq (MHz) 960 band channel Tx Rx low noise amp band filter ωcωc detector channel filter IF amp downconverter 47

Improved receiver - C Example f IF = 100 MHz, f c = 1040, f s = 940 Problem f IF = 100 MHz, f c = 1040, f image = 1140 signal at f s + 2  IF will also go through mixer must filter out ‘image signal’ with band filter/image reject mixer IF f s f c f image low noise amp band filter ωcωc detector channel filter IF amp downconverter 48

Transceiver low noise amp band filter ωcωc detector channel filter IF amp multiplexed source modulator o/p band filter high power amp local oscillator antenna diplexer 49

The Future? Direct Up/Downconversion and Software Defined Radio Intermediate frequency is zero (baseband) Channel filtering and demodulation done by digital processing. Processing can be changed to make radio work with any standard, or even download software for new standards over the air! ωcωc processor a/d converter 50

What next? Attempt the tutorial sheet on transmitters and receivers Next lecture on antennas 51