02-04-2012, 03:30 PM
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Join Date: Dec 2008
Location: Silicon Valley CA, Storm Lake IA
what is "asymmetrical modulation" ? im almost ashamed to ask ........ :(
Time to read first; then you might ask some questions.
There are a LOT of threads on this forum about the NPC mod . . . . Written by SS8541 Richard Byrd-
The npc mod is the most used mod in export radios today. it has been around for yrs, but it is set up wrong and actually labeled wrong (depending on form used) by 90% of the techs using it.
The npc where the limiter is removed and the final/s volted -is- an actual npc. but after reading this i hope you all will see that unless you have the right equipment it is unaceptable for a clean signal (which is the reason for using the npc instead of clipping the limiter). even when set right, a 1w carrier with 35-45w (single or dual final)of swing is not good.
For the npc where the limiter is left intact and and the final/s is not volted, a stock radio set at 100% mod by the factory, needs nothing but the npc components. well, the high power carrier needs reducing to keep the pos peaks from flattopping, but the lower power setting needs nothing but the npc components. this mod is also not truly a npc mod, but a ppe (positive peak expansion) mod. there is no compression of the negative peaks, since the neg peaks still hit 100% mod without the amc being adjusted, but the pos peaks will hit 125-300% pos mod (depending on resistor value used).
The reason is because the amc is only triggered on the neg peaks. the amc is turned off on the post peaks, but it tracks on the positive peaks by an rc circuit.
Here is a galaxy 77hml at 100% mod and a 1.25w carrier as an example....
For a 1.25w carrier the collector of tr51 is at 2.50vdc. the emitter of tr49 is at 2.54vdc. when audio is applied for 100% mod, the collector of tr51 swings from 0-5v (5vp-p). the emitter of tr49 swings from 2.49-2.59v (.1vp-p).keep in mind that the emitter of tr49 only swings .1vp-p as this will be revisited later. the amc samples the neg peaks to keep it at 100% mod.
Now when a 1n914 diode/390ohm res npc (ppe)is used, things begin to change. the banded end of the diode goes to the collector of tr51 and the res goes to the emitter of tr49. the neg peaks still hit the 100% mod i had set the radio for in the last step, but the pos peaks go to about 200%. remember how the voltage on the col of tr51 goes to 0 but the voltage on the emit of tr49 only goes to 2.49v. so when the col of tr51 goes .6v lower than the emit of tr49, the diode is turned on and -tries- to pull the col of 51 back up. but the amc is set to -make- the voltage of the collector of 51 hit 0 (100% neg mod). to do this more audio must be dumped into 49. this is evident by remeasuring the voltages at the emit of 49 with a scope. it was .1vp-p with audio, now it is .2vp-p. since the amc only samples the neg peaks, that same .2vp-p is present on the pos peaks. and since only .1vp-p is needed here, the pos peaks go past 100% (about 200% here). so as you can see, there is no compression of the negative peaks (npc). there -can't- be if the neg mod is hitting 100% before and after the mod without amc being adjusted. but there is positive peak expansion (ppe). this is all done by a way of fooling the limiter/amc. and you can add this to any stock radio and it will work properly as long as it was properly set at the factory for 90-100% mod. you also need to keep the carrier at no more than 3.5w for single final radios and 5w for dual finals for the 390/914 npc(ppe). for a 560/914, 5w carrier for single and 7w carrier for dual finals.
As for the npc where the limiter is removed and finals volted, there -is- neg peak compression since there is no limiter present to react and -force- the neg peaks to 100%. BUT you have to have a scope to set it correctly. and how many of you have that??? you may also have to play with resistor values or mic gain to get it to work properly (once again you still need a scope).you might as well just clip the limiter, since if you don't have the right equipment your in the same shape, but with about 5min-30min (depending on your soldering skill level) more work. also, why volt the finals when with both mods you have to reduce the high power carrier for it to work properly? the reg is being "spared" by reducing the carrier power (which is the reason for "volting" the finals).
Anyone who wishes to post this on other forums has my permission. I'm not doing this to call anyone out or for self recognition, but i'd like to see this get around for techs who don't quite understand what is going on with this/these mods (especially since this is the most popular mod out there). this mod was meant to run clean radios with increased pos mod, so it should be understood how to do it properly. I'm not a "mad scientist" saying it works because I say so. I laid down the facts to prove it.
SS8541 Richard Byrd
BASE: Kenwood TS-2000, GAP Titan DX, Diamond X50A, Sirio SY27-4, and IMAX 2000.
MOBILE: Galaxy DX99V w/RF Limited CR-577 mic, Sirio Z-180. Yaesu FT-8800R & Diamond NR-770.