1. COMMUNICATION SYSTEMS
Assoc. Prof Mohd Fadzil Ain
Room: 1.21
Ext: 5815
Course Contents Based On OBE

2. Please explain the picture

4. 1.2 Communication Systems
The goal of communication systems is to transfer information from one place to another by sending the information as electromagnetic energy through vacuum, air, wire or fiber optic as shown in Figure 1.
The typical communication system consist of :
Transmitter
Channel
Receiver
Noise
Figure 1: Typical block diagram of a communication system

5. 1.2.1 Information Signal
Information signal can be analogue or digital signal. Analogue signal is a function of a discrete time where the amplitude of the signal continuously change with time when the acoustic or optical signal were change to electrical signal. A typical example is the microphone that change a voice signal to a voltage.
Figure 2: Analogue Signal

6. 1.2.1 Transmitter
Information signal is not suitable to be transmitted directly unless the source is in electrical form. In telephony communication, the signal in electrical form can be transmitted directly to the receiver without any processing.
For a long distance communication, the signal must be coded first in a suitable form for transmission and reception. This is called modulation and there are three modulation techniques in analogue communication:
Amplitude modulation
Frequency modulation
Pulse modulation

7. 1.2.2 Channel
Channel is a medium for electromagnetic wave transmission such as transmission line, fiber optic or free space. When the electromagnetci wave travel in a medium, the signal will be distorted because the medium normally are non linear and the frequency response is not perfect.

8. 1.2.3 Receiver
The main task of the receiver is to translate back the original information signal after having distorted during propagation through the medium. This process is called demodulation. In practical applications, the receiver will not be able to reproduce an actual transmitted signal. The quality of the signal will depend to the modulation technique that been used in the system.

9. Electromagnetic Spectrum
1.2.3 Electromagnetic wave spectrum
Electromagnetic wave scattered in a wide frequency range nearly infinity
Electromagnetic Spectrum

10. 1.2.4 Frequency of the wave
Frequency is the number of cycle per unit time of the wave and measured in Hertz. Denoted as:
k = kilo = 1000 = 103
M = mega = = 106
G = giga = = 109
T = tera = = 1012

11. 1.2.5 Wavelength
The wave length of the wave is the distance to complete one cycle and measured in Meter.
The wavelength is in lambda () calculated as follows;
V = Speed of light 3x108 m/s
f = frequency
Example 1.2.5:
Calculate the wavelength of 150 MHz
Solution:

12. 1.2.6 Radio Frequency Transmission
Radio frequency spectrum divided into a few frequency bands given the name under International Radio Consultative Committee (CCIR).

13. 1.2.7 Bandwidth (BW)
Bandwidth is the frequency range covered by a system, in other words the BW is the difference between the lower operating frequency and the upper frequency range of the system.
Example 1.2.7:
Audio frequency range is 300 Hz to 3000 Hz. So, the bandwidth is:
BW = 3000 Hz – 300 Hz = 2700 Hz.

14. RF engineering
RF engineering is very important in our daily activities.
We need radio and television for entertainment and nowadays, we can’t live without the mobile phone.
Why RF is so important?
Wireless system allows the communication of information between two points without the physical connection and can be accomplished using infrared, ultrasonic or radio frequency energy.
Infrared signals can provide moderate data rate but the infrared radiation is easily blocked by the obstruction and limits their use to short-range indoor applications.
Ultrasonic signals has very low data rates and poor immunity to the interference.

15. RF engineering
Ultrasonic signals has very low data rates and poor immunity to the interference.
For these reasons, most wireless system rely on RF or microwave signals, usually in the UHF (100MHz) to millimeter wave (30 GHz) frequency range.
RF and microwave signals offer wide bandwidths and have the advantage of being able to penetrate dust, foliage, building and vehicles to some extent.
Historically wireless communication using RF energy began with the theoretical work of Maxwell and verified experimentally by Hertz during the period from 1873 to 1891 followed by the first practical commercial radio developed by Marconi in the early part of the 20th century.

