what NPC/RC mod looks like on a scope

[cbdoctor]

I've never understood how a circuit that just raises and lowers the static carrier lever in conjunction with the loudness of the modulation ever got named "negative peak compression" True negative peak compression works independantly of the carrier level. On a standard AM transmission, the carrier doesn't change amplitude. The sideband energy is what causes the envelope growth. The only reason the carrier doesn't pinch off with the so called "NPC" mod is because the modulator in any given CB radio is only capable of a finite amount of audio drive. When the NPC mod raises the static carrier level to maximum on high voice levels, it's merely driving the carrier too high for the limited audio available to modulate the transmitter to beyond 100%

That is NOT what is happening! If the negative peaks were not compressed and you modulate the positive peak as much as possible--but still below the saturation level--the negative peak would severly clip. The NPC components, normally just a single diode and a resistor, causes the gain of the modulation circuit to lower during the negative peaks. That is, the more modulation, the less negative peak gain. This causes assymetrical modulation with very nice shapped sine-wave type peaks on both the negative and positive peaks.

What a lot of "so-called" techs have to say is the the negative peaks are trashed, pointing out that they are "thick" in the middle of the modulation envelope. What they do not take into consideration is the fact that if you have the same number of peaks in the positive direction as you do the negative direction. the area that they occupy will be different for the negative peaks than it is for the positive peaks. Of course the center will look thicker because there are so many negative peaks that are compressed into that small area of the screen.

This simply shows that the compression circuit is working properly.

As stated already, the actual carrier does not change as modulation is applied. This can be seen on the spec-an. However, the term amplitude modulation came about because the early fathers of this science thought that the amplitude was being varied in accordance to the modulation applied.

using this idea simply as a teaching tool, it can be seen why they believed this to be so and there are many ham radio operators, and many more cb techs that still subscribe to this erronous thinking.

as modulation is applied to the carrier the carrier "seems" to expand during the positive peaks and go to zero size during the negative peaks.

With NPC-RC (RC = reduced carrier) the negative peaks hit about 98 percent modulation at the same time as the positive peaks PEAK OUT at somewhere above 200 percent.

The compressed negative peaks do not cause clipping and since compression takes over for the limiter, which is removed, there is no problem removing the limiter since compression does a better job anyway. Removing the limiter allows the positive peaks to reach their maximum usuable value WHICH IS JUST BELOW THE SATURATION POINT!

I gotta go--more later.

BILLY DEAN WARD

 

[GLR]

Awesome...

 

[freecell]

baloney:

CHAPTER 5 INTRODUCTION TO WAVEFORM INTERPRETATION
MODULATION MEASUREMENTS
Figure 5-1.—Antenna current increase with amplitude modulation
Figure 5-3.—Rf amplitude percentage modulation wave envelope
Oscilloscope Measurement Methods

 

[loosecannon]

CBdoctor,

i was reading through some old posts between you and Bill Eitner on the cbtricks yahoo group.

i noticed that you used to use a diode when doing the NPC mod to the 148GTL chassis, then after some time, it became increasing the value of a cap from 1uF to 10uF.

am i confused?
are there two ways to do this mod to the 148 chassis?

what is the way that you prefer nowadays?
thanks for any light you can shed on this for me.

oh, how is your NPC mod for the 29LTD chassis coming along?
last i heard, you were thinking of incorporating an echo along with the mod.
i think that an NPC mod board that also contains a GOOD sounding reverb would be a hot ticket item.
most people who use echo want it for the "big radio sound" and not for actual repeating echo. (unless they are just trying to be jerks LOL)

anyway, hope all is well with you.
i look forward to hearing about what you are working on these days.
LC

 

[jazzsinger]

Almost 5 years this thread has been running and still not one single spectrum analyser pic of this beautiful clean signal after NPC-RC mod.Am I the only one who thinks that is suspicious,spectrum analysers don't tell lies,so if there's nothing to hide then why not show the spectral purity of this mod?


Using an EQ system with your audio stereo is a form of distorting the signal.

In answer to that cb doctor,no high end audio manufacturer would ever consider adding an equalisation system to a top end hifi,they even grudge adding bass and treble controls because they know it degrades signal purity but do so because the consumer demands it because they are either deaf,thick or playing music at such volume levels and overdriving their amps that the impurity added by bass/treble controls is minimal compared to the impurity added by overdriving.

Audio distortion doesn't sound good,it is tiresome and causes listening fatigue.

 

[V8Vega]

Parametric & graphic EQ's are used to compensate for acoustically less than perfect rooms in many high end audio systems. I have a Denon PMA-900V (mid-fi IMO) integrated that I use a EQ with for this reason. The adjustment for the room deficiencies will more than make up for a little NOISE (not distortion) being introduced into the signal by a QUALITY EQ. Sorry for getting off topic, but I wanted to respond to the EQ distortion comment earlier.

 

[stonecrusher1]

wheeeee-haaaaawwwww.....they're still arguing about what an npc-rc looks like in 'free-space', that classic 'isotropic-receiver', & i just keyed over 'em & gone with 2 watts carrier from an 8719 chassis & a large tube. again. WHO F@<&!*G cares???? or is that......get a life? can never keep those straight. keep arguing, i'ma going back to the shack.

 

[15minigrass]

Hey DOC, I need this mod for a 146GTL cobra. Can you point me out to where to do the mod????

