Analog Communication - RECIEVERS

Information about Analog Communication - RECIEVERS

Published on August 10, 2014

Author: psureshvenugopal

Source: authorstream.com

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Analog Communication: Analog Communication Suresh P. Nair [AIE, ME, (PhD)] MIEEE Professor & Head Department of Electronics and Communication Engineering Royal College of Engineering and Technology Chiramanangad PO, Akkikkavu, Thrissur, Kerala, India Module 3: Module 3 RECEIVERS Topics to be covered: Topics to be covered Receivers for continuous wave modulation Tuned Radio Frequency (TRF) Receiver Superheterodyne Receiver Double conversion receivers Receiver specifications Frequency Translation - (2.4) Up Conversion & Down Conversion Frequency Division Multiplexing (FDM) – (2.5) FM stereo multiplexing – (Page: 124) Phase locked loop (PLL) operation – (2.14) Synchronous detection and Frequency synthesis FM Receiver Threshold effect General Recevers: General Recevers Receivers Tuned Radio Frequency (TRF) Receiver Composed of RF amplifiers and detectors. No frequency conversion It is not often used. Difficult to design tunable RF stages. Difficult to obtain high gain RF amplifiers Superheterodyne Receiver Downconvert RF signal to lower IF frequency Main amplifixcation takes place at IF Tuned Radio Frequency (TRF) Receiver: Tuned Radio Frequency (TRF) Receiver Composed of RF amplifiers and detectors. No frequency conversion Difficult to design tunable RF stages. Difficult to obtain high gain RF amplifiers Superheterodyne Receiver (Superhet): Superheterodyne Receiver (Superhet) Receiver not only performs Demodulation. Other main works of Receiver are: Carrier Frequency Tuning :- Selecting desired Station Filtering :- Separate desired signal from unwanted ones Amplification :- To compensate the loss of signal power in channel Superheterodyne Receiver (Superhet): Superheterodyne Receiver (Superhet) Demerit – Image interference Mixer as a Converter: Mixer as a Converter Mixer may be used a frequency converter Changes the selected RF frequency to the IF frequency using a tuneable LO signal. Mixers have spurious responses – image frequency, half the RF… LO can be above or below the RF IF can be above or below the RF Image Frequencies: Image Frequencies Image is normally 2x the IF away from the RF frequency On the same side as the local oscillator Image has a band of frequencies that corresponds to tuning range Choice of IF Frequency: Choice of IF Frequency Image is 2x IF away from the wanted frequency Larger IF frequency makes suppression of image easier Tuning range of receiver cannot cross the IF Hence HF receivers often have a very high 1 st IF, >60MHz Hence a second lower IF is often used – DUAL or DOUBLE CONVERSION High 1 st IF gives good image rejection Low 2 nd IF gives good selectivity Double Conversion Receiver: Double Conversion Receiver Receiver Parameters: Receiver Parameters Selectivity Sensitivity Dynamic Range Fidelity Insertion Loss Noise Power Noise Temperature Noise Figure/Factor Selectivity: Selectivity The selectivity of the receiver is its ability to receive the wanted signal and reject unwanted signals in adjacent channels. Receiver selectivity is normally quoted by means of a graph showing the output of the receiver in dB relative to the maximum output, plotted against the number of kHz off-tune, or by quoting some points on this graph. Selectivity is usually measured as a ratio in decibels ( dBs ), comparing the signal strength received against that of a similar signal on another frequency . Selectivity: Selectivity SF = Shape factor (unitless) B (-60 dB) = bandwidth 60 dB below maximum signal level B (-3 dB) = bandwidth 3 dB below maximum signal level Sensitivity: Sensitivity The sensitivity of a receiver is the minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal. It is Expressed in µV Sensitivity in a receiver is normally taken as the minimum input signal ( S min ) required to produce a specified output signal having a specified signal-to-noise (S/N) ratio and is defined as the minimum signal-to-noise ratio times the mean noise power. Sensitivity: Sensitivity Dynamic Range: Dynamic Range The dynamic range of a receiver is defined as the difference in decibels between the minimum input level necessary to discern a signal and the input level that will overdrive the receiver and produce distortion. Expressed in dB Three forms of distortion amplitude frequency phase Fidelity: Fidelity Fidelity is a measure of the ability of a communications system to produce, at the output of the receiver, an exact replica of the original source information. Insertion Loss: Insertion Loss Insertion Loss (IL) is a parameter associated with the frequencies that fall within the passband of a filter. It is defined as the ratio of the power transferred to a load with a filter in the circuit to the power transferred to a load without the filter Noise Power: Noise Power Thermal noise is spread more or less uniformly over the entire frequency spectrum. Therefore the amount of noise appearing in the output of an ideal receiver is proportional to the absolute temperature of the receiver input system (antenna etc) times the bandwidth of the receiver. The convention for the temperature of To is set by IEEE standard to be 290 0 K. Noise Temperature: Noise Temperature T = environmental temperature (Kelvin) N = noise power (watts) K = Boltzmann’s constant (1.38 10 -23 J/K) B = total noise factor (hertz) T e = equivalent noise temperature F = noise factor (unitless) Noise Figure / Factor (NF or F or Fn): Noise Figure / Factor (NF or F or F n ) Electrical noise is defined as electrical energy of random amplitude, phase, and frequency. It is present in the output of every radio receiver. The noise is generated primarily within the input stages of the receiver system itself. Noise generated at the input and amplified by the receiver's full gain greatly exceeds the noise generated further along the receiver chain. Noise Figure / Factor (NF or F or Fn): Noise Figure / Factor (NF or F or F n ) The noise performance of a receiver is described by a figure of merit called the noise figure (NF). where G = Antenna Gain Frequency Translation: Frequency Translation Frequency Translation: Frequency Translation Up Conversion Down Conversion Frequency Translation: Frequency Translation Frequency Division Multiplexing (FDM): Frequency Division Multiplexing (FDM) Frequency Stereo Multiplexing: Frequency Stereo Multiplexing Stereo multiplexing is a form of FDM To send 2 separate signals via same carrier To send 2 different elements of same program Vocal and accompanist of an orchestra Frequency Stereo Multiplexing – Transmitting section: Frequency Stereo Multiplexing – Transmitting section Frequency Stereo Multiplexing – Receiving section: Frequency Stereo Multiplexing – Receiving section Phase Locked Loop (PLL): Phase Locked Loop (PLL) PLL Consist of mainly 3 components: A Multiplier or Phase Comparator A Loop Filter A Voltage Controlled Oscillator (VCO) Phase Locked Loop (PLL): Phase Locked Loop (PLL) Phase Locked Loop (PLL): Phase Locked Loop (PLL) Phase Locked Loop (PLL): Phase Locked Loop (PLL) By taking the Fourier transform Suppose that we design G(f) such that FM Receiver: FM Receiver

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