HF Receiver Using MC3362
The project was designed to provide a viable QRP Superhetrodyne (superhet) receiver for operations in the HF bands. The MC3362 from Motorola was designed as a single chip narrow band v.h.f. f.m receiver but has enough internal functions available at the pins to receive SSB. The present circuit is adapted from a project published in Practical Wireless, July 1996 with the heading "The Spart Transreceiver" by Rev. George Dobbs G3RJV. I claim no originality in the circuit and my only contribution to this circuit is developing a PCB for it. The following figure is the circuit diagram of the project.
The four diodes at the input form a protection circuit should the receiver be used in conjuction with a qrp transmitter. A two pole bandpass filter, around L1 and L2 selects the HF band frequency signals which are coupled , via C to the balanced mixer in the chip. The MC3362 performs all the receiver functions as far as the audio stages. The internal varicap local oscillator (l.o) is tunned for the HF band by using L3 and C as the tank circuit and R with R and R as the tunning control. The MC3362 provides a buffered output from the l.o on pin 20. This output can be used to drive a transmitter. IF selectivity is provided by a three pole 4.433 Mhz. crystal filter. The filter is of the ladder configuration, computer designed with butterworth coefficient for a nominal 500 hz bandwidth. The second l.o. (using pins3 and 4) is used for the beat frequency oscillator (b.f.o). A trimmer capacitor 'pulls' the crystal to vary the beat note.
The recovered audio (at pin 5) passes to Q1 (2N3819) , the audio muting part of the change-over system. The receiver is on in both receive and transmit modes so during transmit, the key mutes the audio output via Q1.
A conventional LM386 audio output circuit gives adequate loudspeaker output under quiet room conditions. The feedback filter provided by R/C reduces the high frequency noise often associated with the LM386.
A double sided glass epoxy pcb is available with me if you wish to try out the circuit. Send me a e-mail and I will send U the details.