10m input unit using tuned resonant circuits
The input circuit shown was part of a DIY radio for the 10m amateur band. It provided very useful properties in terms of sensitivity, intermodulation, mis-reception, and image-rejection, even better as many commercially available devices. Even with an IF of 455 kHz, a image rejection could be achieved, as is normally sufficient. With the specified IF of 10.695 MHz, an almost complete suppression of image frequencies was achieved.
As shown in the diagram, the link filter was designed as a preselector, wich adapts to the frequency, controlled by the main tuning. The required control voltage was generated by a potentiometer mechanically, coupled to the variable capacitor of the tuning VFO. By interconnecting two capacitance diodes, the risk of additional intermodulation was reduced by the tuning circuits.
The RF preamp worked in a brigde circuit, an intermediate circuitry between grounded base and grounded emitter circuit. In such an arrangement, power adaption and noise adation fall together. With the input circuit adjustment to maximum signal at the same time best signal-noise ratio can be achieved. The mixer with the dual-gate MOSFET produce little additional noise. Such circuits also have a much better large-signal behavior, such as mixer circuits with a bipolar transistor.