IN INTRODUCTION TO MOBILE RADIO RECEIVERS ECE 2526 – MOBILE COMMUNICATION Monday, 07 September 2020
FUNCTIONS OF A RADIO RECEIVER The main functions of a radio receiver are: 1. To intercept the RF signal by using the receiver antenna 2. Select the desired RF signal and reject everything else 3. Amplify the RF signal 4. Detect the signal and demodulate to yield the original baseband signal 5. Amplify the baseband signal
CLASSIFICATION OF RECEIVERS Radio Receivers Application Operating Principle AM Radio FM Radio Radar Television Communication Tuned Radio Frequency Super heterodyne Receivers
TUNED RADIO FREQUENCY (T (TRF) RECEIVER Receiver Antenna BASEBAND AMPLIFIER RF STAGE DEMODULATOR/DETECTOR To Display/ Loud Speaker BASEBAND AMPLIFIER RF STAGE DEMODULATOR Baseband signals amplified before being Two or Three RF Amplifiers Signal is demodulated from the RF displayed or fed to a loud speaker frequency to the baseband frequency Contains: input/output tuned circuits and RF amplifiers
DRAWBACKS OF THE TRF RECEIVER/01 1. TRF receivers tend to oscillate due to a small part of the output leaks back to the input. • This is due to the high gain and operation at the same frequency. Intra-stage Inter-stage Coupling Coupling • Avoiding this problem requires great care to shield and decouple each stage from all of the others. X10 X10 X10 • The effects of the feedback worsens as the operating frequency increases, compounding Signal strength, the difficulties of constructing sensitive, 1 𝜈𝑊 adjustable TRF receivers that will operate without oscillation over large frequency ranges.
DRAWBACKS OF THE TRF RECEIVER/02 2. TRF receivers have low selectivity, i.e ability to select the desired signal from the unwanted signal. • This is because the passband of the receiver is broad 3. TRF receivers have higher bandwidth variation over the tuning range.
GENERAL RADIO RECEIVER REQUIREMENTS/01 1. Because the typical signal power level from the receive antenna may be as low as -100 to -120 dBm, the receiver may be required to provide gain as high as 100 to 120 dB. 2. This much gain should be spread over the radio frequency and baseband stages to avoid instabilities and possible oscillation. 3. It is generally good practice to avoid more than about 50 dB gain at any one stage.
GENERAL RADIO RECEIVER REQUIREMENTS/01 1. Good selectivity can be obtained by using a narrow Band Pass Filter (BPF) at the RF stage of the receiver. • However, the bandwidth and cut-off requirements for such a filter are usually impractical to realize at RF frequencies. 2. It is more effective to achieve good selectivity by: a) Down-converting a relatively wide RF BW around the desired signal, and b) Using a sharp cut-off BPF at the Intermediate Frequency (IF) stage to select only the desired frequency band.
GENERAL RADIO RECEIVER REQUIREMENTS/01 • Full-duplex communications systems usually use separate frequency bands for transmit and receive , thus avoiding the difficult (but not impossible) problem of isolating incoming and outgoing radiation at the same frequency. • It is often preferred to use a single antenna for both transmit and receive. In this case it is necessary to use a duplexing filter to provide isolation between the Tx and Rx, while still providing a signal path with the antenna.
RADIO RECEIVER WITH IN INTERMEDIATE FREQUENCY (I (IF) STAGE IF AMPLIFIER DETECTOR/ BASEBAND RF AMPLIFIER MIXER STAGE DEMODULATOR AMPLIFIER STAGE To Display/ Loud Speaker OSCILLATOR
DIR IRECT CONVERSION RECEIVER/01 1. Direct Conversion receiver (also called homodyne receiver) uses a mixer and Local Oscillator to perform frequency down-conversion with a zero IF frequency. 2. Its features are: a) Uses two stage amplifiers b) No need for IF amp and filter c) No need for extra circuit for AM demodulation d) No image filter required e) High stable LO source required. 3. Homodyne receiver is often used with Doppler radars, where the exact receiver Local Oscillator frequency (LO) can be obtained from the transmitter.
DIR IRECT CONVERSION RECEIVER/02 1. Direct-conversion receivers convert an RF signal to a 0-IF signal in one stage. 2. Apart from Radar, they are used in low-cost solutions requiring few components and lend themselves to integrated-circuit (IC) designs. LNA Low Noise Amplifier
SUPER HETERODYNE RECEIVER 1. The superheterodyne (short for supersonic heterodyne) receiver was first evolved by Major Edwin Howard Armstrong, in 1918. 2. It was introduced to the market place in the late 1920s and gradually phased out the TRF receiver during the 1930s. 3. Features of super heterodyne receiver are: a) A midrange IF allows the use of sharper cut-off filters for improved selectivity, and higher IF gain through the use of an IF amplifier. b) Tuning is conveniently accomplished by varying the frequency of the Local Oscillator so that the IF frequency remains constant. 4. Superhetrodyne receivers are used in a majority of broadcast radios and televisions, radar systems, cellular telephone systems, and data communications systems.
