Operation
In the circuit, the grid of the detector (usually a low-mu or medium-mu triode) is connected to the secondary of the final RF or IF transformer through a capacitor (100 pF to 330 pF, with 250 pF being typical). A resistor of a few 100 kΩ – 10 MΩ is connected either in parallel with this capacitor, or from the detector's grid to ground. The resistor-capacitor combination, in concert with the nearly unidirectional current flow of the detector's grid-to-cathode (or grid-to-filament, if it's directly heated) circuit, forms a clamp circuit. The incoming modulated radio frequency signal voltage swings both positive and negative with respect to ground as it emerges from the final RF or IF stage. The detector's grid-to-cathode circuit will only conduct current when the grid is positive with respect to the cathode. The resulting current flow charges the capacitor to maintain a voltage which biases the detector grid negative with respect to its cathode. The amount of bias varies based on the received signal strength. The grid leak resistor - capacitor circuit is designed to have a time constant which is slightly slower than the rate of change of the audio program and much slower than the rate of change of the carrier. Because the bias varies depending on signal strength, plate current will rise on very low signals, and operation near cut-off of the tube will cause distortion. On receivers with battery powered heaters, the heater power supply could be used to bias the tube with a small positive voltage. The nearly unidirectional current flow of the received, modulated signal results in a separate audio signal that is said to be detected or demodulated. The triode detector tube also amplifies this audio signal, thereby providing two processes in one tube. Regenerative detectors often used grid leak, especially in the detection of amplitude modulated signals.
The grid leak detector circuit output, often taken from the plate of the detector tube, may require additional filtering to remove remnants of the carrier portion of the received signal. RF chokes and/or capacitors may be employed.
Read more about this topic: Grid-leak Detector
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