MAIN TOPICS (Part I) • Introduction to Communication Systems • Filter Circuits • Signal Generation • Amplitude Modulation • AM Receivers • AM Transmitters
MAIN TOPICS (Part II) • Single-Sideband Communications Systems • Angle Modulation Transmission • Angle Modulated Receivers & Systems • Introduction To Transmission Lines & Antennas • Mobile Telecommunications
Elements of a Communication System • Communication involves the transfer of information or intelligence from a source to a recipient via a channel or medium. • Basic block diagram of a communication system: Channel Source Transmitter Receiver Recipient
Brief Description • Source: analogue or digital • Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna • Channel: e.g. cable, optical fibre, free space • Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer • Recipient: e.g. person, speaker, computer
Modulation • Modulation is the process of impressing information onto a high-frequency carrier for transmission. • Reasons for modulation: • to prevent mutual interference between stations • to reduce the size of the antenna required • Types of analogue modulation: AM, FM, and PM • Types of digital modulation: ASK, FSK, PSK, and QAM
BANDHz ELF 30 - 300 AF 300 - 3 k VLF 3 k - 30 k LF 30 k - 300 k MF 300 k - 3 M HF 3 M - 30 M BANDHz VHF 30M-300M UHF 300M - 3 G SHF 3 G - 30 G EHF 30 G - 300G Frequency Bands • Wavelength, l = c/f
Information and Bandwidth • Bandwidth required by a modulated signal depends on the baseband frequency range (or data rate) and the modulation scheme. • Hartley’s Law: I = k t B where I = amount of information; k = system constant; t = time available; B = channel bandwidth • Shannon’s Formula: I = B log2 (1+ S/N) in bps where S/N = signal-to-noise power ratio
Transmission Modes • Simplex (SX) – one direction only, e.g. TV • Half Duplex (HDX) – both directions but not at the same time, e.g. CB radio • Full Duplex (FDX) – transmit and receive simultaneously between two stations, e.g. standard telephone system • Full/Full Duplex (F/FDX) - transmit and receive simultaneously but not necessarily just between two stations, e.g. data communications circuits
Time and Frequency Domains • Time domain: an oscilloscope displays the amplitude versus time • Frequency domain: a spectrum analyzer displays the amplitude or power versus frequency • Frequency-domain display provides information on bandwidth and harmonic components of a signal
Non-sinusoidal Waveform • Any well-behaved periodic waveform can be represented as a series of sine and/or cosine waves plus (sometimes) a dc offset: e(t)=Co+SAn cosnw t + SBn sin nw t (Fourier series)
Effect of Filtering • Theoretically, a non-sinusoidal signal would require an infinite bandwidth; but practical considerations would band-limit the signal. • Channels with too narrow a bandwidth would remove a significant number of frequency components, thus causing distortions in the time-domain. • A square-wave has only odd harmonics
Mixers • A mixer is a nonlinear circuit that combines two signals in such a way as to produce the sum and difference of the two input frequencies at the output. • A square-law mixer is the simplest type of mixer and is easily approximated by using a diode, or a transistor (bipolar, JFET, or MOSFET).
Dual-Gate MOSFET Mixer Good dynamic range and fewer unwanted o/p frequencies.
Balanced Mixers • A balanced mixer is one in which the input frequencies do not appear at the output. Ideally, the only frequencies that are produced are the sum and difference of the input frequencies. Circuit symbol: f1 f1+ f2 f2
Equations for Balanced Mixer Let the inputs be v1 = sin w1t and v2 = sin w2t. A balanced mixer acts like a multiplier. Thus its output, vo = Av1v2 = A sin w1t sin w2t. Since sin X sin Y = 1/2[cos(X-Y) - cos(X+Y)] Therefore, vo = A/2[cos(w1-w2)t-cos(w1+w2)t]. • The last equation shows that the output of the balanced mixer consists of the sum and difference of the input frequencies.
Balanced Ring Diode Mixer Balanced mixers are also called balanced modulators.
External Noise • Equipment / Man-made Noise is generated by any equipment that operates with electricity • Atmospheric Noise is often caused by lightning • Space or Extraterrestrial Noise is strongest from the sun and, at a much lesser degree, from other stars
Internal Noise • Thermal Noise is produced by the random motion of electrons in a conductor due to heat. Noise power, PN = kTB where T = absolute temperature in oK k = Boltzmann’s constant, 1.38x10-23 J/oK B = noise power bandwidth in Hz Noise voltage,
Internal Noise (cont’d) • Shot Noise is due to random variations in current flow in active devices. • Partition Noise occurs only in devices where a single current separates into two or more paths, e.g. bipolar transistor. • Excess Noise is believed to be caused by variations in carrier density in components. • Transit-Time Noise occurs only at high f.
Noise Spectrum of Electronic Devices Device Noise Transit-Time or High-Frequency Effect Noise Excess or Flicker Noise Shot and Thermal Noises f 1 kHz fhc
Signal-to-Noise Ratio • An important measure in communications is the signal-to-noise ratio (SNR or S/N). It is often expressed in dB: In FM receivers, SINAD = (S+N+D)/(N+D) is usually used instead of SNR.
