Originally Posted by unit_399
The amplitude of the carrier in an AM
signal is the same with or without modulation. Looking at the familiar "rf envelope" display on a scope is misleading because it can't display the carrier and the sidebands seperately. It only displays the vector sum of the three signals added together. If you look at an am
signal on a spectrum analyzer it shows that the carrier amplitude is the same with or without modulation.
I own a spec an as well as an oscope. What you are saying is textbook, and WILL be what is reproduced WHEN YOU DO NOT EXCEED 100 PERCENT MODULATION in a high level modulated system.
When you OVERMODULATE in the NEGATIVE direction, you actually cut voltage off COMPLETELY to the modulated stage(s).... In effect, BASELINING the modulator... This is what actually causes splatter, IMD, etc. You CAN see this effect on a scope, this is the little bar you get between modulated waveforms (the valleys being extended between peaks, causing actual space between the individual RF cycles.... THIS is 0 watts of carrier, THIS is splatter, AND when you look at BOTH the Spec An AND the Oscope you WILL see this effect.)
What the circuit I presented does is PREVENT the modulator from EVER hitting 0 watts, 0 volts, etc.... However you want to PUT it. It does NOTHING to the modulated waveform (at least, that I can detect), it ONLY keeps a couple volts on the finals AT ALL TIMES, whereas the MODULATED waveform, when modulated in EXCESS of 100 percent will actually CUT ALL VOLTAGE off to the final and driver... PLEASE explain to me how you will still have carrier, when you have 0 volts being delivered to the collector / plate of the amplifying device, to put it in other terms.
If your limiters are STILL intact, and act upon the NEGATIVE going modulated voltage, then YOU ARE CORRECT, and you will NEVER baseline the audio.
HENCE the reason I stated THIS circuit will work on any HIGH LEVEL modulated radio. A low level modulated radio should still use the Eitner induced NPC, which was borrowed and modified version of the BDW CBDoc NPC. The low level modulated radios (balanced mods) will actually take the negative going waveform and 'flip' it, causing the carrier to never baseline, but introducing the 'overmodulated' sound that the lincoln, 2510, jackson, etc have, versus the "louder than hell" overmod sound that the high level radios have (which is actually nothing more than IMD....... Baselining the carrier, etc).
THIS is the type of discussion I hoped to implement...
In a PROPERLY functioning AM
HIGH LEVEL transmitter, that is NOT overmodulated to the NEGATIVE direction, you're right, the carrier won't be cut off... HOWEVER, if you exceed 100 percent modulation, the spec an, the oscope and a fast acting voltmeter with peak and hold detection will show it actually being cut off....
Also, the resistor that feeds this circuit will get noticeably warm under use. If you HAMMER the negative peaks, it will get HOT. Easy way to tell if your overdriving your modulator is to test the heat on the resistor... If you do the math correctly, it will run barely warm...... PROVIDED you're not running a Silver Eagle mic at half throttle, limiters ripped out and dynamic wide open! At that point, you're pretty much baselining EVERY half cycle, and the resistor should be sized in wattage at HALF your total modulator power, or more.
As to LC's question about carrier: You'd want to put MORE diodes in the series diode, ah la the old "dial-a-watt" methods from Secret CB
. Forget about the jumper method. The problem with the jumper and then going to the RC circuit is this: You will NEVER get full drive to the final amplifier with the driver at less than peak output! Then, on modulated peaks, you end up with a slight ripple or valley at the CREST of the mod peak. I'd lower the final AND driver Vcc with a string of diodes, use the baseline preventer here, and you're ALWAYS looking at the proper input and output impedances from the modulator to the driver to the final. I've also played with modulating the pre-driver, but didn't get anywhere near the effect I wanted.... Tried hitting the pre-driver with a variable voltage on the collector, and after losing about 2 watts of carrier (8 watts was where it was when I started), the waveform on the output went all to hell. On the scope and analyzer, the best method thus far is dial a watt.... The 3 watts you lose PEP won't be noticed.
The thing to watch about ripping the limiter out is this: You can STILL overdrive the modulator stage (the audio chip) causing a triangle waveform on it's output. The transformer is GIGO, if you hit it with a non-linear waveform on it's input, the transformer will only amplify it and send it to the finals..... Watching the modulating waveform on channel A and the output on Channel B will let you know exactly how much forward power you can have..... Hint, only HALF the modulator power can go forward... If you want more, you require HARDWARE in your audio chain to actually increase the AVERAGE power input to the radio.
With careful tuning of the radios output section, and a 1969, THIS is the way to get 75 PEP out of a 29.