Adcs Assignment #1

NUST SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE

ANALOG AND DIGITAL CONTROL SYSTEMS ASSIGNMENT #1 118-MEHRAN MUSTAFA-BEE 3A 5/4/2009

IQ MODULATOR One modulation technique that lends itself well to digital processes is called "IQ Modulation", where "I" is the "in-phase" component of the waveform, and "Q" represents the quadrature component. In its various forms, IQ modulation is an efficient way to transfer information, and it also works well with digital formats. An IQ modulator can actually create AM, FM and PM. (It is used in configurable radio). Figure-1: I-Q Modulator (Mixer, Combiner, Differential Input Buffer and Power amplifier

IQ modulators and IQ demodulators change the amplitude, frequency, or phase of a carrier signal in order to transmit information. IQ modulators split an incoming data stream into its in-phase (I) and quadrature (Q) components, mix the resulting signals with local oscillators that are 90º apart in phase, and then combine the outputs vectorially in a 0º mixer When a carrier is modulated with a waveform that changes the carrier’s frequency slightly, the modulated signal is treated as a phasor. It has both a real and an imaginary part, or an in-phase (I) and a quadrature (Q) part. A receiver is designed that locks to the carrier, and the information can be deciphered by reading the I and Q parts of the modulating signal. The information appears on a polar plot as in Fig.1 below. The I/Q plane shows two things: 1. What the modulated carrier is doing relative to the unmodulated carrier. 2. What baseband I and Q inputs are required to produce the modulated carrier. (a)

(b)

Figure-2: Unmodulated (a) and modulated (b) carrier. The positive I axis is arbitrarily chosen to represent 0 degrees relative to the unmodulated carrier. In part (a), since the plot of the modulated carrier is relative to the unmodulated carrier, an unmodulated carrier appears as a fixed vector along the positive I axis. In part (b), a modulated carrier at the same frequency as the unmodulated carrier but offset by 45 degrees appears as a fixed vector at 45 degrees.

To produce the carrier in Fig. 2b, equal dc values would be required at the I and Q modulator inputs. Assuming unity gain in the modulator, to produce a carrier of unity amplitude at 45 degrees, the I and Q inputs must both be dc values of Q dc = Idc = +0.707. The baseband inputs (those producing the information), must obviously vary over time, creating a difference between the modulated and unmodulated carriers. The modulator block diagram is shown below. The signal first goes through an A/D converter, is compressed, checked for errors and encoded, then sent through a filter to the IF and RF mixers:

Figure 3: IQ Modulator and transmitter chain. Baseband signal appears at left. Block "A" to the right of the A/D does compression and error-correction.

Applications of IQ Modulators IQ modulators are used in digital radios and also in reconfigurable radios1. The IQ modulation is also used in modems (Modulator Demodulator)

Collector-Injection Modulator The collector-injection modulator is the transistor equivalent of the electron-tube AM plate modulator. This transistor modulator can be used for low-level or relatively high-level modulation. It is referred to as relatively high-level modulation because, at the present time, transistors are limited in their powerhandling capability. As illustrated in the figure, the circuit design for a transistor collector- injection modulator is very similar to that of a plate modulator. The collector-injection modulator is capable of 100-percent modulation with medium power-handling capabilities. In the figure, the RF carrier is applied to the base of modulator Q1. The modulating signal is applied to the collector in series with the collector supply voltage through T3. The output is then taken from the secondary of T2. With no modulating signal, Q1 acts as an RF 1

IQ Modulators Advance Reconfigurable Radios by Eamon Nash, June 2006.

amplifier for the carrier frequency. When the modulation signal is applied, it adds to or subtracts from the collector supply voltage. This causes the RF current pulses of the collector to vary in amplitude with the collector supply voltage. These collector current pulses cause oscillations in the tank circuit (C4 and the primary of T2). The tank circuit is tuned to the carrier frequency. During periods when the collector current is high, the tank circuit oscillates strongly. At times when the collector current is small or entirely absent, little or no energy is supplied to the tank and oscillations become weak or die out. Thus, the modulation envelope is developed as it was in a plate modulator. As transistor technology continues to develop, higher power applications of transistor collector- injection modulation will be employed. Collector-injection modulation is one of the most commonly used types of modulation because the modulating signal can be applied in the final stages of RF amplification2. This allows the majority of the RF amplifier stages to be operated class C for maximum efficiency. The plate and collector-injection modulators also require large amounts of AF modulating power since the modulator stage must supply the power contained in the sidebands.

Base-Injection Modulator The BASE-INJECTION MODULATOR is similar to the control-grid modulator in electron-tube circuits. It is used to produce low-level modulation in equipment operating at very low power levels. In the figure, the bias on Q1 is established by the voltage divider R1 and R2. With the RF carrier input at T1, and no modulating signal, the circuit acts as a standard RF amplifier. When a modulating signal is injected through C1, it develops a voltage across R1 that adds to or subtracts from the bias on Q1. This change in bias changes the gain of Q1, causing more or less energy to be supplied to the collector tank circuit. The tank circuit develops the modulation envelope as the RF frequency and AF modulating frequency are mixed in the collector circuit. Again, this action is identical to that in the plate modulator. Because of the extremely low-level signals required to produce modulation, the base-injection modulator is well suited for use in small, portable equipment, such as "walkie-talkies," and test equipment3.

2

http://www.tpub.com/content/neets/14184/css/14184_70.htm

3

http://www.tpub.com/content/neets/14184/css/14184_72.htm

Emitter-Injection Modulator This is the transistor equivalent of the cathode modulator. The EMITTER-INJECTION MODULATOR has the same characteristics as the base-injection modulator discussed earlier. It is an extremely low-level modulator that is useful in portable equipment. In emitter-injection modulation, the gain of the RF amplifier is varied by the changing voltage on the emitter. The changing voltage is caused by the injection of the modulating signal into the emitter circuitry of Q1, as shown in the figure. Here the modulating voltage adds to or subtracts from transistor biasing. The change in bias causes a change in collector current and results in a heterodyning action. The modulation envelope is developed across the collectortank circuit.