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Tech548
Moderator
Username: Tech548
Post Number: 309
Registered: 5-2004
| | Posted on Tuesday, October 05, 2004 - 6:12 pm: | 
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How to Properly Adjust Your Modulation Meter!
As we all know, the A/M carrier is stable ONLY with zero modulation. Now, unlike an F/M carrier, this A/M carrier generally increases with modulated voice peaks. And here is where things get a little confusing to some CB operators. When voice audio is introduced to the carrier, this (now modulated) RF carrier is no longer stable. It will now show a RF power increase (aka forward swing) on your power meter. And that's just what we want, right?
Yes, BUT, if your modulation meter was calibrated (set) against a 'dead-key' of lets say, 2 watts, and your RF power meter is showing that you have a forwards "average" swing to lets say, 10 watts, well, your modulation meter just became uncalibrated for any true reading above that originally 2 watt dead-key setting. What you will now see on your modulation meter is a massive amount of needle swing indicating that you have gobs and gobs of audio riding along on your RF carrier. And yes, it certainly does look impressive, but it is a 'totally worthless' modulation reading.
Bummer! Now what do we do? Well, we have a problem here.
First off, you have to understand that 'ALL PEAK READING RF METERS' will show you some upwards RF power whether you actually have it or not. That's just the nature of the beast. The reason why I am now refering to the RF power section of the meter is because we have to determine whether we really have either Upwards or Downwards RF power before we can go any further. But, either way, you will NEVER get an a accurate modulation reading if the modulation meter was 'SET' from a dead-key.
Here is how to determine if you really have Upwards or Downwards RF power when you put some audio to the carrier.
1- Into a dummyload, put your modulation meter in the "SET" mode.
2- With your RF power turned all the way up, dead-key your radio and adjust the "Modulation Calibration Knob" to anywhere around mid-scale. After doing this, DO NOT do anything else with your meter....leave everything as is. Basically, what you are now seeing is just an indication that your radio is keyed.
3- Into the microphone, say a long AAHHHHHHHHH and watch the modulation needle. Does it swing UP or does it swing DOWN? If the needle swings up, that's good because it is more than likely that your transmitter is tuned correctly, and that's exactly what we want to see! But, if it swings downwards, well, then you have a serious tuning problem that needs correcting.
Now, for those of you that have determined that you do indeed have a 'Upwards' RF power swing, now you can find out what your modulation really is. Repeat step #1 and 2, but this time when doing step #2, say a long AAHHHHHHHHH and move the 'Calibration Knob' until the needle is all the way to full scale or the 'Set" position. Once there, turn the Mode control back to the Modulation position. Again say a long AAHHHHHHH and now read your percentage of modulation.
Since modulation cannot be read on a Modulation Meter from a transmitter that has 'Upwards Swing', this method is the only way. The transmitters' modulation can ONLY be read when the RF power is at maximum-carrier "WITH" the audio already riding along with the carrier! In other words, what we're doing here is calibrating (Setting) the meter to a steady modulated carrier.
Another thing to remember. Any modulation that you see on your meter will be incorrect other than what you calibrated for. Or put another way, if you (for example) calibrated for 10 watts using the above method, the meter will be out of calibration for anything under or over that 10 watt calibration.
What I'm attempting to get you to understand here is the fact that 'ALL' Modulation Meters will show you modulation. But not a single one of them will show you your true modulation UNLESS they have a 'constant' modulated RF carrier to sample from. So, by saying that long AAHHHHHHH into your microphone, you are now getting that constant Modulated Carrier and your Modulation Meter now has a reference point to sample from.
As a final point, Modulation Meters are not nearly as accurate or as fast as a scope. The best they can do is give you an idea of what is going on in the modulation department. Even though they can tell you how much, they can't tell you how good your transmitted audio is. But they most certainly do have their use when used correctly and well worth the small investment.
Jeff
Tech548 |
228
New member
Username: 228
Post Number: 2
Registered: 9-2003
| | Posted on Tuesday, October 19, 2004 - 1:39 pm: |
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"Yes, BUT, if your modulation meter was calibrated (set) against a 'dead-key' of lets say, 2 watts, and your RF power meter is showing that you have a forwards "average" swing to lets say, 10 watts, well, your modulation meter just became uncalibrated for any true reading above that originally 2 watt dead-key setting. What you will now see on your modulation meter is a massive amount of needle swing indicating that you have gobs and gobs of audio riding along on your RF carrier. And yes, it certainly does look impressive, but it is a 'totally worthless' modulation reading".
"but it is a 'totally worthless' modulation reading".
as a matter of fact it is measuring accurately. another method of developing additional power in the upper and lower sidebands of a modulated am signal is to increase what is referred to by knowledgeable technicians as the peak-to-carrier ratio, that is, the peak modulated output compared to the resting unmodulated carrier level. 100% modulation usually occurs somewhere between 4:1 and 5:1, all other things being equal. there is no problem. you see gobs and gobs of audio because of the ever-increasing peak-to-carrier ratio as you lower the carrier level (or increase the pep). rightfully interpreted, the meter is doing its job.
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Vtex1
New member
Username: Vtex1
Post Number: 2
Registered: 3-2005
| | Posted on Thursday, March 10, 2005 - 2:54 pm: |
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ok what do you do when it goes back a little??? |
Kid_vicious
Intermediate Member
Username: Kid_vicious
Post Number: 282
Registered: 9-2004
| | Posted on Thursday, March 10, 2005 - 9:35 pm: |
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vtex1, if your needle is swinging backwards when you talk, you have whats called "downward modulation".
this is what happens when you tune the TX coils in your radio with only a dead key instead of saying "ahhhhhhh" into the mic as you tune.
your carrier is set so high that you have no "headroom" for the increased wattage out when you talk. when you exceed the transmitter's capacity like that, it will compensate by lowering the output. hence the downward modulation.
start at the first tuning coil, key the mic, say "ahhhh" in a strong steady voice, as you tune the coil. (you're not using metal screwdrivers are you? bad cb'er!)LOL
while tuning the coil, watch your wattmeter. tune for max wattage out when you talk. repeat this for all the TX coils that are for power output. then go back and do it again. i recommend doing this on ch.20 so that your output is pretty even across the band.
matt |
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