TAILIEUCHUNG - Nguyên tắc cơ bản của thiết kế mạch RF với tiếng ồn thấp dao động P5

Mixers are used to translate a signal spectrum from one frequency to another. Most modern RF/microwave transmitters, receivers and instruments require many of these devices for this frequency translation. The typical symbol for a mixer is shown in Figure | Fundamentals of RF Circuit Design with Low Noise Oscillators. Jeremy Everard Copyright 2001 John Wiley Sons Ltd ISBNs 0-471-49793-2 Hardback 0-470-84175-3 Electronic 5 Mixers Introduction Mixers are used to translate a signal spectrum from one frequency to another. Most modern RF microwave transmitters receivers and instruments require many of these devices for this frequency translation. The typical symbol for a mixer is shown in Figure . LO Figure Typical symbol for a mixer An ideal mixer should multiply the RF and LO signals to produce the IF signal. It should therefore translate the input spectrum from one frequency to another with no distortion and no degradation in noise performance. Most of these requirements can be met by the perfect multiplication of two signals as illustrated in equation V cosrn1t V2 cos 2t i 2 cos i a 2 t cos i -rn2 t Here it can be seen that the output product of two input frequencies consists of the sum and difference frequencies. The unwanted sideband is usually fairly easy to remove by filtering. Note that no other frequency terms other than these two are 236 Fundamentals of RF Circuit Design generated. In real mixers there are a number of compromises to be made and these will be discussed later. Mixing is often achieved by applying the two signals to a non linear device as shown in Figure . Non linear device lout V V V2 Figure Mixing using a non-linear device The non linearity can be expressed as a Taylor series lout Io aV t bV t 2 cV t 3 Taking the squared term bV V bV 2vy2 v It can immediately be seen that the square law term includes a product term and therefore this can be used for mixing. This is illustrated in Figure where the square law term and the exponential term of the diode characteristics are shown. Figure Diode characteristic showing exponential and square law terms Mixers 237 Note of course that there are other terms in the equation which will produce unwanted frequency .

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