[AMRadio] Infinite impedance detector vs diode,

D. Chester k4kyv at charter.net
Fri Sep 5 01:33:18 EDT 2008


Interestingly, the above slew rate discussion explains the fallacy of using 
a diode in series with the modulation transformer secondary to prevent 
overmodulation of a plate-modulated final.

The exact same slew-rate phenomenon caused by the discharge rate at the 
output in a diode detector also occurs in a plate modulated final when a 
diode is placed in series with the modulation transformer secondary to 
achieve negative peak clipping.

Think of  "X" in figure 1 as the modulation transformer secondary in series 
with the rf final power supply.  D1 is the clipper diode, which may be 
placed in series with either the top end or the bottom end of the modulation 
transformer secondary.  C1 is the rf by-pass capacitor in the final, plus 
any additional capacitance in the splatter filter.  R1 is a resistor 
equivalent to the class-C final.

When a negative peak from the modulator exceeds the DC plate voltage on the 
final, thus causing overmodulation in the negative direction, the diode 
should cut off, but the discharge rate of the capacitances associated with 
any splatter filter circuitry and the RF by-pass capacitor in the final, is 
too slow to follow the instantaneous descent of the negative peak, so the 
diode becomes reverse biased and turns off prematurely. The voltage on the 
capacitors in the final continues to bleed off at a  slower rate while the 
negative peak from the now-disconnected modulator resumes its upswing, and 
the descending discharge voltage on the capacitor(s) eventually "collides" 
with the rising positive voltage output from the modulator.  This results in 
the identical sharp point on the tip of the wave form of the negative peak, 
as shown by the green trace in figure 9 and the red and black traces in 
figure 14.

Because of this, even with a good low pass splatter filter (or maybe a 
better term would be modulation transformer destroyer) between the 
modulation transformer and the final, this waveform distortion may cause the 
splatter to be worse than it would be if there were no series diode in the 
circuit and the final were simply allowed to be overmodulated.

Don k4kyv

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