40 m Band Superhet Receiver

  
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By the addition of one integrated circuit and some passive components, I was able to convert my earlier developed 40-meter direct conversion receiver design into a superhet model. Anyone who has listened to 40 meters during the evening hours on the crowded 40 m band will appreciate the advantages of the single signal reception, which this circuit
40 m Band Superhet Receiver - schematic

offers. All simple direct conversion receiver designs suffer from degraded selectivity performance and are more subject to overload by contrasted to even modest superhet designs. Direct conversion designs are simpler by contrast because they are missing an IF stage and any associated filtering, a BFO and the second mixer. In order to modify the design to create a superhet model, it was necessary to increase the 1st oscillator frequency from 7000 kHz - 7040 kHz to a revised range of 10686 kHz -10731 kHz (operating frequency plus the intermediate frequency). The choice of this 1st oscillator frequency turned out to be fairly easy by considering the availability of 3686 kHz crystals. Bandwidth is set to 200 Hz by a series coupled filter (using a single 3686 kHz crystal) in the IF amplifier section. This choice is highly suitable for CW operation. The choice of 3686 kHz IF allows use of readily available 10. 7 mHz ceramic resonator in the 1st oscillator circuit. The first part of the circuit to be described is the 1st oscillator (main tuning). It is a variation of the Colpitts type oscillator. It uses a 10, 7 ceramic resonator as its frequency-determining component. Unlike a quartz crystal based VXO, the ceramic resonator Q1 results in a much broader tuning range (approximately 45 kHz) while still maintaining a desired level of frequency stability at the same time. A second ceramic resonator with a center frequency of 7020 kHz...



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