Question on negative peak limiting

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AB2EZ:
All

Thanks for your inputs... which are very much appreciated.

During spare moments... when I don't want to think about other stuff... I've been thinking about some of the subtleties of a negative peak limiter. The fact that it is a non-linear circuit makes for a lot of subtle effects. Here are a few that might be important:

1. Diode switching transients

The diodes will generate some switching transients because the keep-alive diode won't turn on at exactly the same time, and with exactly the same speed as the other diode turns off (and vice-versa). Depending upon the details of the modulator (how the modulator responds to this change in load) and the capacitance across the output of the modulation transformer (see below)... this effect could add high frequency noise that is correlated with the modulation... and therefore perceived by others as "splatter". For some transmitters, the effect would be negligible. For other transmitters, the effect could be large. I think that this effect, if large enough, could also cause fireworks in some transmitters if one were to push the negative peak limiting too far.

2. Capacitive load on the modulation transformer

The capacitive load on the modulation transformer secondary will be lower during the keep alive period... because the plate choke r.f. bypass capacitor and the capacitance associated with the tank circuit (usually dominated by the plate DC blocking capacitor) at audio frequencies will not be across the modulation transformer during the keep alive period. Discontinuities in the load on the modulator (caused by discontinuities in the load on the modulation transformer secondary) can cause transients in the modulator that persist beyond the keep-alive period... and which are perceived as splatter.


For example, if the rf bypass capacitor on the modulated B+ side of the plate choke is .001 uF, and if the DC coupling capacitor leading to the output tank circuit is .0022 uF, (and assuming there is an RF safety choke at the output), then the total capacitance loading the modulation transformer secondary at audio frequencies is .0032 uF. At 5kHz, the impedance of this capacitance is ~ 10k ohms. Some transmitters have more capacitance bypassing the plate choke and/or have a larger DC blocking capacitor.

3. Is it necessary to keep a constant load on the modulator?

For some modulators, a discontinuity of the load on the modulator... during the keep alive period... will result in transients that persist beyond the keep alive period. In those cases, it is important to maintain a reasonably constant load on the modulator by using a resistor in the keep alive circuit that is matched to the load presented by the rf output stage during normal (non-keep-alive) periods.

However, some modulators... such as a modern audio power amplifier driving a backward-connected audio output transformer... may not produce much of a transient response if their load is removed during the keep alive period. They are more like ideal voltage sources. Therefore, the benefits of maintaining a constant load during the keep alive period may be much smaller in those cases.

Best regards
Stu

w4bfs:
hi Stu and others ... Astabula Bill reported to me that in his pair 4-250 mod by pair 833 transmitter that he had tried the all solid state 3 diode limiter and eventually had to replace one of the diodes with a thermionic rectifier (sorry can't remember tube type or placement) to get proper operation ... all who have talked with Bill running this circuit can attest to its effectiveness on the air ... seems that this circuit is placing similar turn on/off time constraints as a low freq switcher ... maybe FREDs or something similar are called for ...just my thinking ...73 ...John

k4kyv:
Try using type 836 rectifiers.  Those are high-vacuum equivalents to type 866's (not 866A's; they don't have the higher piv rating of the mv types) and have been abundant as WW2 surplus.  They should still be easy to find.  Some voltage drop will occur with high plate current, but this shouldn't be a problem when using those tubes with 100-watt class transmitters.

steve_qix:
Hmm.. As far as diodes switching, I believe the problem arises not necessarily in the turn ON time, but the turn OFF time...   Here again, we are dealing with relatively low frequencies (under 30kHz), and I presume the diodes can at least go this fast.

Now, if they can't, there are fast recovery, high voltage diodes available (and I have used some of them in negative peak limiters), and these definitely work.

Anyway, at audio I have to say I haven't seen any switching transients or other anomalies.

Regards,

Steve

kc2ifr:
Quote

I agree with Bill on processing at low level if If IF (and only if Cheesy  ) the transmitter is capable of passing a square wave (or close to one), and properly.

Steve,
Could u explain why this is? Im not talking about hard clipping.
Bill

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