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Chapter 1

Chapter 1. Introduction to Communications Circuits. Introduction. Radio frequency integrated circuit (RFIC) is one of the fast developing research areas RFIC circuits were designed as discrete in the past and now integrated into single chip

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Chapter 1

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  1. Chapter 1 Introduction to CommunicationsCircuits

  2. Introduction • Radio frequency integrated circuit (RFIC) is one of the fast developing research areas • RFIC circuits were designed as discrete in the past and now integrated into single chip • RFIC finds wide use applications as in cordless phones, cell phones, WLAN, GPS systems remote tags, assets tracking, key less entry for cars, remote sensing and tuners in cable modem • The increasing interest in radio frequency (RF) communications has resulted in an effort to provide components and complete systems on an integrated circuit (IC) • Many researchers aimed at putting a complete radio on one chip which is called System On Chip (SOC)

  3. Introduction • SOC circuits can be realized using either complementary metal oxide semiconductor (CMOS) technology or bipolar transistor design • The CMOS technology has the advantage of lower power consumption and lower cost compared to bipolar technology • The bipolar technology have the advantage of being older than CMOS and therefore better modeled • The objective of radio communication system is transmit or receive a signal between a source and destination with acceptable quality and without incurring a high cost

  4. Lower Frequency Versus Radio Frequency Designs • At low frequency, regular circuit theory deign rules can be applied to RFIC design • As the operating frequency increase to the microwave region, the component dimensions became closer to the signal wavelength. • Therefore basic circuit design rules are no longer valid • Instead electromagnetic theory need to be employed when designing the RF circuits • The traces connecting between different circuit parts are treated as transmission lines • However advances in technology resulted in the manufacturing of very small dimensions circuit components (resistors, inductors, capacitors and transistors) • This advances in technology make it possible to deign RFIC with discrete components at RF frequency ranging from (0.1-5) GHz

  5. Impedance Levels for Microwave and Low-Frequency Designs • In the low frequency design, the input impedance is usually high • On the other hand the output impedance is usually low • For example a given op-amp circuit has almost an infinite input impedance while almost have zero output impedance • The impedance properties of the op-amp makes it good driving device for measurement equipment • On the other hand, if circuits are connected using transmission lines, an input and output matching circuits are required to mach the device I/O impedances of the transmission line

  6. Units for Microwave and Low-Frequency Analog Design • In microwave circuits, signals, noise, or distortion are measured with power • The typical unit of measure used is the decibels above 1 milliwatt (dBm) • Since infinite or zero impedance is allowed in RF circuits, power levels became meaningless • Therefore voltages and current are usually chosen to describe the signal levels • Voltage and current are expressed as peak, peak-to-peak, or root-mean-square (rms) • Power in dBm, PdBm, can be related to the power in watts, Pwatt , as shown in (1) • The voltages when computing power are assumed across 50 ohm resistors (1)

  7. Units for Microwave and Low-Frequency Analog Design • Assuming sinusoidal signal, the power in watt is given by • Where R is the resistance where the voltage is developed across • Vrms is related to the peak voltage according to

  8. Units for Microwave and Low-Frequency Analog Design • The following table lists different values for vpp, vrms and the associated power in what and in dBm across 50 Ω resistor

  9. Blocks of communication transceiver • Any communication transceiver is composed from the transmitter and the receiver

  10. popular receiver architectures • In the early communications system there were two different receiver architectures • The first was the tuned radio frequency receiver • The second was the super heterodyne receiver • The third type is the direct conversion receiver

  11. Tuned radio frequency receiver The tuned radio receiver is composed from three tuned amplifiers These three amplifiers are tuned to the desired signal frequency before the signal is fed to a detector The detector recovers the information signal from the carrier Tuned Radio receiver

  12. Tuned radio frequency receiver • The tuned radio frequency receiver has the disadvantage of tuning the three tuned amplifiers to the same carrier frequency • It was replaced by the super heterodyne receiver which has a better filter sensitivity

  13. The super heterodyne receiver The super heterodyne eliminates the need for tuning all the RF amplifiers to the frequency of the RF input It works by shifting the frequency of the RF input signal to the frequency of the receiver IF filter This means that a fixed frequency IF filter can built which has better performance compared to the variable tuned RF amplifiers Super heterodyne receiver

