HOW TO READ YOUR MODULATION

First of all there are several different ways of using an oscilloscope to analyze your modulation on a AM carrier. Some methods are easier, some harder, some methods require expensive, calibrated and high end oscilloscopes.

The techniques I am going to show you will give the exact same results. While maintaining a very high level of accuracy without the expense and complexity of other methods of measuring modulation.

A lot of people ask me about using the trapezoidal way of reading modulation, this is another way of looking at modulation. If you are setting your radio to be linear or symmetrical, Which means your positive and negative peeks are the same. This method does work very well. However, audio loudness can be achieved with asymmetrical modulation. Meaning that the positive peeks are bigger than the negative peeks and therefore the trapezoidal way of looking at symmetrical modulation will not read properly.

First off, you will need an oscilloscope at least 15 MHz, single channel is fine. I get mine from E-Bay, I do not ever pay more than $ 50.00. If your scope is uncalibrated that's OK, we will be calibrating it when we read the modulation anyways.

NOTE: A 500MHz scope is NOT better than 15MHz scope when using these methods of monitoring modulation, DO NOT WASTE YOUR MONEY ON A BIGGER SCOPE. Leave your small peepee syndrome at home, we are in the radio room now !

You will need to build a inductive pickup. To do this you will simply need to wrap a single piece of 10 to 20 gauge standard insulated wire around your coax, one loop right next to each other, side by side, 12 to 18 times. Use some black tape to secure your new pick up so it will not come unwrapped. Then solder a piece of thin coax to your new inductive pick up. Center wire on one end and shield on the other, it does NOT matter which end goes where, because you are making a circle. On the other end of the coax you will need a BNC connector, this will plug into your scope on the front where its says input or CH1. Please refer to picture "A" below.

PICTURE A


As far as setting up your scope, there are a lot of knobs and buttons and without having the scope in front of me I cannot tell you how to set it exactly. I can tell you that you can NOT hurt the scope by simply pressing buttons or turning knobs. The only thing you can NOT do with your scope is use it as a dummy load. YOU SHOULD NEVER PUT RF DIRECTLY INTO THE SCOPE.

Always have a dummy load or antenna hooked up to your coax when using your scope or the magic smoke will come out of the back and the screen will go dim. ( this is bad )

There are 2 main knobs that you will be adjusting. This is the VOLTS / DIV knob and the SECONDS / DIV knob. The volts per division knob is usually above or below the BNC connection where your inductive pick up is attached. This knob adjusts the height of the image across the scope. The seconds / division knob adjusts the speed at which the image appears across the screen. There should also be a AC/DC lever, select AC.

To calibrate your scope to read modulation, key your mic with the mic gain turned all the way down, or make sure their is NO audio going into the mic. Adjust the volts / division knob until the line on the scope fills one division above and one division below the center line as shown in picture " B ". The only time you adjust your volts / division knob is when calibrating. After it's set to one division up and down, you do NOT adjust the volts / division knob when reading modulation. When you get your scope to look like this " Picture B " you are now calibrated and ready to turn on your audio or turn up the mic gain and look at modulation.

PICTURE B


When you look at modulation, the human voice is most powerful when pronouncing the vowels, A E I O U, this is why you hear people tuning up their radios saying foooooooooooooour, or aaaaaaaaaudio. Do the same thing and look at your scope with the mic keyed up, to get the picture to look steady, simply adjust the seconds / division knob until you can see clearly what will look like a sign wave or some nice peeks and valleys similar to the picture below " C ". DO NOT adjust volts / division. You must ALWAYS start with a dead key one division up and down from the center line!

EVERY TIME YOU MAKE AN ADJUSTMENT OR READ MODULATION, YOU MUST FIRST VERIFY THAT YOU ARE EXACTLY ONE DIVISION ABOVE AND ONE DIVISION BELOW THE CENTER LINE !!!!!!!!!

