Simple Shortwave Superhet With Regenerative IF
A Google Image search for Direct Conversion receivers using the NE602 (NE612 SA612) double balanced mixer will reveal numerous, perhaps over 100 similar schematics designed for the 80 to 10 meter amateur bands. Kits used to be sold by Vectronics and Ramsey.
I was never happy with their performance and for this reason I built a simple superhet using an LM386 audio amplifier as a regenerative IF detector-amplifier.
Using 10 turns on a ferrite rod as a preselector enables the receiver with the values shown to use both high side and low side injection to tune from 3.4 MHz to 10.2 MHz in 2 bands.
No external antenna is necessary for casual listening and selectivity and performance are comparable to my best commercial portable shortwave receivers using their built in whip antennas.
I use it most evenings to listen to SSB on 80 and 40 meters up and down the East coast of the U.S.
The IF transformer is a red medium wave local oscillator transformer which in my case is tuned to 1.7 MHz. A variable local oscillator frequency of 5.1 to 8.5 MHz permits 2 band coverage using both low side and high side injection by tuning the front end preselector variable capacitor. Image rejection is good and by properly arranging the IF and LO frequency values there is little chance of overlap. Unlike a pure regenerative receiver, a superhet using a regenerative IF permits tuning across the bands without having to vary the regeneration control.
The LM386 audio amplifier is set up as a high gain regenerative RF envelope detector which when oscillating allows the reception of CW and SSB signals. Although adding varactor fine tuning would be an asset, the regeneration control varies the receive frequency slightly and is useful to fine tune SSB reception.
The photo shown below is that of a modified QRP Kits EASY Direct Conversion receiver which has a varactor fine tuning control and after modification uses the original audio volume control for regeneration.
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Link to EASY receiver construction and schematic:
https://www.qrpkits.com/files/EZRX20170527.pdf" -="window.open(this.href);return false;
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