Chapter 9 - 1 Chapter 9 Mixers 2 Duplexer Phase Locked Loop...

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Unformatted text preview: 1 Chapter 9 Mixers 2 Duplexer Phase Locked Loop Oscillator Low noise amplifier Power amplifier Up- converter Down- converter Rx IF Tx IF Ctrl Bus 3 What’s Microwave Mixer z A nonlinear three-port circuit (two inputs and one output) that generates a output frequencies equal to the sum (or difference) of the two input frequencies or their harmonics as shown in Figure 1; z For down-converters, the two input ports are referred as RF and LO whereas the output is called the IF port; z For up-converters, the two input ports are referred as IF and LO whereas the output is called the RF port; z When a mixer is used as an up-converter in a transmitter, the “sum frequency” is utilized and the “difference frequency” is rejected; z When a mixer is used as an down-converter in a heterodyne receiver, the “difference frequency” is used and the “sum frequency” is filtered out; 4 IF LO RF r f i f o f Upper side band filter IF LO RF r f i f o f Lowpass filter G IF amplifier Figure 1. Frequency conversion process: (a) Up-conversion in a transmitter, (b) Down-conversion in a heterodyne receiver (a) (b) o i r f f f + = o r i f f f − = Upper side band-pass filter IF band-pass-filter or IF low-pass-filter 5 What Devices Generally, we have below three devices for mixers: 1. Schottky Diode; 2. Field Effect Transistor (FET); 3. Bipolar Transistor (BT). These devices are all nonlinear so that new frequencies can be generated. 6 Advantages of Mixer in Applications (1) Use of a mixer as a down-converter in a heterodyne receiver has several advantages: 1. The receiver can share the same IF channel for amplification, filtering, and demodulation; 2. The receiver can use the same IF band-pass-filter for channel selection; 3. At very high frequency (millimeter-wave and sub-millimeter wave), there is no amplifier or detector, mixer is the only device can operate at these frequencies, which converts the high frequency to low frequency so that can be manipulated by other devices. Figure 1 (c) The mixer for down converter. Explorer to High Frequency 7 (2) Use of a mixer as a up-converter in a transmitter 1. The receiver can share the same LO with receiver. 2. It is easy to change the transmitting channel by changing the LO frequency; 3. By employing a sub-harmonic mixer as an up-converter, a high transmitting frequency can be achieved by low frequency LO. Figure 1 (d) The mixer for up-converter. 8 Mixer Types In general, mixers utilize one or more nonlinear device(s), properly pumped with a relatively large signal (called LO), to mix with a small signal in order to generate new frequencies based on the sum and difference of the harmonics of the small signal and LO frequencies. The result can be simply written as: o r i n m ω ω ω ± ± = , where m and n are positive integers. (1) Point of Caution: The pumping signals (LO) should have a relatively large signal level to turn the device on and off (the most common devices are diodes), thereby causing the device to behave nonlinearly, which is...
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This note was uploaded on 05/01/2011 for the course ELECTRICAL EE5602 taught by Professor Xuequan during the Spring '11 term at City University of Hong Kong.

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Chapter 9 - 1 Chapter 9 Mixers 2 Duplexer Phase Locked Loop...

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