16. Introduction to RF engineering
The term wireless dates back to this early period although replaced by the word radio for most of this century, wireless is again the preferred description for most of today’s cellular phone, data links and satellite system.
The wireless system is catagorised according to the nature and placement of the users.
Point-to-point radio system
A single transmitter communicates with a single receiver using high-gain antennas in fixed position to maximise received power and minimise interference from other radio operating nearby at the same frequency, e.g wireless telemetry for data communications.

17. Introduction to RF engineering
Point-to-multipoint radio system
A single transmitter station broadcasting to a large number of possible receivers. The most common examples are commercial AM and FM broadcasting radio and television broadcasting.
Multipoint-to-multipoint radio system
This system allow simultaneous communication between individual users (static or non-static users). Such system generally connect two users using base stations or repeater station. e.g police walkie-talkies.

18. Introduction to RF engineering
Another way to characterise wireless system is in term of the directionality of communication:
Simplex system
Communication occurs only in one direction, from the transmitter to the receiver. e.g television broadcasting and radio broadcasting.
Half-duplex system
Communication may occur in two directions but not simultaneously. Normally the receiver and the transmitter sharing the single channel and rely on the push-to-talk button. e.g walkie-talkie.

19. Introduction to RF engineering
Full-duplex system
The system allow simultaneous two-way transmission and reception through duplexing technique to avoid interference between transmitted and received signals. This can be realised using separate frequency bands for transmit and receive or by allowing users to transmit and receive only in certain predefined time intervals. e.g cordless phone and mobile phone.

20. Receiver Architectures
Receiver is the most important component of a wireless system and it should reliably be able to recover the desired signal from wide spectrum of transmitting sources, interference and noise.
The receiver must be sensitive enough to detect very low signal level, as low as -110 dBm, while not being overloaded by much stronger signals.
The well designed radio receiver must provide the following requirements:
High gain approximately 100 dB to restore received signal to its original baseband value.
Selectivity, in order to receive the desired signal while rejecting adjacent channels, image frequencies and interference.

21. Receiver Architectures
Down-conversion from received RF frequency to an IF frequency for processing.
Detection of received analog or digital information.
Isolation from transmitter to avoid saturation of the receiver.
The typical signal power level receive from the antenna may be as low as -100 to -120 dBm, so that the receiver may required to provide gain as high as 100 to 120 dB.
This much gain should be distributed over the RF,IF and baseband stage to avoid instabilities that can cause oscillation.

22. Receiver Architectures
It is a good practice to avoid more than 50 to 60 dB gain at the front end and the fact that the amplifier cost generally increases with frequency.
Downconverting from high frequency RF signal to a low frequency IF signal offer the advantages on:
Selectivity:
Selectivity can be obtained by using a narrow bandpass filter at the RF stage of the receiver, however, it is impractical to realise such a filter to meet the specified requirement at RF frequency. It is more effective to achieve selectivity using a sharp cutoff bandpass filter at the IF stage to select only the desired frequency band.
Cost effective
It is easier and less expensive to construct high-gain and stable amplifiers for low frequency signal.

23. Tuned Radio Frequency Receivers
The simplest radio receiver called a crystal set is useful to explain the basic principle of the radio receiver. As shown in Figure 12, a crystal set consisting of a tuned circuit, a diode (crystal) detector and earphones.
Figure 12

24. Tuned Radio Frequency Receivers
The antenna picks up the signal and causes current to flow in the primary winding of the coupling transformer and induces the voltage into the secondary.
The diode rectifies the signal and the capacitor C2 filters out the carrier leaving the audio signal which heard in the earphones.
Capacitor C1 parallel with the coupling transformer form the tuned circuit and can be used for frequency tuning.
The crystal set does not provide the selectivity and sensitivity, only the strongest signal can produce an output.

