If a low-pass filter is used after the series-diode clipper, the filter
itself will generate phase shift distortion, resulting in a tilt in the
waveform. A simple pi section (constant-K) filter produces less phase shift,
but the rolloff may not be sufficient to reduce the splatter to an
acceptable level. An "M-derived" filter produces a better cutoff, but more
phase shift.
It looks to me like Steve's (WA1QIX) circuit is a variation of the old
series-diode limiter, which simply uses a diode in series with the
modulation transformer secondary (or mod reactor). It has the addition of
the "keep alive" circuit to allow adjustment the clipping point to slightly
less than 100% to avoid overshoot, and maintains the correct load on the mod
transformer during clipped peaks.
There is a major flaw in the old series diode/splatter filter method that is
usually overlooked. It involves "slew rate" of the waveform following the
clipper. I'll try to explain it non-mathematically. Beyond a certain
frequency the low-pass filter, by definition, tries to limit the rate of
change of the instantaneous voltage feeding the final. (The higher the
frequency, the greater the rate of change of instantaneous voltage of any
a.c. waveform and thus the modulated B+ voltage). The problem with the
series clipper lies with the negative half of the audio cycle, during which
the instantaneous voltage is decreasing towards zero.
Beyond a certain frequency, the modulated B+ as it exits the modulation
transformer, is decreasing at a faster rate than the splatter filter will
allow it to drop. In other words, the stored energy in the filter circuit
tries to keep the instantaneous voltage at the filter more positive than the
instantaneous voltage output from the mod transformer/clipper circuit. The
result is that the clipping diode cuts off, allowing the mod transformer
output voltage to drop precipitously, while the B+ line to the final is
temporarily floating free, and as the energy stored in the filter section
decays, the instantaneous voltage to the final is decreasing at a slower
rate than the instantaneous voltage output from the mod xfmr. As the mod
xfmr output voltage bottoms out at the crest of the negative peak, the B+
line voltage is still decreasing. As the instantaneous voltage from the mod
xfmr begins to rise after passing the crest of the negative peak, a point is
reached where the mod xfmr output voltage equals the B+ line voltage (which
is still decreasing as the filter energy continues to decay). At that point
the clipper diode turns back on, and the B+ line once again follows the mod
xfmr output. The result of all this is that the modulated waveform at the B+
line to the final has a sharp peak at the crest of the negative peak, at a
point somewhat less than 100% negative modulation, and the "pointy" negative
peak is lopsided since the upswing of the peak is more rapid than the
downswing; this represents a discontinuity of the rate of change of the
waveform, exactly as occurs with overmodulation. Even though the negative
modulation is limited to less than 100%, splatter still occurs at the point
where the still-decaying B+ line voltage "crashes head-on" with the rising
mod xfmr output voltage.
From my experience, this is readily observable with an o'scope envelope
pattern. While the series diode negative peak clipper may reduce splatter
under severe overmodulation conditions, it actually INCREASES splatter on
negative cycles that would normally approach, but not exceed 100%
modulation.
Don k4kyv
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