[AMRadio] Infinite impedance detector vs diode,


John Coleman jc at pctechref.com
Fri Sep 5 10:04:47 EDT 2008


I agree Don:
	The series diode thing hit me wrong when I was 18 years old and I
have seen patterns on the scope of resultant audio after over modulation and
it ain't pretty.  

	Besides the sharp change in waveform creating audio harmonics,
another big problems of over modulation is ringing of the modulation circuit
during the time interval of the over modulation.  During this time interval
there is no load for the modulator and the transformer and chokes will ring
("undamped wave train caused by resonates circuits with very little load"
for those not familiar with the term).  It seems to me that adding another
filter circuit to the equation might only cause more ringing.  One of the
best ways to stop this is a shunt diode circuit from the final modulation
point.  The cathode of the diode goes to the final modulation point where it
is reverse bias by the DC on the final.  The anode goes to a resistor
approximately equal to Ep/Ip of the final (although not critical).  The
other end of the resistor goes to ground.  The diode stays reverse biased
most of the time. But on occasional over modulation intervals, when the
plate of the final goes to negative voltage on audio negative peaks, the
diode will conduct and the audio load is switched from the RF final to the
resistor that is in series with the diode.  The modulation circuit is never
without a load.  The diode of course, most be rated at more than 2 times the
final supply voltage.  If it is a tube rectifier, high vacuum types are best
rather than mercury vapor or gas because the later has a threshold of turn
on voltage.  There are a number of SS HV diodes available that have
extremely high PIV and low forward drop.  A meter or LED detection circuit
can be added to show over modulation.      

John, WA5BXO




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