AMC removal

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I have had so many people ask me what my views on AMC removal are that I decided to devote a page to the topic, and avoid having to repeat myself constantly. I'm not even going to bother replying to any more AMC questions, other than to give this page address.


O.K., here's the short version.

Removing the AMC from a radio (or disabling it or maxing out the AMC control) will allow the radio to over-modulate when running at high mic levels (high mic levels are needed to completely modulate the radio with the softer parts of the human voice, such as long "E" and "I" vowel sounds). This over-modulation causes audio distortion (see below), as well as interference with other channels and frequencies (see below), and other electronic equipment. It also leads to premature wear and tear, and sometimes failure, of finals, drivers, and modulator circuit parts.


Here's the long version (refer to short version for intro).

Distortion:

The audio distortion caused by over-modulation can range from a raspy or blaring sound to a muffled and muddy sound. I've even heard signals that sounded as if they were on side-band, or an adjacent channel. What happens to cause the distortion is a bit complicated, but here's a rough idea. A large part of the distortion is caused by "clipping" of the audio signal in the amplifying transistors or audio chip. This is also referred to as "saturation". This just means that a circuit can only amplify up to a certain point, and if more input is added, it doesn't get any stronger, only distorted. (Think of a sponge that is completely soaked. Adding more water only makes a mess). This distortion is then passed along to the output of the radio during modulation, and shows up on the receiving radio. The distortion can often be eliminated by turning down the mic control to keep the audio amplifier from clipping, but the overall loudness of the radio always suffers by doing this, because of the fact that the softer aspects of the human voice need to be delivered to the modulation section of the radio at much higher levels than the loud parts of the human voice. If the mic control is turned back up to maintain full modulation with the softer parts, you go right back into clipping with the louder portions. IF the amc is left intact, and adjusted properly, the mic control can be left at maximum to keep the softer parts fully modulated, while the AMC keeps the louder parts from over-modulating. The result is no audio distortion, and a louder over-all sound. It "levels out" the transmit audio, much like a compressor does for the vocals in a recording. It can also, in some cases, help compensate for a cheap-sounding microphone. Another part of the audio distortion is caused by short periods of no output at all during over-modulation. If you look at an over-modulated radio on an o-scope while injecting a steady tone into the mic jack, you will see brief periods where the radio's output signal "flat-lines". These flat spots are periods where the radio is putting out 0 power. If the AMC is intact, there will be no flat lines, only a tiny, brief instant where the output drops to minimum during its negative peak. The result, with the AMC intact and the mic gain up, is a higher average output from the radio.

Interference:

The interference caused by over-modulation generally shows up as heavy bleed-over onto other channels, even at longer ranges. At close range, the bleed over can become severe. I'm sure most of you have had the experience where you've been a mile or more away from someone on another channel, and have that person bleeding into your radio even from many channels away. This is usually caused by a radio with its AMC disabled. (A cheap amp can sometimes cause similar problems, but in general, its a chopped radio. After all, the amp can only amplify what you put into it, I.E. garbage in garbage out.) The technical cause of these garbage signals is complicated, but includes inter-modulation distortion, harmonic distortion, and other types (a bit of bleed over is normal, even from a properly-modulated radio, but generally not at long ranges unless the radio is running through an amp, and generally not more than a channel or two away).

Basically, here's what happens to cause the bleed over.

Lets define a couple things first.

  • Harmonic - A harmonic is a multiple of a frequency. Example, the 1st harmonic of 3kz is 6kz, the 2nd harmonic is 9khz, etc, etc.
  • LSB - Lower Side Band, part of any AM (CB) signal
  • USB - Upper Side Band, part of any AM (CB) signal
  • VF - Voice Frequency, defined as between 300 hz and 3.5 khz for our discussion
  • CF - Carrier Frequency. Example, channel 19's carrier = 27.185 mhz
  • IMD - inter-modulation distortion. Too complicated to go into detail here, but normal even in a properly-modulated radio

A normal, properly tuned signal will only show up on its correct channel, with a bit of interference one or two channels away (due to normal levels of IMD). This is because the total bandwidth of the signal, including its sidebands (USB and LSB), is only about 6 or 7khz wide, keeping it within the proper channel (CB channel spacing is 10khz, so the actual signal should be a max of about 7khz wide). The sidebands are a mathematical function of adding and subtracting the VF to and from the CF. For example, take a channel 19 carrier of 27.185 mhz, and then add and subtract the typical 3.5khz-wide VF band to and from that CF, and you get a USB of 27.1885 and an LSB of 27.1815. The total signal width is the USB minus the LSB, which is 7khz. You'll find this signal width to be true on any CB channel, since the math is always the same.

Now, lets take a "clipped" radio, and look at the audio. The VF coming in from the mic is not monitored by the AMC, and will go through the main modulator portion of the transmitter at levels which cause the clean sine waves of the VF to become squared off, generating a lot of odd (1st, 3rd, 5th, etc) harmonic frequencies. Those harmonic frequencies can be quite high, and in extreme cases, hundreds of khz of the harmonic "trash" can be found. All of this trash then shows up in the signal's sidebands. For example, lets take the 11th harmonic of the highest VF of 3.5khz, which works out to 38.5khz. Using the math from above, we add and subtract that to and from the CF, generating an USB of 27.2235mhz, and a LSB of 27.1465mhz. That now means your signal is splattering from about channel 15 up to channel 22. This is a very minor scenario, because the harmonics in the squared-off VF usually cover a much wider frequency range. For another example, lets take the 101th harmonic, which is roughly 350khz. Now we have a USB of 27.5mhz, and an LSB of 26.8mhz. That now means your signal is splattering across all 40 CB channels, and further. All of that "bleed-over" will be seen by the meter, and the meter will move higher when there really isn't any more power on the channel you want.

The examples mentioned don't even cover the increased  IMD distortion from a hacked radio. IMD will cause a much wider wider bleed-over bandwidth, often 1mhz or higher.

The problem of extra wear and tear (and premature failure) is also related to the output interference. When your CB over-modulates, it generates signals on frequencies outside of the CB channels. Because your antenna is not designed/tuned to operate at these signal frequencies (high swr on these signals), the power contained in them is bounced right back into the radio, and can cause overheating of final/driver, as well as premature failure of these parts. If a final or driver fails, it can take out parts of the modulation section of the radio, as well.

A "myth" associated with clipping limiters is a direct result of this same problem of extra generated frequencies. The reason you see extra "swing" on a power meter when you clip a radio is that power meters do NOT distinguish between signals, they only add the total power seen and give a single reading. That extra swing is NOT more power from your radio on the channel you are on, it is the meter adding the power from the garbage signals generated by over-modulation to the power of the real, usable signal. If you view the signal on a spectrum analyzer (device which shows each separate signal and its level), you will see what I mean. Clipping the limiter yields negligible gain in output power of the desired signal, but DOES cause lots of other, "garbage" signals which rob power and cause problems. Proper adjustment of the AMC circuit will yield max power on-channel without the garbage.

Experience has shown me that the best way to get a louder transmit (not taking into account various NPC mods) is to set the AMC for 100% modulation, then increase the mic amplification until the AMC is engaged all the time. This means that the entire range of levels of your voice are fully amplified/modulated, giving maximum transmit loudness. I also find that decreasing the response time of the AMC circuit (lower the value of the AMC timing capacitor)  will yield even better transmit audio, because the AMC tracks the shifting voice level much more closely.