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  1. 1.1 Classification of Microwave Integrated Circuits
  2. 1.2 Microwave Circuits in a Communication System
  3. 1.3 Summary
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1.2 Microwave Circuits in a Communication System

Microwave integrated circuit classification has been discussed previously. The microwave integrated circuit was classified according to the method of implementing the planar transmission lines for the purpose of connecting active and passive devices. The functions of microwave integrated circuits vary greatly and we will now consider several important microwave integrated circuits, the designs of which will be discussed in later chapters. Some examples of these circuits are low-noise amplifiers (LNA), power amplifiers (PA), oscillators, mixers, directional couplers, switches, attenuators, and filters, among a host of other microwave-integrated circuits. Among these, directional couplers, switches, attenuators, filters, and so on, are basically passive microwave circuits, although they are very widely used. Thus, they are not covered in this book because they are considered to be outside its scope. In addition, although components such as switches, variable attenuators, phase shifters, and other control circuits are important and are composed of semiconductor devices, they are generally not regarded as the basic building blocks of a wireless communication system. Therefore, this book will only cover low-noise amplifiers, power amplifiers, oscillators, and mixers, which are the most widely used circuits in the construction of wireless communication systems. The basic design theory of these circuits as well as the devices related to them will be explained in this book.

As an example of a wireless communication system, Figure 1.7 shows a block diagram of an analog cellular phone handset (Rx frequency is 869–894 MHz and Tx frequency is 824–849 MHz).2 A general transceiver used for the transmission and reception of analog signals (usually voice) has a similar block diagram that is shown in Figure 1.7. A weak RF signal with a typical power level of about -100 dBm (0.1 nW) received from an antenna first goes through a filter called a diplexer and the signal is received only in the receiver frequency band. The filtered signal is too weak for direct demodulation or signal processing, and a low-noise amplifier (LNA) with a gain of 20–30 dB is required to amplify the received signal. Too much gain may cause distortion and an LNA with a gain of 20–30 dB is usually employed. Chapter 8 provides a detailed explanation of the design of an LNA.

Next, because the received signal frequency is so high, the first mixer shown in Figure 1.7 translates the carrier frequency to a lower frequency band called first IF (intermediate frequency). A double-conversion superheterodyne receiver is more widely used than a single-conversion super heterodyne receiver in a communication system. The filter in front of the first mixer again suppresses both the image frequency signal and other signals at the outside of the receiving frequency band. Since multiple users in service are using the same frequency band, multiples of other user signals generally coexist with the signal in the first IF. Intermodulations among the multiple signals are one of the crucial issues in mixer design. Chapter 12 describes the typical topologies of various mixers for suppressing such spurious signals. In order to filter out possible spurious signals that appear at the first mixer output, the signal is passed through a narrow bandpass filter that has a bandwidth of about the signal bandwidth. The first IF filter removes many unwanted spurious signals although it may not be completely sufficient. The first IF output is converted again through the second mixing. Now the center frequency of the second IF is low enough, the highly selective filter is available, and the spurious signals can be sufficiently suppressed through the second IF filter. In addition, the signal frequency is low enough and can be demodulated for the recovery of the original signal. The demodulator is an FM demodulator and is almost the same as the FM demodulator that is commercially popular.

Figure 1.7

Figure 1.7 A block diagram of an analog mobile phone handset (AMPS standard). Tx_EN stands for Tx enable and ALC stands for automatic level control. Tx_ and Rx_data are required to set the programmable frequency dividers in Tx and Rx synthesizers. LE stands for Load Enable. When LE is high, the digital channel data are loaded to the corresponding programmable frequency divider in PLL IC. Synthesizers are explained in Chapter 11. Lock signal indicates that the synthesizer using PLL is in a locked state.

Note that the mixer requires the input signal from a local oscillator (LO) for the translation of the signal frequency to the IF. The two LO signals are supplied from the two Rx-synthesizers and each Rx-synthesizer consists of a voltage-controlled oscillator (VCO) and a commercial PLL (phase-locked loop) IC (integrated circuit). Since the frequency of most VCOs is not stable enough to be used in such communication systems, the frequency of a VCO must be stabilized using a stable crystal oscillator (XO in Figure 1.7) with a typical temperature stability of 2 ppm (parts per million) and a phase-locked loop (PLL). Furthermore, the LO frequency should be moved up and down according to the base station commands. Such frequency synthesis and stabilization can be achieved by a phase-locked loop (PLL). To build a frequency synthesizer using PLL, the VCO frequency as well as the crystal oscillator frequency must be divided by appropriate programmable frequency dividers in the PLL IC. The signals CLK, Rx_ChDATA, Rx_ChLE, and Rx_Lock, shown in Figure 1.7, are the digital signals between the PLL IC and the system controller. The clock signal CLK is used for the timing reference signal that is generated by the system controller using the crystal oscillator. Rx_ChDATA sent from the controller represents the digital data to set the programmable frequency dividers. The signal Rx_ChLE selects the corresponding programmable divider for Rx_ChDATA to be loaded among several frequency dividers in the PLL IC. When phase lock is achieved, the PLL IC sends the signal Rx_Lock to the system controller to inform the phase lock completion. The two Rx synthesizers are necessary for the double-conversion superheterodyne receiver. The commercial PLL IC generally includes the necessary components to achieve the phase lock for two VCOs in a single PLL IC. Thus, the LO signal for the second conversion is similarly synthesized using a single PLL IC. The design of the Tx and Rx VCOs in Figure 1.7 as well as the other microwave VCOs are described in Chapter 10, while the PLL’s operation is explained in Chapter 11.

In the transmission operation, the modulation input signal (usually voice) goes to the modulation input of a Tx synthesizer. The Tx synthesizer is similarly composed of a VCO and a PLL IC. Through the PLL IC, the desired carrier center frequency is similarly synthesized as in the Rx synthesizer. The digital signals CLK, Tx_ChDATA, Tx_ChLE, and Tx_Lock are similarly interpreted as in the Rx synthesizer. The modulation signal has a generally higher frequency than the PLL loop bandwidth and thus can modulate a VCO without the effects of a PLL. Therefore, the frequency-modulated (FM) signal appears at the Tx synthesizer output with the synthesized carrier frequency. The modulated signal then passes through the bandpass filter that removes unnecessary or spurious signals. The average output power level of the modulated signal is generally low; thus, in order to obtain the desired RF power output level, the signal must be amplified by a power amplifier (PA) whose typical maximum output power level is about 1W. The function ALC (Automatic Level Control) is generally built in to control the transmitting power level. When a user is close to the base station, the transmitting power level is set to low; otherwise, it is set to high for a better quality of communication. The PA output signal is then passed through a diplexer without affecting the receiver and radiated via the antenna. A power amplifier is important in this type of communication system because it consumes most of the DC power supplied from a battery. Furthermore, because a power amplifier operates in large-signal conditions, significant distortion arises. In Chapter 9, we will discuss the design and linearity evaluation of a power amplifier.

Given the preceding discussion, the key circuits in building a communication system are a low-noise amplifier, a power amplifier, oscillators, and mixers. With that in mind, this book will discuss in detail the design and evaluation method of these circuits.

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