RADIO RECEIVERS ECE 3103 WIRELESS COMMUNICATION SYSTEMS

RADIO RECEIVERS ECE 3103 WIRELESS COMMUNICATION SYSTEMS

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 Operating Principle Application AM Radio FM Radio Radar Television Communication Tuned Radio Frequency Receivers Super heterodyne

TUNED RADIO FREQUENCY (TRF) RECEIVER Receiver Antenna RF STAGE DEMODULATOR/DETECTOR BASEBAND AMPLIFIER To Display/ Loud Speaker RF STAGE Two or Three RF Amplifiers Contains: input/output tuned circuits and RF amplifiers DEMODULATOR Signal is demodulated from the RF frequency to the baseband frequency BASEBAND AMPLIFIER Baseband signals amplified before being displayed or fed to a loud speaker

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. Inter-stage Coupling Intra-stage Coupling X10 X10 X10 Avoiding this problem requires great care to shield and decouple each stage from all of the others. Signal strength, 1μV The effects of the feedback worsens as the operating frequency increases, compounding the difficulties of constructing sensitive, 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 INTERMEDIATE FREQUENCY (IF) STAGE RF AMPLIFIER STAGE MIXER IF AMPLIFIER STAGE DETECTOR/ DEMODULATOR BASEBAND AMPLIFIER To Display/ Loud Speaker OSCILLATOR

OPERATION OF A MIXER A mixer is basically a down-frequency translator in a radio receiver. Down converter Up converter Asin(2πf 1 t) Asin 2πf 1 t Bsin(2πf 2 t)= AB 2 cos 2π(f 1 f 2 ) cos 2π(f 1 + f 2 Bsin(2πf 2 t) Amplitude AB 2 f 1 f 2 f 1 + f 2 frequency

DIRECT 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.

DIRECT 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.

SINGLE CONVERSION SUPERHETERODYNE RF signal is mixed with a local oscillator signal to produce sum and difference frequency The lower frequency difference component called the intermediate frequency (IF), is separated from the other components by fixed tuned amplifier stages

MULTIPLE CONVERSION SUPERHETERODYNE 1. Receivers in HF and the VHF 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 FILTERING Filtering is required in a superheterodyne receiver to provide 1. interference rejection, 2. image rejection, 3. selectivity, 4. suppression of Local Oscillator radiation

FILTERS IN SUPERHETERODYNE RECEIVER Pre-select filter Rejects out-of-band interference thus preventing strong interference signals from saturating the RF amplifier or mixer. Low-insertion loss Image reject filter Used reduce the effect of possible harmonic distortion from the RF amplifier IF Filter Sets the overall noise bandwidth of the receiver, as well as removing most unwanted mixer products.

LOCAL OSCILLATOR SIGNAL 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.

RECEIVER SENSITIVITY 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-tonoise 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: N o = KT = 290 1.38 10 23 = -174 dbm/hz

MINIMUM DETECTABLE SIGNAL 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.

MINIMUM DETECTABLE POWER

RECEIVER VOLTAGE SENSITIVITY OR SENSITIVITY The Minimum Detectable power, S imin can be converted to a minimum detectable signal voltage, for a given receiver input impedance

RECEIVER 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 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 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.

IMPLEMENTING AGC 1. AGC consists of a variable voltage controlled attenuator or variable gain amplifier (VGA) with a detector to convert a sample of the IF voltage to a DC value. 2. The rectified signal is then compared with a reference level, and passed through a LPF to provide a time-constant long enough to avoid having the AGC following low-frequency components of the modulated signal.

Variable gain amplifier (VGA) Tracks the signal strength in order to fit the input signal range into the baseband processing range, for a wide range of input signal levels. DC AMP Sets threshold to avoid having the AGC following low-frequency components of the modulated signal

WHAT IS THE AGC DOING?

DUPLEXING Duplexer must be used to allow both the Tx and Rx to be connected to the antenna, while preventing the transmit signal from directly entering the receiver. Isolation between the Tx and Rx is required to be greater than 100 db.

HALF DUPLEX Half-duplex 1. If transmission and reception are not simultaneous, then T/R switch is used. 2. Diode switch can operate in microseconds and offer 40 db isolation, limiters and filters to increase isolation.

FULL DUPLEX To transmit and receive at different frequencies, Bandpass Filters are required for duplexer. The function of the filter is to: a) Provide some preselected filtering on receive. b) Attenuate spurious out-of-band signals from the transmitter. Duplexing filters often have insertion losses on the order of 1-3 db which degrades the Noise Figure of the Receiver.

DIPLEXER Vs DUPLEXER A diplexer is a three-port network that splits incoming signals from a common port into two paths depending on frequency. These frequency bands are usually wide apart. Example. Separation EGSM and GSM1800 Tx-Rx isolation is usually in the range 50-60 db. Smart phones always use the LC Diplexer. A duplexer uses single antenna for both Tx and Rx at the same time. Both Tx and Rx paths have freq. bands very nearer, hence narrow BPF are used. Example: GSM uplink and downlink. Two types of duplexer, i.e one by using PIN diode switches and the other using circulators. Tx-Rx isolation is usually in the range 90-95 db. Smart phone always use the SAW Duplexer

FM RECEIVER 1. Radio frequency range 88MHz - 108 MHz 2. Radio Station frequency 88 + n 75 KHz, n=1,2,3.. 20 3. Sensitivity - 10 μv 4. Intermediate Frequency (IF) 10.7 MHz f IF 10.7 MHz 88 108 MHz

AM RADIO RECEIVER 1. Radio frequency range 535KHz - 1600KHz 2. Radio Station frequency 535 + n 10 KHz, n=1,2,3.. 3. Intermediate Frequency (IF) 455 KHz

ANALOGUE TV RECEIVER TUNER TV Receiver tuner amplifies the RF signal picked up by the antenna and converts the carrier frequencies and their associated bands into the intermediate frequencies and their sidebands

The standard intermediate frequencies for the 625-B system are- Picture IF = 38.9 MHz, Sound IF = 33.4 MHz

GSM Transmitter/Receiver (Showing only Receive) LNA 900-MHz RF input is filtered, amplified and applied to the first stage mixer. Transmit/Receive Oscillator Usually controlled by the 26MHz crystal Frequency Synthesizer 26MHz Crystal 1 st Mixer LO must have a range of 640 MHz to 675 MHz in order to cover the RF input band, i.e 890 915 MHz. f m = 240 229.3 MHz = 10.7 MHz