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25-11 Block diagram of a direct-conversion receiver.
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changes in frequency. The incoming signal is first passed through a tunable, sensitive front end. The output of the front end is mixed with the signal from a tunable, unmodulated LO. Either the sum or the difference signal is amplified. This is the first intermediate frequency (IF), which can be filtered to obtain a high degree of selectivity. If the first IF signal is detected, the radio is a single-conversion receiver. Some receivers use a second mixer and second LO, converting the first IF to a lower-frequency second IF. This is a doubleconversion receiver. The IF bandpass filter can be constructed for use on a fixed frequency, allowing superior selectivity and facilitating adjustable bandwidth. The sensitivity is enhanced because fixed IF amplifiers are easy to keep in tune. A superheterodyne receiver can intercept or generate unwanted signals. False signals external to the receiver are called images; internally generated signals are called birdies. If the LO frequencies are carefully chosen, images and birdies do not cause problems during ordinary operation of the receiver. Figure 25-12 is a block diagram of a generic single-conversion superheterodyne receiver. (Individual receiver designs vary somewhat.) Here s what each stage does. Front end: The front end consists of the first RF amplifier, and often includes LC bandpass filters between the amplifier and the antenna. The dynamic range and sensitivity of a receiver are determined by the performance of the front end. Mixer: A mixer stage converts the variable signal frequency to a constant IF. The output is either the sum or the difference of the signal frequency and the tunable LO frequency. IF stages: The IF stages are where most of the gain takes place. These stages are also where optimum selectivity is obtained. Detector: The detector extracts the information from the signal. Common circuits are the envelope detector for AM, the product detector for SSB, FSK, and CW, and the ratio detector for FM. Audio amplifier: Following the detector, one or two stages of audio amplification are employed to boost the signal to a level suitable for a speaker or headset. Alternatively, the signal can be fed to a printer, facsimile machine, or computer.
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25-12 Block diagram of a single-conversion superheterodyne receiver.
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In a superhet, the stages preceding the first mixer must be designed so they provide reasonable gain, but produce as little noise as possible. They must also be capable of handling strong signals without desensitization (losing gain), also known as overloading.
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Preamplifier All preamplifiers operate in class A, and most employ FETs. An FET has a high input impedance that is ideally suited to weak-signal work. Figure 25-13 shows a simple RF preamplifier circuit. Input tuning reduces noise and provides some selectivity. This circuit produces 5 dB to 10 dB gain, depending on the frequency and the choice of FET. It is important that the preamplifier be linear, and that it remain linear in the presence of strong input signals. Nonlinearity results in unwanted mixing in RF amplifiers. The mixing products produce intermodulation distortion (IMD), or intermod. That can wreak havoc in a receiver, producing numerous false signals. It also degrades the S/N ratio by generating hash, the result of complex mixing of many false signals over a wide range of frequencies. The Front End At low and medium frequencies, there is considerable atmospheric noise, and the design of a frontend circuit is simple. Above 30 MHz, atmospheric noise diminishes, and the main factor that limits the sensitivity is noise generated within the receiver. For this reason, front-end design becomes increasingly critical as the frequency rises through the VHF, UHF, and microwave spectra.
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428 Wireless Transmitters and Receivers
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The front end, like a preamplifier, must be as linear as possible; the greater the degree of nonlinearity, the more susceptible the circuit is to the generation of mixing products. The front end should also have the greatest possible dynamic range.
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Preselector The preselector provides a bandpass response that improves the S/N ratio, and reduces the likelihood of receiver overloading by a strong signal far removed from the operating frequency. The preselector provides image rejection in a superheterodyne circuit. Most preselectors have a 3-dB bandwidth that is a few percent of the received frequency. A preselector can be tuned by means of tracking with the tuning dial, but this requires careful design and alignment. Some receivers incorporate preselectors that must be adjusted independently of the receiver tuning. IF Chain A high IF (several megahertz) is preferable to a low IF (less than 1 MHz) for image rejection. But a low IF is better for obtaining good selectivity. Double-conversion receivers have a comparatively high first IF and a low second IF to get the best of both worlds. Intermediate-frequency amplifiers can be cascaded with tuned-transformer coupling. The amplifiers follow the mixer and precede the detector. Double-conversion receivers have two chains of IF amplifiers. The first IF chain follows the first mixer and precedes the second mixer, and the second IF chain follows the second mixer and precedes the detector. The selectivity of the IF chain in a superheterodyne receiver can be expressed mathematically. The bandwidths are compared for two power-attenuation values, usually 3 dB and 30 dB. This gives an indication of the shape of the bandpass response. The ratio of the 30-dB selectivity to the 3-dB selectivity is called the shape factor. A rectangular response is desirable in most applications. The smaller the shape factor, the more rectangular the response.
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