25. Tuned Radio Frequency Receivers
One of the earliest types of the commercial radio receiver is the tuned radio frequency receiver (TRF) as shown in Figure 13.
Figure 13

26. Tuned Radio Frequency Receivers
In the TRF the sensitivity has been improved by adding the RF amplification stages between the antenna and the detector. Detected audio was then amplified by the audio amplifier.
Another design improvement is that the RF amplifiers is a cascaded tuned circuit amplifier in which all the tuned circuit tuned to the same resonant frequency and overall selectivity is improved. The unwanted signal is attenuated by the first tuned circuit and further attenuated by the next tuned circuit.
The effect is to steepen the skirts of the overall frequency response of the receiver and the selectivity is improved.

27. Tuned Radio Frequency Receivers
The main problem of the TRF is the tracking of the tuned circuit, in which the tuned circuit must be made variable so that they can be set to the desired RF frequency.
In the early receivers, each tuned circuit had a separate capacitor and multiple dials need to be adjusted to tune in a signal.
The solution was to gang the capacitor and the tuning would be done simultaneously. This improved the performance but tracking errors still occurred because different capacitor caused slight deviation of the resonant frequency of each tuned circuit.
This problem is solved by connecting the trimmer in parallel with the tuning capacitors and a fine tuning adjustment can be performed.

28. Superheterodyne Receivers
The problem mentioned in the TRF receiver design led to the development of the superheterodyne receiver.
Heterodyne means to mix two frequencies together in a nonlinear device or to translate one frequency to another using nonlinear mixing.
Superheterodyne receiver is the most popular type of receiver used today.
A typical block diagram of the single conversion superheterodyne receiver is shown in Figure 14.

29. Superheterodyne Receivers
Figure 14

30. Superheterodyne Receivers
RF Amplifier:
The first element in the block diagram is an RF amplifier operating in the frequency range being received. The amplifier must have a reasonably low noise figure and enough gain to dominate the system noise.
The amplifier must operate in the linear region to minimise the intermodulation distortion as previously explained. Intermodulation distortion is a nonlinear distortion that increase the magnitude of the noise figure by adding correlated noise to the total noise spectrum.
Typical RF amplifier is a single transistor providing a voltage gain in the 10 to 30 dB range.

31. Superheterodyne Receivers
Bipolar transistor amplifier are used at the lower frequencies, while at VHF, UHF and microwave frequencies field effect transistor (FETs) are more preferred.
Mixer/Converter:
Mixer is a nonlinear device and it purpose is to convert RF signal to intermediate frequencies (RF to IF frequency translation).
The amplified RF signal from the RF amplifier mixed with the local oscillator frequency and produced low frequency IF signal.

32. Superheterodyne Receivers
The IF frequency is related to the RF and LO frequencies by:
fIF = fRF-fLO
The mixer also responds to an RF image frequency separated by twice the IF frequency:
fIM = fRF-2fIF
To ensure that the image frequency is outside the RF bandwidth of the receiver (filtering of image frequency will not affect the RF response), it is necessary to have:

33. Superheterodyne Receivers
Where BRF is the RF bandwidth of the receiver.
Although the carrier and the sideband frequencies are translated from RF to IF, the shape of the modulation envelope remains the same and therefore the original information contained in the envelope remain unchanged.
It is usually helpful to use an IF frequency less than 100 MHz because of component cost and availability considerations. Filter with reasonable size and good cutoff characteristic is easily obtained at this frequency. The most common IF frequency for the broadcast AM is 455 kHz and for the broadcast FM is 10.7 MHz.

34. Superheterodyne Receivers
Figure 15: NE602 commercial mixer

35. Superheterodyne Receivers
Local Oscillator:
Local oscillator (LO) in the superheterodyne radio receiver is made tunable so that its frequency can be adjusted over a desired receiving frequency band.
As a local oscillator frequency changed, the mixer translates an input frequency to the fix IF frequency.
The LO can be as simple as a tunable LC oscillator or frequency synthesizer.
Figure 16 shows a typical LC oscillator circuit that can be used as a LO. The circuit is a JFET Colpitts oscillator with a feedback made up of C5 and C6.

