The principle of signal mixing

Chapter The principle of signal mixing

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • number of possible states. The DSP circuit gets rid of confusion between digital states.The result is an output that is essentially free from interference.Digital signal processors are available from several commercial sources. They canbe installed in existing communications receivers.The benefits of DSP are improved signal-to-noise ratio, superior intelligibility, andenhanced fidelity or image clarity. In addition, DSP can make a CW, AM, or SSB receiverless susceptible to interference from atmospheric noise and ignition noise. This is be-cause a digital signal is as different from noise as a watermelon is from an apple.The principle of signal mixingA mixer is a circuit that combines two signals having different frequencies, producinga signal whose frequency is either the sum or the difference of the input frequencies.One of the signals is usually an unmodulated carrier, so that the mixer has the effect ofconverting a modulated signal at one frequency to a modulated signal at some otherfrequency. A mixer has two inputs and one output.Up conversion versus down conversionSuppose the signals to a mixer are at frequencies f l and f 2, with f 2 being the higher fre-quency. The mixer has a nonlinear active element, such as a diode or a class-B-biasedtransistor. The circuit generates new signals, in addition to passing the original two sig-nals. Outputs will appear at f l, f 2, f2 + f l, and f 2f l.A typical mixer circuit is diagrammed in Fig. 27-10. The output has a tuned circuitthat is set to either f 2 + f l or at f 2f 1. If the output is tuned to the sum of the two in-put frequencies, the output frequency, fout, is higher than either of the input frequen-cies. If the output is tuned to the difference frequency, the new signal is either inbetween the two inputs, or else lower than both.Let f 1 represent the frequency of a signal that you want to convert via mixing. Letf 2 be the signal from a local oscillator (LO). If the sum output frequency is used, youhave up conversion. If the difference frequency is used, and if f2 is selected so that foutis less than f 1, you have down conversion.A VLF/LF converterUp conversion is sometimes used for reception of very low-frequency (VLF) andlow-frequency (LF) radio (9 kHz to 300 kHz). The VLF or LF input is mixed with an LOto provide an output that falls within the range of a shortwave receiver, say over therange 3.509 MHz to 3.800 MHz. A block diagram of an up converter for VLF/LF is shownin Fig. 27-11A. The LO frequency is 3.500 MHz.This VLF/LF converter produces mirror-image output duplicates, one above3.509 MHz and the other from 3.200 to 3.491 MHz. Does this seem like a super-broad-banded AM signal, having an overall bandwidth of 600 kHz, with USB at 3.509 to3.800 MHz and LSB at 3.200 to 3.491 MHz? If you think so, you’re right. Up conver-sion produces AM. When you tune from 3.509 to 3.800 MHz, thereby hearing VLF/LFsignals from 9 to 300 kHz, you are actually listening to little “slices” of the USB of awideband AM signal.The principal of signal mixing511