1. 1 Chapter 1 Introduction to Communications Circuits

2. 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. 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. 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  Therefore basic circuit design rules are no longer valid  Instead electromagnetic theory need to be employed when designing the RF circuits  The traces connecting between different circuit parts are treated as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. 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. 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. 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. 8 Units for Microwave and Low- Frequency Analog Design  The following table lists different values for v pp, v rms and the associated power in what and in dBm across 50 Ω resistor

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

10. 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 11 Tuned Radio receiver

12. 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 13 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 14 Super heterodyne receiver

15. 15 Image frequency  What is the image frequency?  When a signal with carrier frequency f c1 is fed to the input of a super heterodyne receiver then the frequencies appears on the IF stage are f IF = f lo -f c1 and f IF = f lo+ f c1 (assuming that f c1 is less than the f lo )  If another signal of frequency f c2 appears at the same time at the input of the mixer then  Another tow frequency components appears on the IF stage these are f IF = f c2 -f lo and f IF = f c2 +f lo (assuming that f c2 is greater than the f lo )  By adding the equations f IF = f lo -f c1 and f IF = f c2 -f lo  The result is 2f IF =f c2 -f c1

16. 16 Image frequency  The previous result shows that if f c2 is spaced in frequency 2f IF from f c1 then the mixer will bring both signals ( f c1 and f c2 to the IF stage)  This results in unwanted distortion in the Rx  The signal with f c2 is called the image of the signal with frequency f c1  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 f IF >( f c2 - f c1 )  Another solution can be realized by the use of tow mixers and tow different local oscillators

17. 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 f image =f signal +2f IF OR f image =f IF +f lo f image =1 MHz+2*455 kHz or f image =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 18 Direct conversion receiver

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+ 2f LO (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 19 Direct conversion receiver

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. 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. 22 A modern Communications Transceiver  A typical block diagram typical super- heterodyne communications transceiver is shown below

23. 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. 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. 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. 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