  14. The super heterodyne receiver The main advantage of the super heterodyne receiver over the TRF is that the same high quality filter can be used for all input signals The frequency selection is made by varying the local oscillator frequency The super heterodyne receiver suffers from the image frequency problem Super heterodyne receiver 14

  15. Image frequency • What is the image frequency? • When a signal with carrier frequency fc1 is fed to the input of a super heterodyne receiver then the frequencies appears on the IF stage are fIF=flo-fc1 and fIF=flo+fc1 (assuming that fc1 is less than the flo) • If another signal of frequency fc2 appears at the same time at the input of the mixer then • Another tow frequency components appears on the IF stage these are fIF=fc2-flo and fIF=fc2+flo (assuming that fc2 is greater than the flo) • By adding the equations fIF=flo-fc1 and fIF=fc2-flo • The result is 2fIF=fc2-fc1

  16. Image frequency The previous result shows that if fc2 is spaced in frequency 2fIF from fc1 then the mixer will bring both signals (fc1 and fc2 to the IF stage) This results in unwanted distortion in the Rx The signal with fc2 is called the image of the signal with frequency fc1 The purpose of the image reject filter will be to prevent such an action One of the solutions to this problem is to select the fIF>(fc2-fc1) Another solution can be realized by the use of tow mixers and tow different local oscillators 16

  17. Image frequency example Consider a receiver with the IF filter centered at 455 kHz. If it is desired to receive a 1 MHz input signal, the local oscillator is tuned at 1.455 MHz determine the image frequency that correspond t the 1 MHz signal. Solution The image frequency is determined from the relation fimage=fsignal+2fIF OR fimage=fIF+flo fimage=1 MHz+2*455 kHz or fimage=1.455 MHz+0.455 MHz =1.91 MHz

  18. The direct conversion receiver The direct conversion receiver is an immediate extension of the super heterodyne Rx In the direct conversion receiver the IF section is eliminated The receiver works by converting the input signal directly to current base band Direct conversion receiver 18

  19. The direct conversion receiver The conversion is done by setting the local oscillator frequency to the input signal frequency The mixer output contains signal at the base band frequency and another signal located at the base band frequency+2fLO (high frequency signal) The high frequency signal can be removed by using the LPF The advantage of this receiver is that the LPF filter is much easier to build compared with the IF band pass filter The disadvantage of this receiver is the local oscillator drift Direct conversion receiver 19

  20. The direct conversion receiver Also this receiver has a DC offset as an another problem Another problem with such receiver is that a complete synchronization between Tx and Rx local oscillator is needed Direct conversion receiver finds applications in many battery operated systems 20

  21. Transmitters • Transmitter does the following tasks • Modulates the information signal by the carrier • Does frequency up conversion • Amplify the signal using power amplifier • Finally route the signal to the antenna prior to transmission Direct conversion receiver

  22. A modernCommunications Transceiver • A typical block diagram typical super-heterodyne communications transceiver is shown below

  23. Reciever building blocks • The communication system is composed from a transmitter and a receiver as shown in the previous slide • The receiver is composed from the • Antenna • Preselect filter • Low noise amplifier • Image reject filter • Mixer • Frequency synthesizer (Local oscillator) • IF filter • Automatic gain control (AGC) unit • Analog to digital converter and DSP processing unit

  24. Transmitter building blocks • The receiver is composed from the following blocks • Base band modulation plus Digital to Analog (D/A) converter • Mixer • Frequency synthesizer (Local oscillator) • Power amplifier (PA) • Antenna

  25. Description of various transceiver blocks • The transmitter and the receiver are connected to a single antenna through a duplexer • The duplexer can be viewed as a switch or a filter depending the communication standard to be used • The pre-select filter removes the signals not in the band of interest • This may be required to prevent overloading of the (LNA) by out-of band signals • The LNA amplifies the input signal without adding much noise

  26. Description of various transceiver blocks • LNA is used in the first amplification stage to strengthen the weak signal detected by the receiver • The LNA does not add much noise to the amplified signal • The use of LNA reduces the effect of noise added to the signal by the other electronic components in the receiver • The image reject filter removes the image signals and the noise before the down frequency conversion stage

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