Thanx. 1-5

 

[cbdoctor]

I have been gone from the internet for quite a few months. I Now am back in Arkansas and have had a new shop going for almost 3 months. I JUST read the last few posts. I am startled at the response buy such seasoned ham radio operators. Such as "static carrier level" I guess very few have actually looked at the subject of this thread on a scope.

And, BTW, I never answered one of the questions asked me. The reason the center of the modulation envelope is brighter at the negative peaks is NOT because they are "trashed". is is because the negative peaks are all taking place in 10 to 20 percent of the space that the positive peaks are happening in which causes the phospherous in that area of the screen to stay "lit up" for a longer time period.

Well, this thread was about through, I guess, since I was the main one defending the NPC-RC mod so it is not important to carry it any further than it went a few months ago. I just saw a couple of things that I felt like addressing since I didn't do so months ago when they were brought up on this thread.

73,

BDW

 

[cbdoctor]

Interestingly enough, I HAD already answered the question that I thought I had not answered. LOL!

However, I did not answer loosecanon's question about the 148. I have always done that mod by using the exponiential curve caused by the capacitance being increased instead of the diode curve for that radio only.

BDW

 

[cbdoctor]

This thread!

Wow! Believe it or not, I took the time tonight to read this entire thread which started almost 6 years ago.

It is absolutely incredible how many, apparently--very knowledgeble cb'ers AND extremely well informed Ham Radio operaters ever got that well informed.

The reason I say that is that it seems to me that NO one paid any attention to anything that I said. That is not very flattering to my ego to say the least. Of course, I say that with a large amount of humer. I am a big boy and do not get my feelings hurt easily.

However, I do get frustrated when someone makes a statement to me or asks me a question and then proceeds to go about as though I never even responded to them.

Not that I am some kind of expert but they did ask and it seems that they could have, at least, agreed with or rebuttled my answer.

It is as though they didn't even listen to my answer. It would have been fine if someone would have said, "You are wrong because............" But they continued to preach their philosphy with NO REGARD to what I said.

If I am wrong about something, then I want to know the reason someone thinks so--OR KNOWS so!

What I am talking about is the fact that it seems that almost no one on this thread has a clue as to the how or why of the NPC-RC modification!

Some guys talk about the fact that it is bad BECAUSE it is NOT symetrical. Interestingly, that is EXACTLY why it is GOOD!

Others talk about how bad it is BECAUSE the negative peaks do not come all the way to the zero line.
Interestingly, that is EXACTLY why it is GOOD!

Still others talk about how the center of the modulation envelope is brighter than the rest of the envelope.
Interestingly, that is EXACTLY why it is GOOD!

I decided tonight to come up with a website that will be 100% dedicated to the NPC-RC modification. It will have the modification for EVERY radio that I can create it for with life-like pictures of the steps required.

It will have an antimated tutorial showing everything that I can think of that will cause one to understand the merits of the mod.

It will also have pictures of geniune oscilloscope waveforms associated with the mod and explain how to interpret what you are looking at.

It will also have pictures of the spectrum analyzer showing that the harmonics, when properly done are down greater than 60 dB. and that is on a spectrum analyzer that originally cost $32,000 dollars.

One of the biggest misunderstandings is caused by the fact that a lot of the modifications done as though they are the NPC-RC mod that I use when in fact they are NOT what I am talking about.

I know of NO modification that works as well as the one I use. Of course, if I did, I would be using it instead of the one that I am using.

In fact if any of the advocates of the NPC-RC can show me a different version of the mod, I would welcome you to send me the information. Of course, I don't expect that to happen because there are NO ill effects caused by the mod that I do.

What other folks are doing, I don't know. I only know for sure that what I am doing works and EVERY customer leaves my shop happy.

The ONLY complaints I have EVER had came because some
other "tech" "informed" my customer that I didn't know what I was doing and that I "have been burning up radios for years." Then he talked my customer into letting him take my mod out and put the radio back to stock so that it would not burn up--And that will only cost $85.00 to do so.

HUMMMMMM! wonder why the "so-called" expert would want to do that?

 

[15minigrass]

whether they like it or not I love that mod. You say any radio? Do you have one for a 146? I found one online but lost it and cant find it anymore. Thanx Doc. Aloha, Jim....

 

[Robb]

CBDoctor:
Excellent analysis.
Excellent defense.

Now; are you going to post that info on this site so that we can test it for ourselves? At least info on the more popular and common radios, say the Cobra 29 and the 148 and a few more? Of course, we don't have the spec analyzer to check the work. But we can see if a radio will burn up, as you claim that it won't. Are you willing to take this challenge? This can be a means to test the info and dispel the claims of 'foul'. Best way to test the claims of the evidence to your credit...

 

[cbdoctor]

The best thing to do, until I get the NPC-RC website going is to google the radio that you are interested in doing the mod to. I haven't done one for the Cobra or any of the transformer modulated rigs. That is way I am talking about doing the kit for. A kit to convert a transformer modulated rig to transistor-modulated!

You might want to google this} "the red wire on the bottom" also.

 

[cbdoctor]

I don't remember if I have put this link on this forum or not but if not here goes:

NPC-RC Negative Peak Compression-Reduced Carrier

This is from a very old websigte of mine that is defunct so don't expect the site to work very well.

If I have already posted this link, forgive me but there is NEW interest.

73's