Diodes for crystal receivers

  
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Diodes with a low voltage drop, also have a high reverse current (leaking current), this will load the detector circuit heavier, the Q of the detector circuit reduces, and with that also the voltage across the LC circuit. At a lower input voltage the diode will give much more losses, and it can happen that despite the lower voltage drop of the
Diodes for crystal receivers - schematic

diode, you have less voltage at the load resistor. The diode resistance RD of germanium diodes is most times rather low, and only useable in crystal receivers with a low Q (low sensitivity and low selectivity). If the input voltage is high enough (well above the voltage drop of the diode at 1 mA), the output voltage of the diode will be about proportional to the input voltage. In the square law region the output voltage of the diode will be much lower then the input voltage, the diode gives much power loss between input and output. Via this link you find a measurement on several schottky diodes, which shows detection in the square law region takes place at input voltages below 200 mVpp. In the following table some types of schottky diodes, with the values for n, Is and Rd, the maximum reverse voltage and the diode capacitance at zero voltage. Between the two connections of the diode there will be a certain capacitance (capacitor value), when this capacitance is fairly high (e. g. 10 pF) the tuning range at high frequencies is limited.



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