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THE C. B. Doctor's Study Group Dot Com

NPC-RC Tutorial by Billy Dean Ward AKA the CB Doctor

NPC-RC For The 3600 Board

NPC-RC For The 6900 Board

NPC-RC For The RCI-2950

NPC-RC For The Cobra 148

NPC-RC For The Magnum S9

with

Dragon Communications Products

THE C. B. Doctor's Study Group Dot Com

NPC-RC Tutorial by Billy Dean Ward AKA the CB Doctor

NPC-RC For The 3600 Board

NPC-RC For The 6900 Board

NPC-RC For The RCI-2950

NPC-RC For The Cobra 148

NPC-RC For The Magnum S9

As done by Billy D. Ward

(AKA The C. B. Doctor)

The NPC-RC Conversion--What is it?

There has been quite a bit of concern about the NPC-RC conversion since I first introduced those initials on another forum here on the internet. They stand for Negative Peak Conversion-Reduced Carrier. While I take no credit for the concept of NPC-RC I am fairly certain that I was the first to coin these initials as a common name for it. NPC was what I originally called it but there was some controversy over the fact that if the carrier was set to high that it could cause early failure of driver and final amplifier transistors. I had always recommended that the carrier be set for 2 watts but I guess that this was not always adhered to, thus the renaming to NPC-RC which by definition would now demand a lower than normal carrier.

The concept of Negative Peak Compression among ham radio operators and commercial broadcast stations goes back to at least the 40's, and possibly it was known and appreciated earlier than that. I have used the concept in my personal radios almost since the advent of C. B. Radio, starting with a home-built tube unit in 1960 which used a 6146B beam-power pentode As the Rf final amplifier. I was running 10 watts dead-key swinging to about 75 watts on peaks. Later, while working for General Radiotelephone, in 1969, I experimented with the concept in a Super MC-11-A with excellant results. When we started Golden Eagle Linears in 1971, we manufactured a unit which I coined as a "swing carrier" amplifier. That amplifier used a noval input design that would allow the carrier to be set to about 10 watts and swing all the way to 180 watts on peaks without noticable distortion.

Notice that the carrier signal pictured here takes up two vertical divisions on the screen. If the carrier is modulated exactly 100% on the positive peak it will be 100% larger or 4 divisions during the peak of the positive half of the modulated cycle.

If the carrier is modulated exactly 100% on the negative peak it will be 100% smaller or 0 (ZERO) divisions during the peak of the negative half of the modulated cycle.

Note: Be sure and understand that when speaking of the RF carrier, everything above the zero line is the positive 1/2 of the cycle and everything below the zero line is the negative 1/2 of the cycle. THIS IS NOT THE CASE when viewing the modulation envelope. The top peaks and the bottom peaks are caused by the positive 1/2 of the audio cycle and the center valleys are caused by the negative 1/2 of the audio cycle. The bottom peaks are NOT the negative peaks of the modulation envelope, they are part of the positive peaks. The negative peaks of the modulation envelope are in the center of the oscilloscope screen. The Positive 1/2 of the audio cycle cause the width of the carrier to double during 100% modulation-The carrier is 100% larger. During the negative 1/2 of the audio cycle and under the condition of 100% modulation, the carrier is reduced to ZERO which, of course, is 100% smaller.

Note that the carrier signal pictured here is considerably more than 100% modulated on the positive peaks. It is 275% larger, during the positive peak, than the unmodulated signal.

However, It is impossible to be more than 100% less during the negative peak as there can be no reduction of a real quantity to a point below zero. But, if you will look closely, you will see that during the time that the signal "tries" to be lower than zero, in order to satisfy the symetrical shape of a sine-wave, it is forced to "ride" along on the zero line. This results in severe clipping of the negative peak which is responsible for distortion of the audio signal.

Pictured here is a carrier that is being 275% modulated on the positive peak while also being compressed on the negative peak. The compression of the negative peak causes the negative peak to reach the (approximate) 98% modulated state at the same time that the positive peak reaches the 250% modulated peak. Any time that the signal differs from the original signal, which is the case here, there will be distortion to some degree. If the compression is done at an exponential rate, the degree of distortion is negligible and unheard by the human ear.

Pictured here is a carrier that is being 100% modulated on the positive peak and 100% on the negative peak. I'm sure that you can appreciate the fact that the final outcome of the output signal is the same as far as power output strength. However the percentage of modulation is considerably LESS in this case. So the pictures show the truth of the matter! Running a lower carrier is not only easier on the final RF transistor but easier on linear amplifiers. But, simply turning down the carrier with no NPC conversion will also lower overall power.

THE C. B. Doctor's Study Group Dot Com

NPC-RC Tutorial by Billy Dean Ward AKA the CB Doctor

NPC-RC For The 3600 Board

NPC-RC For The 6900 Board

NPC-RC For The RCI-2950

NPC-RC For The Cobra 148

NPC-RC For The Magnum S9

with

Dragon Communications Products

THE C. B. Doctor's Study Group Dot Com

NPC-RC Tutorial by Billy Dean Ward AKA the CB Doctor

NPC-RC For The 3600 Board

NPC-RC For The 6900 Board

NPC-RC For The RCI-2950

NPC-RC For The Cobra 148

NPC-RC For The Magnum S9