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OVER 130% MODULATION!!!
- Thread starter space cowboy
- Start date
In commercial broadcasting we routinely ran 120% positive peak modulation and it was clean as a whistle on the 'scope. I believe the legal limit is 125% IIRC so as to not have a loudness war on the band. Special audio processor units ensure the positive peaks are always the peaks that get expanded and not the negative peaks. Properly set, a transmitter of any type can sound good at greater than 100% positive peak modulation as long as there is enough headroom to prevent flat-topping and as long as the negative peaks do not exceed 100% as that is what causes carrier cutoff and distortion.
Exceeding 100% is a mathematical impossibility. Once you reach 100%, you have reached the limit of the channel's capability to handle more information. Any amount over 100% spills over into the adjacent channels, causing bleedover.
You cannot bend physical laws.
You cannot bend physical laws.
No part of that statement is true, lol.
Your signature line says it all: "AntiSquid Disclaimer: All information provided in my messages constitute *personal opinion* only, and may or may NOT constitute actual fact."
LOL
You need to learn the difference between positive peak modulation and negative peak modulation. I worked in commercial broadcasting for 22 years and routinely ran 120% positive peak modulation. When I had problems with a Nautel AMPHET 1 transmitter I tested it to in excess of 200% positive peak modulation (on a dummy load of course)
all the while not exceeding 100% negative modulation. It is possible trust me. There is a difference between positive and negative peaks.
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Here's a picture to illustrate what CK is referring to. Granted, this is a little extreme, as the positive peaks are in excess of 150%. However, the negative peaks are around 100% with no clipping or unusual waveform. It IS possible to achieve over 100% modulation.
This first pic is a carrier of 2 watts, no modulation.
The 2nd pic is the carrier modulated.
This was done on a President Madison with the MB8719 SSB chassis. No modulation limiters were removed, and I maintained full adjustability of the AMC circuit. This is a form of asymmetrical modulation.
(Edited in carrier pic, thanks CK!)
~Cheers~
This first pic is a carrier of 2 watts, no modulation.
The 2nd pic is the carrier modulated.
This was done on a President Madison with the MB8719 SSB chassis. No modulation limiters were removed, and I maintained full adjustability of the AMC circuit. This is a form of asymmetrical modulation.
(Edited in carrier pic, thanks CK!)
~Cheers~
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Edited in the carrier pic, CK.
~Cheers~
Hey Exitthirteen!
what mod did you do to that madison to get 150% pos and 100% neg?
also, doesnt the madison have a non-adjustable AMC resistor instead of a pot?
just asking because you are always doing cool mods and was wondering how involved what you did to the madison was.
LC
Hmmm . . . Venture a guess here and say that he might have used a electrolytic cap to keep the negative peak from distorting.
Again . . . Just a guess . . .
Then turning up the AMC w/o clipping the limiter should get a higher positive peak before it gets flat topped.
Again . . . Just a guess . . .
Heya LC!
I'll give you a quick rundown of what I did make this mod possible. Gonna post a couple pics below.
The first pic shows the AMC control. You are correct, the Madison has a fixed resistor for the AMC. However, it is screened on the PCB for a diode (1N914/1N4148) and a vertical PCB mount trimmer pot. I removed the fixed resistor and installed the diode and trimmer pot as shown. 5 to 10k for the value of the trimmer pot works well.
This allows you to then control your AMC.
For the mod to increase positive modulation peaks while maintaining negative peaks, you'll need a 10uf, 25V electrolytic capacitor. In the second photo, you'll want to install the capacitor across the two points shown. The positive lead of the capacitor goes to pin 9 of the audio IC (TA7222P or TA7222AP), the negative lead of the capacitor goes to the junction of D63, R194, and R228.
With this capacitor value, I got approximately 160~165% positive peaks and 100% negative peaks by adjusting the trimmer pot in the first picture. Some people might not like that much positive, you can change the value of the 10uf capacitor to 4.7uf and get about 130~140% positive peaks while keeping negative peaks at 100%.
The scope pictures I posted is with the 10uf capacitor. If you need anymore information, I'll answer it the best I can.
~Cheers~
I'll give you a quick rundown of what I did make this mod possible. Gonna post a couple pics below.
The first pic shows the AMC control. You are correct, the Madison has a fixed resistor for the AMC. However, it is screened on the PCB for a diode (1N914/1N4148) and a vertical PCB mount trimmer pot. I removed the fixed resistor and installed the diode and trimmer pot as shown. 5 to 10k for the value of the trimmer pot works well.
This allows you to then control your AMC.
