Hello again,
Well first of all, the term "negative peak compression" or NPC refers to compressing the negative peaks, while allowing you to modulate the positive peaks more than 100%. They're reasoning behind this is that
in theory , and they are correct in this aspect, that you cannot modulate negative peaks beyond 100%, as at 100% the envelope is at zero potential. Let's first begin, by stating that adding a diode in series with a resistor DOES NOT "magically" provide negative peak compression, contrary to popular belief. It would appear so on a scope, but in reality it's much deeper in theory than that.
Let's start with the basics. AM = amplitude modulation, which by definition, amplitude meaning strength, or level, and modulation being a term to describe how intelligence (with the exception on some cb channels, he he) is added to a carrier. and yes, even in SSB it's added to a carrier, it's just that the carrier isn't actually transmitted. Actually, SSB is a form of AM believe it or not, by raw definition. AM consists of positive peaks and negative peaks, both which are rectified or detected in an AM receiver, usually some equivalent of a half wave rectification process. Newer more sophisticated AM receivers use different methods, but I have yet to see them be adopted into the CB or amateur radio world.
The problems with this theory is simple. You are getting away from real amplitude modulation. Any time you change AM, you increase distortion. For instance, in the CB radio world, I see people all the time refer to "swing". What they are referring to is either positive carrier shift, OR a measurement of positive peaks. They seem to think that this makes them "get out" better. I have talked to several broadcast engineers that are friends of mine, and you would not believe the response that you get. some of the responses would get one banned from this forum for sure! But back on subject.
Basically, it's like this: When you reduce the carrier, which I know is a waste of RF, to a point that is so low it's ridiculous, you're approaching DSB, or double side band. People are thinking "sideband!!" well even if it did sideband's better, right? Well yes, but not if you're trying to listen to it on an AM receiver.
Back to NPC... now, when an AM signal is PROPERLY modulated, a proper AM signal (meaning no silly diode / resistor changes here people) will sound excellent, and be a near lifelike reproduction of the original AF) Keep in mind, when people hear you on the radio, they are not hearing your voice. Think instead about it this way: They are listening to an electronic reproduction of your voice!
If you compress the negative peaks, it appears on a watt meter, that you are getting more power, but in reality, the envelope contains the same amount that it did before. The difference is that when the envelope approaches zero, for an indefinite amount of time, then goes positive, as a duplicate of the AF signal it's modulated with, the meter is going to reflect this, and therefore, if you're reading RMS, which is not the standard for reading AM, (even if they do in CB world, name one other type of service that uses this standard for reading ANY amplitude modulated RF signal, AM broadcast, television, etc) it will either not "swing" as high, or even may show that it's "back-swinging", after you've gone beyond the linear region of the transmitter. The ideal situation under 100% modulation, is to have a little bit of forward deflection, and then stop. If you read it on a true peak reading watt meter, it will show about the same, or maybe a little more if it's reading peak-to-peak, when the transmitter is operating correctly.
I've seen it posted places where they used this theory in older broadcast transmitters, and they did. But, keep in mind that a tube is different, and you didn't work magic by using a diode resistor combo. Also a tube is a voltage device, and using plate modulation, when done correctly in the AF stage, or the modulator, you could achieve the desired result... sometimes.
Another thing that I don't like, is the fact that alot of the so called NPC - RC mods call for "volting" the final. If you remember, a transistor is a current device, not a voltage device. (unlike a tube)
What people are not paying attention to, or WON'T pay attention to is that 99.999% of the time, this is not needed at all. I see people all the time changing the 2SB754 to another part, in alot of export radios. A couple of comments on this... first off, you WANT the final to be modulated by a current regulator, this is called high level modulation, and the final transistor (s) will last forever. People try changing the regulator to something else, and I hear all day... my lights on the radio don't dim any more...well think about it, it's because you aren't delivering near as much current to the final output section as you were before. or they change it because it will run cooler.... geez.. it's not because the part is "better" it's because it's real-world dissipation is lower. Sure a 2SC945 won't run as hot as a %%WORD651%$7, but you're comparing apples and oranges here.
In my opinion, what people should pay more attention to is the way that the final output section is designed. Did you know that in a Galaxy 99 for example, the bias circuit is bypassed so poorly, that RF finds a path directly back in, and actually causes the bias to go negative, which increases distortion, and power output??? What about cheap, lossy ceramic capacitors used throughout the circuit, and often no where near the correct values? What about the type of coupling they use between final output transistors, and then couple them to the antenna jack? It's funny, because no one thinks of these. How about instead of butchering the modulator section, adding some proper AF clipping if you really want your audio to have some distortion and "cb style loudness" to it?
I typically get 45-50 watts peak out of a Galaxy 66, and over 50 out of a Galaxy 99. I don't volt the final, nor change regulators, etc. I simply engineer the circuit to work effectively. I don't jumper diodes or clip diodes in cobra 29's, and get over 50 watts. In fact, moleculo, I believe that you saw over 55 watts today at the shop out of one on a Bird 43P, and a $20,000 hewlett packard RF analyzer, with the calibration sticker on it.
My point to all of this, is, expand your horizons, don't do what other people say to do, just because they say to do it. I see it all the time in the CB world, people wanting that one more watt... driving an amp into an amp into an amp trying to swqeeeeezzzee <img src=http://www.ezboard.com/intl/aenglish/images/emoticons/eek.gif ALT=":eek"> every last watt out... why not............. buy a bigger amplifier? I try to run all of my equipment at about 80% of whatever it will do, and you know what... I can honestly say I've never had to replace a final transistor, on any of my equipment ever in my life.
Take care,
Justin
EDIT: one more thing, if people want a detailed explanation of what they have done to their radio, etc, I will answer in detail on three types of radios, not per person but total, if they give me the component designators, as there are alot of junk internet mods floating around out there, and I cannot explain every one on every type of radio, as I'm sure you can understand.
</p>
Edited by: <A HREF=http://p067.ezboard.com/bworldwidecbradioclub.showUserPublicProfile?gid=justindepolis2>JustinDePolis2</A> at: 8/16/04 2:05 am