36. Superheterodyne Receivers
Figure 16: LC Colpitts oscillator circuit

37. Superheterodyne Receivers
The oscillator is set to the desired centre frequency by a coarse adjustment of trimmer capacitor C1 or adjustable slug-tuned ferrite core of L1. The main tuning is accomplished with variable capacitor C3. The tuning can be done mechanically or electrically if C3 is replaced by a varactor diode. The output of the oscillator is taken from the emitter follower buffers isolating the oscillator from load variations, which can change the frequency.
Most modern receivers used a frequency synthesizer as a local oscillator. Frequency synthesizer offer high degree of frequency stability and tuning is accomplished in incremental rather than continuous frequency change.

38. Superheterodyne Receivers
Intermediate Frequency section:
The IF section consists of a series of IF amplifiers and bandpass filters and often called the IF strip. Most of the receiver gain and selectivity is achieved in this section.
The IF centre frequency and bandwidth are constant for all stations and are chosen so that their frequency is less than any of the RF signals frequencies.
It is easier and less expensive to construct high gain and stable amplifiers for low frequency signals.
Therefore it is common to see a receiver with five or more IF amplifiers and a single RF amplifier or no RF amplification at all.

39. Superheterodyne Receivers
Figure 17: Commercial IF amplifier with built-in detector and audio pre-amp

40. Superheterodyne Receivers
Detector section:
Detector section is used to convert an IF signals back to the original source information.
The detector can be a simple diode detector in AM broadcasting receiver or as complex as a phase locked loop in FM receiver.
For a diode detector, the rectified signal is passed to the RC filter in which the capacitor charges quickly to the peak value of the carrier and discharges through the resistor when the carrier amplitude drop to zero.

41. Superheterodyne Receivers
Because of the time constant of the RC chosen to be long compared to the period of the carrier, the capacitor will discharge slightly before the next peak of the carrier.
The resulting waveform across the capacitor is a close approximation to the original modulating signal.
The recovered signal has a small amount of ripple on it, however because the carrier frequency is much higher than the modulating frequency, these ripple variations are not noticeable.

42. Superheterodyne Receivers
Figure 18: Typical AM diode detector

43. Superheterodyne Receivers
Figure 19: Amplitude Modulation detection

44. Direct Conversion Receivers
The direct conversion receiver are based on the superhet receiver topology, it uses a mixer and a LO to perform frequency downconversion with a zero-IF.
The LO frequency is set to the same frequency as the received RF frequency and then converted direct to the baseband signal.
For this reason, the direct conversion receiver sometime called a homodyne receiver or zero-IF receiver.
Direct conversion receiver is simpler and cost-effective since there is no IF amplifier, IF bandpass filter and IF local oscillator (in case of dual-conversion superhet).

45. Direct Conversion Receivers
Another important advantage of the direct conversion receiver is that there is no image frequency because the mixer difference frequency is effectively zero.
A big attraction of the direct conversion receiver is that the IF filter is replaced by the low pass filter. At baseband frequency, an active filter having greater selectivity and better gain can be designed.
A number of pager design and a single chip GSM receiver already use the direct conversion approach. Recently, this technique has been used in WCDMA-based WLL transmitter and receiver.

46. Direct Conversion Receivers
Figure 20: Direct conversion or homodyne receiver.

47. Direct Conversion Receivers
In spite of advantages, a serious disadvantage is the LO must have a very high degree of precision and stability to avoid drifting of received signal.
Self mixing comes from the leakage of LO signal to the mixer generates DC and saturating the following filter and gain amplifier. The leakage LO signal also can radiate through the receiving antenna violate spurious emission regulation. (LO radiation should be below 2nW or -57 dBm).
Flicker effect (1/f noise) from the mixer can be a problem in distinguishing between the original signal and the noise.