Site hosted by Angelfire.com: Build your free website today!

The NPC-RC Conversion. . . The NPC-RC Conversion. . . The NPC-RC Conversion. . . The NPC-RC Conversion. . .

As done by Billy D. Ward (AKA The C. B. Doctor)

@ The C. B. Clinics Of America

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 Compression-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 now demands 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 Linear Amplifiers 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 NPC effect to take place. With the carrier set to 10 watts the amp would swing all the way to 180 watts on peaks while holding the negative peaks at approximately 95% modulation.

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.

Notice that the carrier signal pictured here is considerably more than 100% modulated on the positive peak. It is 350% 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 350% modulated on the positive peak while also being compressed on the negative peak. As the audio increases on the negative peak (we see it as going down), the rate that it could increase is modified by the NPC components in a way that causes it to compress at an exponential rate which means that the further negative it goes the less it will increase per time period. The percentage of modulation that it applys to the carrier during the negative peak will be less and less than the positive peak causing the signal to be asymetrical but with extremely low distortion as far as the human ear is concerned. So, instead of the proverbial clipping of the negative peak that would be expected under normal conditions of over-modulation, the compression causes the negative peak to reach the (approximate) 98% modulated state at the same time that the positive peak reaches the 350% modulated peak.

Take note that the while the modulation envelope, during the fully modulation condition,looks identical to the one pictured above, the carrier here is that of a signal that is 4 times the dead-key of the one above and 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 by reducing the carrier and keeping the modulated positive peaks at the same point as the full-carrier signal, the output transistor will run cooler with the NPC-RC conversion at the same time giving a much larger percentage of modulation. So, the pictures show the truth of the matter! Running a radio using the NPC-RC Conversion is not only easier on the radios final RF stage but easier on linear amplifiers too!
Billy