SIN INGLE CONVERSION SUPERHETERODYNE The lower frequency difference component called the intermediate RF signal is mixed with a frequency (IF), is separated from the local oscillator signal to other components by fixed tuned amplifier stages produce sum and difference frequency
MULTIPLE CONVERSION SUPERHETERODYNE 1. Receivers in HF, VHF, and UHF bands usually employ two (or more) stages of frequency conversion. 2. The lowest frequency IF channel provides the selectivity or bandwidth control that is needed. 3. The highest frequency IF channel is used to achieve good Image rejection.
FREQUENCY FIL ILTERING Filtering is required in a superheterodyne receiver to provide 1. interference rejection, 2. image rejection, 3. selectivity, 4. suppression of Local Oscillator radiation
FIL ILTERS IN IN SUPERHETERODYNE RECEIVER Image reject filter IF Filter Pre-select filter Used reduce the effect of Sets the overall noise bandwidth • Rejects out-of-band interference possible harmonic distortion of the receiver, as well as thus preventing strong from the RF amplifier removing most unwanted mixer interference signals from products. saturating the RF amplifier or mixer. • Low-insertion loss
LOCAL OSCILLATOR SIG IGNAL LEAKAGE 1. Local Oscillator (LO) signal lies in the RF pass band of the Receiver, and may pass back through the RF stages to be radiated by the antenna. 2. LO signal leakage usually minimized by: 1. combined attenuation of the preselect and image reject filters, 2. LO-RF isolation, 3. the reverse attenuation of the RF amplifier .
RECEIV IVER SENSITIV IVITY • Receiver sensitivity describes how well a receiver can process very weak input signals. • It is usually quantified as the weakest signal level that a receiver can detect to meet a given requirement, such as a specified signal-to- noise and distortion ratio or bit-error-rate (BER).
SENSITIVITY AND NOISE/01 • Thermal noise represents the fundamental limit on achievable signal sensitivity. It is a result of the vibrations of conduction electrons and holes due to their finite temperature. • Power delivered by a thermal source into a load is defined as: P = kTB where: k = Boltzmann’s constant (1.38 x 10 -23 Joules/K); T = temperature in degrees Kelvin (K); B = noise bandwidth. • The standard source noise temperature, or To, is 290° K. Therefore , the thermal noise generated in a 1-Hz bandwidth is: 𝑂 𝑝 = 𝐿𝑈 = 290 × 1.38 × 10 −23 = -174 dBm/Hz
MIN INIMUM DETECTABLE SIG IGNAL 1. Reliable communication requires a receive signal power at or above a certain minimum level, which we call the minimum detectable signal (MDS). 2. MDS determines the minimum SNR at the demodulator for a given system noise power.
MIN INIMUM DETECTABLE POWER
RECEIV IVER VOLTAGE SENSITIVITY OR SENSITIVITY The Minimum Detectable power, 𝑇 𝑗𝑛𝑗𝑜 can be converted to a minimum detectable signal voltage, for a given receiver input impedance
RECEIV IVER DYNAMIC RANGE Rx dynamic range is given by: The range depends on noise, modulation scheme, and required minimum SNR. The maximum allowable signal power could alternatively be defined by the third-order intercept point, P3, at the input to the receiver, as this would be the maximum input power before intermodulation distortion becomes unacceptable.
AUTOMATIC GAIN IN CONTROL/01 1. There is always need for about 80-100 dB of receiver gain to raise the minimum detectable signal to a usable level of approximately 10 mW (about 1 V peak at 50 ohm) . 2. This gain is usually placed at the IF stage because: a) Amplifiers and other components are cheaper at lower frequencies. b) High input signal levels may exceed the P1dB,or IP3, of the front-end components if the gain of the early RF stages is too high. c) Moderate level of gain at the RF stage sets a good NF for the Rx system.
AUTOMATIC GAIN IN CONTROL/02 1. The power gain through the receiver must vary as a function of the input signal strength in order to fit the input signal range into the baseband processing range, for a wide range of input signal levels. 2. This variable-gain function is accomplished with an automatic gain control (AGC) circuit. 3. AGC is most often implemented at the IF stage.
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