CBWI September 1996 Proper Power & Modulation Adjustments
Everyone seems to be looking for more power and modulation. The 40 channel radios have their modulation limited on AM by the AMC (automatic modulation control). Most have a potentiometer (POT) that controls the amount of maximum modulation the limiter allows the radio to put out.
The reason it has been limited is to reduce the adjacent channel splash on the band. The primary cause of splash is by over modulation distortion. But many radios are turned down well below the point of distortion.
Also the AMC may be slow and allow distortion during some syllables. The radios without an adjustment pot are quickly modified by clipping out a resistor, diode, or transistor, but this eliminates the AMC.
This is OK if the output is constantly monitored and good sense used. The AMC constantly monitors the output and automatically reduces the input sensitivity to maintain a constant maximum modulation.
Below are illustrations of an AM transmitter under dead key and audio test. These illustrations are representations of what you would see on an oscilloscope if you were monitoring the RF output of the transmitter under test.
Notice the over modulated signal is clipped off at the top and bottom. There is much audio information missing. The ideal situation is to get the maximum modulation out without loosing any part of what is going into the audio circuit.
Above is an un-modulated AM transmission.
Above is an under modulated AM transmission
Above is an over modulated AM transmission
Above is a 100% modulated transmission.
How many times have we heard or said, "You're breaking my speaker?" Over modulation distortion does sound like a breaking or broken speaker. But everyone thinks that they're so powerful that they are overloading the other parties receiver.
This isn't true. Yes, you're loud, but the more you over modulate the more voice information is being lost.The dead key power should be one third of the peak power.
Say the radio is capable of 18 watts peak without distortion, then the dead key should be 6 watts. Less if you're driving a linear amp. We'll cover amps in more detail later in this issue.
This ratio will make your signal loud and clear. Not only will that impress other operators, it will increase your range of transmission.If you don't have access to a scope, you can get an idea of what the peak level of modulation is available without distortion.
With the radio tuned for maximum whistle loudly into the n@dc and take a reading on your peak reading watt meter. Usually reducing the peak by 15% with the AMC will give you clear audio. Then reduce the carrier to 1/3 of the peak power.
For example we have a radio the produces 20 watt peak modulation. Reduce it by 15% by adjusting the AMC to read 17 watts peak with a whistle. Then adjust e carrier to 5.5 watts. Check the peak for original 17 watt setting. Now you have approximately 100% clean modulation.
An ALC (automatic level control) is incorporated on sideband radios to limit the maximum PEP output. A radio with a miss adjusted or defeated ALC can clip in a manner similar to the illustration of over modulation on an AM transmitter.
When the signal clips, information is being lost. It may look good on the meter, but it doesn't sound right at the receiver's end. Sideband is a different animal from AM.
It doesn't take as much audio from a mic to carry a great distance. Therefore sideband is more susceptible to background noise and an overpowering mic gain adjustment is counter productive.
In understanding how to adjust your audio on sideband you must first know what the receiver AGC (automatic gain circuit ) does as it receives a sideband signal.
The AGC has the job of varying the receivers gain to prevent overloading from strong signals. Without it, signals from around S-5 and above would start or completely overload the receiver rendering the signal unintelligible.
In controlling the gain the AGC keeps the audio compressed to a somewhat constant maximum level.
The AGC has a recovery delay time that varies from radio to radio. This delay keeps the gain according to last signal received.
If a weak signal comes in after a strong one, the first word or two may be very low or completely missed.Now, understanding that to increase a signal by 3db the power must double and a calibrated "S" meter reads I- S unit for every 6db.
We can use the reverse for an example. If we cut the output power in half we loose 3db of signal. Receiving an 18 watt signal at S-9 we would receive a S-8 at 4.5 watts, a S-7 at 1.125 watts, a S-6 at .14 watts, a S-5 at .07 watts and a S-4 at .018 watts.
That's 18 milliwatts to give a S-4 signal and 1000 times that (18 watts) to give a S-9 on the meter. Background noise that wouldn't be close to registering on your RF meter would be received with a decent signal strength. Imagine if the background was flicking the meter at around I watt, approximately 20% of full output.
Your background noise would be giving a S-7 and your peak signal only 2 "S" units more. The background is desensitizing the receive with the AGC kicking in, making your peaks have less impact or punch. The background would get mixed in with the voice and make it difficult to understand what is intended to be heard.
On sideband the name of the game is signal to noise ratio, the less noise the less distraction from the intended transmission. There's enough noise out there to get through without creating your own to overcome.
In setting the ALC it is good practice, with a loud whistle, to open the ALC up all the way and then back it down until you see a reduction of about 2 watts. This will give you peak performance without clipping in most cases.
Some radios won't distort with the ALC wide open while others will distort terribly once the mic is cranked up. If you control the mic gain carefully you can leave the ALC open.For true reproduction of your voice a good quality mic is essential.
Using only the amount of gain needed to amplify your voice to a respectable level, holding the hand mic or desk mic close to your mouth and talking into the side or talking across the screen will give your transmission the most natural sound