Two Thresholds Progressive Negative Peak limiter with ALC

[IN3IEX]

Dear AMers, after more efforts to improve the original circuit, I included in the HV ALC circuit the progressive negative peak limiting concept as found in this forum.

http://www.ing.unitn.it/~fontana/AM%20ALC.pdf

I am very happy with it. With no feedback it is a very good negative peak limiter. With feedback it is a very efficient and simple ALC.
With feedback I can talk at "any" distance with a hand microphone and fixed mic gain with always near 100% modulation, with no transistor or IC in the circuit....

If the feedback system is overloaded for any reason, the progressive peak limiter takes control and fixes the situation. Obviously the overall dynamic range is the summation of the dynamic ranges of the two systems taken separately.

I hope it will give good results to all those will try it.

Giorgio

[WD5JKO]

Georgio,

  This is an exciting development! I wonder about your modulator, and what the rise in P-P load impedance does when that diode gets backbiased.

   I was thinking about what you've done, and I wonder about a change. The change would base the LED current from the upward modulation peak instead of the downward modulation peak. So here if the modulator could peak 130% upward before overload, then adjust the LED/LDR to reduce the gain before the overload, and then rely on the two stage negative peak limiter to do its job on the downward modulation.

  The approach would result in more distortion, but less than you might think. It also would boost your talk power considerably which would enhance the communication effectiveness. There would be a positive carrier shift as well.

Keep up the good work Georgio!
CIAO!!

Jim
WD5JKO

[IN3IEX]

Hmmm....

Going high voltage to positive peaks with the LED is possible (it is indeed a high voltage photocoupler) but I need a high voltage threshold more than two times the HV supply. Maybe using a diode voltage tripler on the same HV transformer? As soon as you confirm that it is possible to have that voltage, just let me know and I will do a run with the simulator.

Giorgio

[KZ5A]

I am interested in adapting this circuit for use in my BTA 1R1 conversion.  There are a couple of areas where my meager knowledge is not up to the task, that I could use some help with.   The first is just determining appropriate values for the various R's as appropriate for 3200 VDC plate voltage.  The other is figuring out how/where to apply the feedback considering the there is no single ended circuit or gain pot to work with.  Any suggestions or comments will be greatly appreciated.

[KZ5A]

Cockpit trouble.... the schematics didn't remain attached.

73 Jack KZ5A

[IN3IEX]

6400 V peak is not within my expertise.
The schematic is not complete, it seems that there is a modulation choke somewhere.
I need help from someone that has successfully built a negative peak limiter for such a transmitter, then we can start designing the ALC over it.
Alternatively we could start with a big voltage divider (by three) feeding my circuit, using only the ALC function - no peak limiting.
You will certainly use a mike preamplifier for it. The photoresistor could be located there, in a high impedance low voltage audio circuit.

Let's see what happens.

Giorgio

hmm: there might be an error in the schematic: near the lower 4-400A, IR327 should be connected to IC604, not to the grid of the upper 4-400A. What do you think?

[KZ5A]

Giorgio,

I also think the R326/R327 wiring in the schematic is wrong, however the TX is actually wired that way.  I will be putting it back together the way I think it should have been, resistor to each grid from the common C.   

There is a 50H modulation reactor.  I attached the remaining piece of the drawing.

My rack gear is solid state and probably surface mount, it might could be mod'ed but not by me.

One thought I'm mulling over is to use the varistor to pull down the screen voltage on the 2E26 drivers to reduce gain.  Any thoughts on that?

73 Jack KZ5A

[KB5MD]

Remove the connection of IR326 from the junction of 1R503 & IC604, place IR326 into the grid lead of the upper 4-400, then connect the junction of IR326 and IR327 to the junction of IC604 and IR327.  This modification will keep  IR326 from running hot.
  I questioned the correctness of the wiring also and tried changing it.  It works fine and without the heating of IR326.

[Opcom]

Giorgio,

Do you mean for the six 1N4007's to provide some voltage drop across them, to keep the LED ready and conducting at 0.1mA during silence?