PICTURE C


Now there are basically 2 different ways people set their radios. First as a competition type radio meaning Maximum watts and little concern of what the audio quality sounds like and second, as a good sounding type of station with more emphasis on audio quality rather than watts as you CAN NOT have both when comparing 2 equal stations.
Let's look at a example station consisting of a stock cobra 29, and a stock 2 transistor driver in class Ab2 into a stock SB-220 in class Ab2.
For competition, you would peek the 29 to 25watt PEP, and drive the 2 transistor box to about 275 PEP into the SB-220 and get about 2200 PEP at about 10% modulation. This station will get out and have a big signal, but the audio is soft and really high and peeky sounding.
For good audio quality, using the same equipment, one would turn down the 29 to about 5 W PEP, into the 2 transistor box getting about 80W PEP, and then into the SB-220 getting about 1200 W PEP and 100% modulation. This is going to sound a lot better as far as audio quality is concerned.

The next statement that would be said is " but to sound good I'm loosing HALF of my power, RIGHT? 2200W PEP @ 50% audio down to 1200W PEP @ 100% audio WTF ! Isn't power more important?

LOUDNESS is gained by higher percents of modulation or audio processing or both. NOT BY WATTS. Percent of modulation is referring to how much audio is on the carrier. The more you have the louder you are PERIOD.

So the description below is what you want IF you are looking for good audio quality.

Read % of modulation by referring to picture " A ", also you want to make sure that you are not " FLAT TOPPING " the top of each sine wave and more importantly NOT going past 100% in the negative direction, or flat topping in the negative direction.
ANY flat or square waves at the top or bottom of the sign wave is distortion, pinching the carrier is a abrupt "cutting off" of the carrier and this is also distortion and 95% reason your neighbors hate you! Bleed-over usually does not come from running power, it comes from square waves and pinching the carrier.

This picture is what 20Hz should look like,

NOTE : if your using a standard CB radio, you will NOT be able to pass 20Hz or 7500Hz or create 200% + modulation. However you will be able to read and see what your radio is doing or not doing for yourself, instead of trusting your technician. They have been known to lie. I hope this information is helpful to you as my goal is to provide information to help the average CB operator understand the relationship between audio and watts.

This picture above shows what pinching the carrier looks like, in this photo the carrier is slightly pinched, as more audio is added the flat area expands wider and wider. Also the peeks are defined again, we do not read anything below the center line when reading modulation. pinching the carrier creates backwards swing on your watt meter as your power is being reduced buy excess audio.

When a AM carrier is properly modulated at 100%, your PEP watt meter should rise exactly 4 times the the dead key. Assuming your not pinching nor flat topping the sign wave. 1 watt dead key modulated to 100% = 4 watts PEP.

When a AM carrier is properly modulated at 50%, your PEP watt meter should rise exactly 2 times the the dead key. Assuming your not pinching nor flat topping the sign wave. 1 watt dead key modulated to 50% = 2 watts PEP.

When a AM carrier is properly modulated at 150%, your PEP watt meter should rise exactly 6 times the the dead key. Assuming your not pinching nor flat topping the sign wave. 1 watt dead key modulated to 150% = 6 watts PEP.

So you can also work this formula backwards, assuming that your not pinching or flat topping, your dead key is 10 watts and your PEP is 40 watts, one could assume you are producing 100% modulation. ( 4 times difference )



Questions and Answers

Q : How much modulation should I get on a standard radio?
A : Typically I see about 85% in the positive direction without pinching.

Q : I can't get my modulation to go higher than 30%, what can I do ?
A : Lower the dead key wattage and % of modulation will go up. Don't forget to recalibrate the scope.

Q : My radio is putting out 85% mod, but when I turn on the amp it only gets to 30%, what am I doing wrong?
A : lower the input wattage into the amp. Whats happening is the amp is cutting off the audio because it can't handle the input wattage and the audio at the same time. Don't forget to recalibrate the scope.

Q : My scope shows slight flat topping of the carrier, is this bad?
A : technically yes, anything square is distortion, and your neighbour's will hear your every word.

Q : Why not use a commercially available RF pick up for the scope ?
A : you can use one like that but remember that each PL-259 does cause a loss in Db. 2 of them even more. I like low loss ! so I use the inductive pick as shown above and save the Db's for the antenna. This applys for watt meters also.



spelling and grammer checked by Rikenna age 9