The product detector

Chapter The product detector

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • A UHF/microwave converterDown conversion is often used to allow reception of ultra-high-frequency (UHF) andmicrowave signals (above 300 MHz). The UHF or microwave input is mixed with an LOto provide an output that falls within the tuning range of a shortwave VHF receiver. Ablock diagram of a down converter for UHF/microwave reception is shown in Fig.27-11B.This converter has an output that covers a huge band of frequencies. In fact, a sin-gle frequency allocation at UHF or microwave might be larger than the entire frequencyrange of a shortwave receiver. An example is a UHF converter designed to cover 1.000GHz to 1.100 GHz. This is a span of 100 MHz, more than three times the whole range ofa shortwave radio.To receive 1.000 to 1.100 GHz using a down converter and a shortwave receiver, theLO frequency must be switchable. Suppose you have a communications receiver thattunes in 1-MHz bands. You might choose one of these bands, say 7.000 to 8.000 MHzand use a keypad to choose LO frequencies from 0.993 GHz to 1.092 GHz. This will pro-duce a difference-frequency output at 7.000 to 8.000 MHz for 100 segments, each 1MHz wide, in the desired band of reception.If you want to hear the segment 1.023 to 1.024 GHz, you set the LO at 1.016 MHz.This produces an output range from 1023 − 1016 = 7 MHz to 1024 − 1016 = 8 MHz.The product detectorFor the reception of CW, FSK, and SSB signals, a product detector is generally used. Itworks according to the same basic principle as the mixer. The incoming signal combineswith the signal from an unmodulated local oscillator, producing audio or video output.512 Data reception27-10A passive mixer.