Looking at the SA612

The SA612 is a wonderful circuit! It combines a balanced mixer with an oscillator. Its internal circuits is this:



Internally the circuit has three Bias circuits which set it up in the right conditions. It has an oscillator at pins 6 & 7, inputs at pins 1 & 2, and outputs at pins 4 & 5. The inputs are grounded through 1k5 resistors and the outputs are loaded by 1k5 resistors also. This makes the whole circuit extremely easy to use with minimal external components.

First the oscillator. This has two pins, 6 & 7. To use this as an oscillator external components have to be connected like this:



The inputs are on pins 1 & 2, and are balanced, but can be used a single end inputs by connecting the unused one to ground via a small capacitor, like this:



The outputs on pins 4 & 5 are also balanced, but may be used unbalanced like this:



To complete the circuit just add power and ground, pins 8 & 3:



Now to connect some external circuits, for example to make an RF mixer. Here the input comes from a crystal filter and the output is audio frequency, this would be suitable for a PSK32, SSTV or SSB detector.



The crystals "X" have the same frequency, e.g. 14070 kHz for PSK31 or 14230 kHz for SSTV reception. The oscillator is pulled to the upper sideband by the 30pF trimmer. The RF input would typically come from a preamplifier which we will add next, and the Af output would go to an AF amplifier (see previous posting for an AGC controlled amplifier).



CORRECTION: The 2K2 resistor in the collector of the BF245A should be 1K

The SA612 needs a 5V supply, so a 7805 regulator (TO92 size is OK) is used. So there we have it, correct use of the SA612 to make simple 14MHz, fixed frequency, receiver. Here's the complete circuit, now to get the veto board wired and try it...