The AM Forum
March 29, 2024, 05:18:00 AM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: 1 2 [3]   Go Down
  Print  
Author Topic: Negative peak limiter revisited - Better SOLUTION FOUND - will keep this  (Read 54855 times)
0 Members and 1 Guest are viewing this topic.
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8308



WWW
« Reply #50 on: January 08, 2012, 01:07:55 AM »

A sure way to destroy your modulation transformer.

Like the old spatter filters but faster!

Those wanting to avoid solid state could try mercury vapor tubes. There was an article using 866's it posted here in the past, and someone had found an adapter and shown pictures of it. But I agree that peak limiting should be done before the modulator.

It looks like negative peak limiters/transformer loaders/keep alives should be intended as a safety valve, not a controller.

A problem I had with keep-alive experiment was that when the modulation ws forced negative enough to activate it, it created some splatter as the modulated B+ transitioned from the downward-slope of the modulating wave to hit the hard floor of the keep-alive voltage. The only way I can describe it is that when the switching point was crossed, that little 'corner' in the waveform was full of high frequencies. Maybe I did it wrong but I couldn't avoid it.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
Gito
Guest
« Reply #51 on: January 08, 2012, 01:51:00 AM »

Hi


Yes,when I read this article ,i have the same opinion,but when i reread it  
,with no load the modulation transformer has a high peak voltage developed,/transient voltage

but does not the audio choke inductance  ,"makes" a load /blocks  the transient voltage ,since the choke is connected between the Mod transformer and the RF transmitter tube.


Gito.n
Logged
Opcom
Patrick J. / KD5OEI
Contributing
Member
*
Offline Offline

Posts: 8308



WWW
« Reply #52 on: January 08, 2012, 02:26:12 AM »

If a filter is used, yes the transients will be prevented from reaching the RF amplifier.

In my experiments I did not use a low pass filter because of opinions here that the transient voltages can possibly damage the modulation transformer.
Logged

Radio Candelstein - Flagship Station of the NRK Radio Network.
Gito
Guest
« Reply #53 on: January 08, 2012, 02:44:31 AM »


Yes ,You are right,but is it possible that ,this audio low pass filter (limiting the audio to 6 khz maybe)also absorb this transient voltage .
limiting the transient voltage develop across the modulation transformer,since the L blocks the high frequency(high frequency load)  and the C bypass the high frequency to ground.

Gito

Logged
WD5JKO
Member

Offline Offline

Posts: 1996


WD5JKO


« Reply #54 on: January 08, 2012, 10:23:52 AM »


A problem I had with keep-alive experiment was that when the modulation ws forced negative enough to activate it, it created some splatter as the modulated B+ transitioned from the downward-slope of the modulating wave to hit the hard floor of the keep-alive voltage. The only way I can describe it is that when the switching point was crossed, that little 'corner' in the waveform was full of high frequencies. Maybe I did it wrong but I couldn't avoid it.

Patrick,

   You might remember when you and I were doing modeling of the three diode circuit, and variations thereof. I was using a LPF for the reasons you and Gito have mentioned. When using SS diodes, the reverse recovery time is a consideration as well. When I actually ran a multi-diode circuit years ago, I did have a LPF, and splatter was not an issue even at 200% positive peaks. I did use HV fast diodes surplus from HV switching power supplies.

  More recently, I developed a Ultra-Mod circuit for the SS Retro-75 XCVR. Here I was able to get by with only one diode, and still had a keep alive supply which acted somewhat as a LPF as well. This arrangement with a larger Mod transformer, allowed clean peaks to 150% while limiting the negative peaks to just under 100%. I attach that circuit here. I wonder if this idea would be workable when scaled to high impedance tube circuitry.

  Also, the idea to process the audio prior to going in the rig might work when the modulator is transparent input to output. Broadcast transmitters might do this, as well as some of the Class E setups. On the other hand, take the average Ham transmitter like a Viking II, DX-100, etc. where we have multiple transformers in the audio chain, along with frequency limiting (hi and lo) R-C circuits. Here we simply cannot gain much from upstream processing, especially if we wish to enhance the positive peaks by embracing asymmetrical modulation. Therefore we must 'process' the audio at a point where there will be little to no phase shift, or level shift following the processing point in the audio chain. With amateur transmitters like the ones I mentioned, the only place to minimize the phase shift, and level shift problems is AFTER the modulation transformer.

Jim
WD5JKO



* Retro75_Neg_Peak_Limiter.jpg (119.25 KB, 838x387 - viewed 1116 times.)
Logged
Gito
Guest
« Reply #55 on: January 08, 2012, 09:07:10 PM »

Hi

I am thinking about the transient voltage that is developed across the mod.Transformer,Yes there must be a transient voltage if the peak modulation goes below zero.

But there's a difference with loaded and unloaded Mod.Transformer.
When a transient voltage appear (positive pulse) it  is absorb/loaded by the RF tube itself ,like a " B+" supply as long there's a grid drive in this RF tube
So the transient voltage may not be too high.
It;s just my thinking.

the high level clipper is used in the military transmitter.
here's a PDF file ,about this High level Clipper.

Gito

* Splatter_Choque_Chicago.pdf (1205.46 KB - downloaded 551 times.)
Logged
Steve - K4HX
Guest
« Reply #56 on: January 10, 2012, 12:46:05 AM »

Measure the impulse response of that filter circuit. I probably creates some nasty transients vice reducing them.
Logged
Pages: 1 2 [3]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.053 seconds with 18 queries.