For the mod to increase positive modulation peaks while maintaining negative peaks, you'll need a 10uf, 25V electrolytic capacitor. In the second photo, you'll want to install the capacitor across the two points shown. The positive lead of the capacitor goes to pin 9 of the audio IC (TA7222P or TA7222AP), the negative lead of the capacitor goes to the junction of D63, R194, and R228.
With this capacitor value, I got approximately 160~165% positive peaks and 100% negative peaks by adjusting the trimmer pot in the first picture. Some people might not like that much positive, you can change the value of the 10uf capacitor to 4.7uf and get about 130~140% positive peaks while keeping negative peaks at 100%.
The scope pictures I posted is with the 10uf capacitor. If you need anymore information, I'll answer it the best I can.
~Cheers~
Most of these simple NPC mods look good on the scope under a single tone test but not nearly as well under average modulation were the frequency response in varying. Consider this capacitor mod that placed the cap directly across the output transistors of the audio IC.
At the low range of the audio bandwidth this 10 uf cap has a reactance of over 50 ohms. Not enough loading to do anything. On the other hand, by the time you reach the high end of the stock bandwidth, the cap is nearly a dead short under 5 ohms.
If we were to load the circuit more evenly across the bandwidth by using resistor and much larger cap just to block the DC present, we would see a significant increase in distortion the more dependant you become on the negative cycle loading.
We are already expecting this audio stage to have the extra headroom to produce a larger positive peak without distorting. Then to control the negative peak, we load it harder than the positive? That has to add a lot of distortion by overloading the current output of the IC and is probably why the cap value is low enough to not function at the lower frequencies were it will distort first.
At the low range of the audio bandwidth this 10 uf cap has a reactance of over 50 ohms. Not enough loading to do anything. On the other hand, by the time you reach the high end of the stock bandwidth, the cap is nearly a dead short under 5 ohms.
If we were to load the circuit more evenly across the bandwidth by using resistor and much larger cap just to block the DC present, we would see a significant increase in distortion the more dependant you become on the negative cycle loading.
We are already expecting this audio stage to have the extra headroom to produce a larger positive peak without distorting. Then to control the negative peak, we load it harder than the positive? That has to add a lot of distortion by overloading the current output of the IC and is probably why the cap value is low enough to not function at the lower frequencies were it will distort first.
Suppose the next step would be to volt the final and re-adjust the bias for the driver and final for the best wave form. As a guess, the little extra voltage should keep the final from 'running out of gas' on the positive peak. Less chance of flat-topping the positive peak. That would be the best part of the NPC mod; between adding the cap for the negative peak and the final/volting for more positive peak. Others will also change out a resistor and clip the AM Limiter, which only pushes it too far into distortion and IMO not really necessary.
From what ExitThirteen is saying, the circuit has potential but I wouldn't "volt" the final. The ONLY time raising the operating voltage at the final makes sense is if you're willing to modulated that higher DC voltage too. Otherwise the loss in modulation is not worth the gain in watts.
The part used to load the circuit should not change its resistance as the frequency of the audio applied changes. The loading should be placed after a series dropping resistor so that excess negative audio peaks are not forced to drop across the output transistors of the audio IC.
The part used to load the circuit should not change its resistance as the frequency of the audio applied changes. The loading should be placed after a series dropping resistor so that excess negative audio peaks are not forced to drop across the output transistors of the audio IC.
I don't currently have this mod in my radio. I pulled it out, only did for testing purposes. The radio is set at 100% modulation on the scope at a 2W deadkey. I seldom talk AM, and when I do, I figure that if people can't hear me, that's their problem.
(edit) You're correct, Shockwave, it does have potential, but it's not something I'd do, considering it's quite complex to shape the wave (i.e. higher than 100% positive peaks and 100% or lower negative peaks) and not cause other issues. This was just merely a test to say that it can be done, and it can look good on a scope.
(2nd edit) I might also add that I never went "on the air" with this mod, I never got a radio check. I did all the testing on a dummy load, so I have absolutely no idea what it would've sounded like on the airwaves. For all I know it could have been nasty, or it could have been amazing. *shrug*
~Cheers~
(edit) You're correct, Shockwave, it does have potential, but it's not something I'd do, considering it's quite complex to shape the wave (i.e. higher than 100% positive peaks and 100% or lower negative peaks) and not cause other issues. This was just merely a test to say that it can be done, and it can look good on a scope.
(2nd edit) I might also add that I never went "on the air" with this mod, I never got a radio check. I did all the testing on a dummy load, so I have absolutely no idea what it would've sounded like on the airwaves. For all I know it could have been nasty, or it could have been amazing. *shrug*
~Cheers~
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