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IF unit for double-superhet with AM demodulator

If there is insufficient filtering in front of the first converter, radio receivers will easily be disturbed by image frequency reception. For simple superhets with a low IF of e.g. 455 kHz, a preselection with two HF circuits tuned to the input frequency is completely insufficient already in the upper SW range. Even worse, the conditions are for VHF receivers, e.g. for the 2m band. On the other hand, a very good IF selection can be achieved on an IF of 455 kHz even with simple LC circles, so that it is possible to build receivers with good selectivity. Furthermore, for intermediate frequencies around 455 kHz, cheap ceramic filters with even better selection properties are available. As a result, the use of the principle of double conversion may be advantageous.

With cheaper or many older receivers, you often find the dual conversion superhet principle with a first IF in the range around 10.7 MHz. But a lower first IF at e.g. 5.5 MHz allows to achieve a completely sufficient stability for AM and FM without the use of a crystal oscillator for mixing from the first to the second IF. With careful construction with temperature compensation and stabilized supply voltage, such a first IF, rather than at 10.7 MHz, also gives very usable results for SSB and CW. This principle used to be found on the first devices and assemblies from the manufacturer Semcoset. The circuit shown here works in this way. It is reduced to minimum effort and is well suited for experiments with the double super principle.

A similar self-oscillating mixer, as used to be found in the AM ranges of most transistorized radio receivers, converts the 5.5 MHz input signal to the second intermediate frequency of 455 kHz. There, the signal is amplified in two stages and then demodulated. With a suitable tuner and a following audio amplifier you get an AM receiver that can be designed about to receive aviation radio. For this purpose e.g. the RF unit of my dual conversion super for 144-146 MHz is suitable. The input selection in the range of 118 to 136 MHz for this should be tuned by means of a triple rotary capacitor synchronously with the oscillator (123.5 ... 141.5 MHz). To redimensionate the 10.7 MHz IF filter for 5.5 MHz, it is sufficient to connect a parallel capacitor with approx. 100 pF in parallel to the resonant circuit.

Similarly, the use in a 10m amateur radio receiver is conceivable. In this area work, even today occasionally stations in the operating mode AM, mostly from the US. With good propagation conditions, they can be heard in Europe with good field strengths and consequently also with simple receivers. For reception in FM mode, a suitable demodulator could be connected behind. How this is done can be found on my page elsewhere. Of course, the circuit presented here could also be used for a 2m amateur radio receiver.

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