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Old 9th Nov 2017, 6:13 pm   #25
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Infinite Impedance detectors.

Quote:
Originally Posted by kalee20
With an infinite impedance detector, AC/DC load is not an issue as long as there is a sufficient cathode current drain to allow the cathode capacitor voltage to decay between HF cycles so that it can always follow downward modulation.
The capacitance is still being discharged by a resistor, so it still suffers from the negative peak problem. The valve cannot do anything further beyond being cutoff. AC/DC load is less of an issue because the II detector cathode circuit can use lower impedances than a diode detector, but it is not a non-issue.[/QUOTE]

With the diode detector, at the modulation trough of 100% modulation is negative audio peak, you have vanishingly small voltage for the resistor to discharge the capacitor, so the capacitor voltage has great trouble following the modulation envelope.

But with the II detector, a typical valve may have its cathode sitting at 5V or so above chassis. So the cathode resistor always has some voltage across it to discharge the capacitor. A signal will take it more positive, and at modulation peak it can be quite a lot more positive, but it will never go less than this quiescent value. Get the values right, and you can be assured of following the modulation envelope. You can even use a constant-current cathode load if you want.

So, yes it's not a non-issue, but while the diode inherently can't be made distortion-free from this point of view, the II can. (It still leaves other forms of distortion, of course!)
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