Cathode rectification
The cathode rectification is a method for demodulation of amplitude-modulated signals.
construction
The circuit corresponds to a cathode follower whose output for the carrier frequency is (almost) short-circuited by a sufficiently large capacitor C k . The decisive factor for the function is the time constant T = R k C k :
- T must be much longer than the oscillation period of the carrier frequency.
- T must be much smaller than the oscillation period of the highest modulation frequency.
The function can be easily explained: The capacitor C k is charged very quickly when the grid becomes more positive due to the increased anode current. As soon as the grid voltage becomes more negative, the tube blocks and the capacitor slowly discharges through the resistor R k . The tube can advantageously be replaced by a junction field effect transistor with a considerably lower volume and energy requirement.
This circuit offers significant advantages over an envelope demodulator :
- In the absence of a rectifier diode, the threshold voltage also disappears . Therefore, smaller signals can be demodulated.
- Because grid current never flows, the high-frequency source (usually an oscillating circuit ) is not loaded. This increases the quality factor , which in turn reduces the bandwidth and increases the resonance voltage.
Individual evidence
- ^ Author collective, elektronikum, Deutscher Militärverlag , Berlin 1967
- ^ Conrad, Basic circuits of radio technology, Fachbuchverlag Leipzig, 4th edition, 1958