As designers sought to optimise their crystal radio circuits, a variety of different designs appeared.
In reality there may have been little to choose between the different crystal radio circuits, but each one optimised the performance in a different way.
Basic crystal radio circuit
The circuits for crystal sets could be very simple. Some simply employed a tuned circuit, a crystal detector and a pair of headphones.
The circuit shown below shows the most basic form of crystal radio circuit. It consists of just four components: inductor and variable capacitor (to form the tuned circuit); diode (to act as the envelope detector); and headphones.
This circuit, whilst easy to make did not provide any matching for the antenna and therefore the performance would not have been as good as some.
In this circuit the combination L1 with the range of VC1 would have been chosen to cover the required frequency range, typically the medium wave band.
Originally the diode detector, D1, would have been a Cat’s Whisker assembly, but today a simple signal diode would suffice. Either a germanium diode or a Schottky diode would be ideal as these types have a lower turn on voltage than silicon devices. (silicon diode turn on voltage is around 0.6 V whereas both the germanium and Schottky diodes are around 0.2 – 0.3V).
Crystal radio circuit employing input transformer
In order to ensure the maximum signal transfer from one circuit to another it is necessary to match the impedance. In some crystal receiver circuits this was achieved simply by having an additional coil on the same former so that the often lower impedance of the antenna could be matched to the higher impedance of the crystal radio receiver itself.
This circuit can be seen to have an input matching transformer, T1. This provides an impedance step up to provide better matching for the antenna to the crystal radio circuitry.
Carborundum crystal radio circuit
Carborundum detectors were more reliable than the ordinary galena or other types available. The cat's whisker wire was generally of steel to allow more force to be applied to the carborundum. This made the junction less prone to vibration and also some impurities. This often required a different holder to that used for the other types of crystal.
However the junction did need some forward bias to enable it to operate to its best. This was applied by a batteries and the voltage level needed to be adjustable to ensure the best performance.
The use of carborundum in crystal radios tended to be reserved for the more advanced or specialist radios. As they needed batteries to bias the crystal into its operating region, it added additional cost because batteries were not cheap in the early 1920s and 1930s.
By Ian Poole
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