http://tonnesoftware.com/appnotes/demodulator/diodedemod.html
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 ______________________________________________________________ Our Main Website: http://www.amfone.net AMRadio mailing list Searchable Archives: http://www.mail-archive.com/[email protected]/ List Rules (must read!): http://w5ami.net/amradiofaq.html List Home: http://mailman.qth.net/mailman/listinfo/amradio Help: http://mailman.qth.net/mmfaq.html Post: mailto:[email protected] To unsubscribe, send an email to [EMAIL PROTECTED] with the word unsubscribe in the message body.