Noise Figure • Noise Factor is a figure of merit that indicates how much a component, or a stage degrades the SNR of a system: F = (S/N)i / (S/N)o where (S/N)i = input SNR (not in dB) and (S/N)o = output SNR (not in dB) • Noise Figure is the Noise Factor in dB: NF(dB)=10 log F = (S/N)i (dB) - (S/N)o (dB)
Equivalent Noise Temperature and Cascaded Stages • The equivalent noise temperature is very useful in microwave and satellite receivers. Teq = (F - 1)To where To is a ref. temperature (often 290 oK) • When two or more stages are cascaded, the total noise factor is:
High-Frequency Effects • Stray reactances of components(including the traces on a circuit board) can result in parasitic oscillations / self resonance and other unexpected effects in RF circuits. • Care must be given to the layout of components, wiring, ground plane, shielding and the use of bypassing or decoupling circuits.
Narrow-band RF Amplifiers • Many RF amplifiers use resonant circuits to limit their bandwidth. This is to filter off noise and interference and to increase the amplifier’s gain. • The resonant frequency (fo), bandwidth (B), and quality factor (Q), of a parallel resonant circuit are:
Narrowband Amplifier (cont’d) • In the CE amplifier, both the input and output sections are transformer-coupled to reduce the Miller effect. They are tapped for impedance matching purpose. RC and C2 decouple the RF from the dc supply. • The CB amplifier is quite commonly used at RF because it provides high voltage gain and also avoids the Miller effect by turning the collector-to-base junction capacitance into a part of the output tuning capacitance.
Wideband RF Amplifiers • Wideband / broadband amplifiers are frequently used for amplifying baseband or intermediate frequency (IF) signals. • The circuits are similar to those for narrowband amplifiers except no tuning circuits are employed. • Another method of designing wideband amplifiers is by stagger-tuning.
Amplifier Classes An amplifier is classified as: • Class A if it conducts current throughout the full input cycle (i.e. 360o). It operates linearly but is very inefficient - about 25%. • Class B if it conducts for half the input cycle. It is quite efficient (about 60%) but would create high distortions unless operated in a push-pull configuration.
Class C Amplifier • Class C amplifier operates for less than half of the input cycle. It’s efficiency is about 75% because the active device is biased beyond cutoff. • It is commonly used in RF circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the non-conducting portion of the input cycle.
Input fi Frequency Multipliers • One of the applications of class C amplifiers is in “frequency multiplication”. The basic block diagram of a frequency multiplier: High Distortion Device + Amplifier Tuning Filter Circuit Output N x fi
Principle of Frequency Multipliers • A class C amplifier is used as the high distortion device. Its output is very rich in harmonics. • A filter circuit at the output of the class C amplifier is tuned to the second or higher harmonic of the fundamental component. • Tuning to the 2nd harmonic doubles fi ; tuning to the 3rd harmonic triples fi ; etc.
Neutralization • At very high frequencies, the junction capacitance of a transistor could introduce sufficient feedback from output to input to cause unwanted oscillations to take place in an amplifier. • Neutralization is used to cancel the oscillations by feeding back a portion of the output that has the opposite phase but same amplitude as the unwanted feedback.
Review of Filter Types & Responses • 4 major types of filters: low-pass, high-pass, band pass, and band-reject or band-stop • 0 dB attenuation in the passband (usually) • 3 dB attenuation at the critical or cutoff frequency, fc (for Butterworth filter) • Roll-off at 20 dB/dec (or 6 dB/oct) per pole outside the passband (# of poles = # of reactive elements). Attenuation at any frequency, f, is:
Review of Filters (cont’d) • Bandwidth of a filter: BW = fcu - fcl • Phase shift: 45o/pole at fc; 90o/pole at >> fc • 4 types of filter responses are commonly used: • Butterworth - maximally flat in passband; highly non-linear phase response with frequecny • Bessel - gentle roll-off; linear phase shift with freq. • Chebyshev - steep initial roll-off with ripples in passband • Cauer (or elliptic) - steepest roll-off of the four types but has ripples in the passband and in the stopband
Low-Pass Filter Response Gain (dB) BW = fc 0 Vo Ideal -20 1 -20 dB/dec -40 -60 dB/dec 0.707 -40 dB/dec Passband -60 BW 0 f f fc fc 10fc 100fc 1000fc Basic LPF response LPF with different roll-off rates
High-Pass Filter Response Gain (dB) 0 Vo -20 1 -20 dB/dec -40 0.707 -40 dB/dec -60 dB/dec Passband -60 0 fc f 0.01fc 0.1fc fc f Basic HPF response HPF with different roll-off rates
Band-Pass Filter Response Centre frequency: Vout 1 Quality factor: 0.707 Q is an indication of the selectivity of a BPF. Narrow BPF: Q > 10. Wide-band BPF: Q < 10. BW f fc1 fo fc2 Damping Factor: BW = fc2 - fc1
Band-Stop Filter Response • Also known as band-reject, or notch filter. • Frequencies within a certain BW are rejected. • Useful for filtering interfering signals. Gain (dB) 0 -3 Pass band Passband f fc1 fo fc2 BW
Filter Response Characteristics Av Chebyshev Bessel Butterworth f
Damping Factor The damping factor (DF) of an active filter sets the response characteristic of the filter. Frequency selective RC circuit Vin Vout + _ R1 R2 Its value depends on the order (# of poles) of the filter. (See Table on next slide for DF values.) General diagram of active filter
Active Filters • Advantages over passive LC filters: • Op-amp provides gain • high Zin and low Zout mean good isolation from source or load effects • less bulky and less expensive than inductors when dealing with low frequency • easy to adjust over a wide frequency range without altering desired response • Disadvantage: requires dc power supply, and could be limited by frequency response of op-amp.