Percentage of modulation

[Robb]

I suppose......not disagreeing with you or anything but was just curious. Really you are ball parking it but since there are a couple variables to deal with such as mike gain and internal adjustments then ball parking is close enough since the user operator will be able to adjust it anyway. The important thing is to set the max modulation limit properly and ensure that it cannot be exceeded by advancing the mike gain on the front panel too far.

Yup.
And if we were to be totally honest about scope use, we also both know that gauging 30% modulation is also ball parking it. I mean; how many of us use a gauge on the scope screen and measure? Not I. But we can see distortion and cannot count it in as part total % of modulation. That's recognizable, and also a trap for new players. But voltage/amperage/freq settings; those are easily more definable.

 

[Tallman]

It is 30 millivolts not 30%.

You inject a 1 kHz 30 millivolt signal and adjust for 100% by turning VR4.

View attachment 20045

 

[LeapFrog]

Ummm . . .yes.
But the other part is recognizing distortion too. Seems this will answer many of your own questions about modulation mods and adjustments once this well understood.

Some of the questions I have are not so cut-and-dry as is this thread.
I measure my graticule, that's how far I go.
Nothing will help you spot distortion, like experience. It takes a keen eye, and you have to know what to look for, obviously.

Yup.
And if we were to be totally honest about scope use, we also both know that gauging 30% modulation is also ball parking it. I mean; how many of us use a gauge on the scope screen and measure? Not I. But we can see distortion and cannot count it in as part total % of modulation. That's recognizable, and also a trap for new players. But voltage/amperage/freq settings; those are easily more definable.

How is "gauging 30%" as you say "ball parking it", when you measure the signal. The graticule is the reference point, so no guess work needed! This is "well-understood".

Explained in this link:

30% AM modulated signal, time domain view. (Horizontal axis time, vertical axis amplitude)

This photo & link are relevant to your question:

The measured modulation is quite close to the signal generator's 30% setting.

With all this talk about 30%... Don't lose sight of the fact: The end goal is to have a radio that will not over-modulate. (As Captain Kilowatt said)

 

[ExitThirteen]

When I do all my modulation testing I direct inject the signal to the mic jack, rather than using an audio tone thru a mic. That way you can't "corrupt" the tone. I also sweep it from 500hz to 4khz to see if there's any major changes in the harmonics on the spectrum analyzer.


~Cheers~

 

[Tallman]

When I do all my modulation testing I direct inject the signal to the mic jack, rather than using an audio tone thru a mic.~Cheers~

That's the way I do it when the specification calls for it. I have the B&K 2040 and the 1040. I can do it in any way required.

 

[Robb]

When I do all my modulation testing I direct inject the signal to the mic jack, rather than using an audio tone thru a mic. That way you can't "corrupt" the tone. I also sweep it from 500hz to 4khz to see if there's any major changes in the harmonics on the spectrum analyzer.


~Cheers~

Ahh, a SA.
Like to have one of those.

 

[ExitThirteen]

I direct inject it at the mic jack when I am doing my asymmetrical modulation mod on a radio, so I can fine tune the mod itself, and it also allows me to adjust the 54mhz trap coil to minimize harmonics. IMD is never really an issue because from an RF standpoint, I am operating all of my RF stages within the limits of their transistor specs.

Last radio I did my mod on, the worst reading I got for harmonics was -52dB down. That was on the 1st order. Everything else was below -52dB. That's checking 1st thru 4th order harmonics on the spectrum analyzer, and sweeping the tone frequency from 500hz to 4khz.

~Cheers~

 

[Robb]

I peak TX coils with 30-40% in AM/only radios.
In this way, I don't drive the final for too hard and long and possibly pop it.

 

[LeapFrog]

I direct inject it at the mic jack when I am doing my asymmetrical modulation mod on a radio, so I can fine tune the mod itself, and it also allows me to adjust the 54mhz trap coil to minimize harmonics. IMD is never really an issue because from an RF standpoint, I am operating all of my RF stages within the limits of their transistor specs.

Last radio I did my mod on, the worst reading I got for harmonics was -52dB down. That was on the 1st order. Everything else was below -52dB. That's checking 1st thru 4th order harmonics on the spectrum analyzer, and sweeping the tone frequency from 500hz to 4khz.

~Cheers~

In an older but recent thread, we were discussing an NPC modification, and IMD or *something* and a spectrum analyzer.
Only you could not detect the issue the modification produced, unless you specifically tested for it.
(The analyzer had to be adjusted in certain manner or you would not notice the problem)
Someone said "okay, the distortion is there, what to do about it"
I cannot find the thread, would someone please point me to it.
Guess I'm out of the friggen' circle. It'll take me a while to search through 600 messages to find the thread

 

[Robb]

One thing I noticed in your video that might help you, is to adjust the time base so that you can see two or three complete cycles, rather five or six cycles. Then, make the peaks higher by changing the voltage setting. In this way you can get a larger, more detailed look at each cycle. Makes it easier to detect ambiguities visually. Of course you cannot see IMD or 2nd harmonics on a scope like you could with a SA, but you will be able to see if the sine wave is uniform and not clipping. That'll be something you can do. That should help keep IMD down some too. Keeping the sinusoidal shape pristine as the signal coming in is important too.

BTW - did you peak the TX coils?

 

[LeapFrog]

That should help keep IMD down some too. Keeping the sinusoidal shape pristine as the signal coming in is important too.

BTW - did you peak the TX coils?

This was in my early days (I'm still new) I now have a much better grasp on how to use an oscilloscope. I will get a tri-pod and demonstrate one day. it takes a little bit to learn how to use one.
I need a proper signal gen. the problem has already been explained to me, it's because I used the speaker method (a sloppy approach at that). The mic stage was "OVER-driven" by the speaker.


Didn't want to quickly "ruin" (make inoperable) the radio by touching cans I had no business toying with, after all I don't have a FSM for this one and it's a big difference between vco alignment and TX peaking.

 

[Captain Kilowatt]

Yup.
And if we were to be totally honest about scope use, we also both know that gauging 30% modulation is also ball parking it. I mean; how many of us use a gauge on the scope screen and measure? Not I. But we can see distortion and cannot count it in as part total % of modulation. That's recognizable, and also a trap for new players. But voltage/amperage/freq settings; those are easily more definable.

Well maybe if you want to split hairs between 30% and 31% you could call it ball parking but actually using a sine wave and using the reticles on the scope screen is quite accurate. I used to have to do it and be accurate every time I installed a new set of modulator tubes in a transmitter but most of the time it was using program content and not a sine wave. A bit harder but a good scope display and a darkened room helped a lot. In any event we are talking CB radios here and for 99% of the time ball parking is closer than most operators will ever need anyway especially with a variable gain at their fingertips.

 

[2FB327]

If this will help anyone else out, I found some of the factory manuals say 80% and othe 30mv, but I was able to hook a DVM to the output terminals and use the fine tune knob to get it to 30mv. Confirmed with a frequency counter at the other end with 1kHz tone, Problem solved. Thanks to everyone.

 

[Robb]

Congrats.
I sure hope that thing works better as an audio sig gen than it did for an HF sig gen. It really sucked at doing that. Real unstable. BT; DT.

 

[LeapFrog]

I sure hope that thing works better as an audio sig gen than it did for an HF sig gen. It really sucked at doing that. Real unstable. BT; DT.

Robb, this unit looks like an AF sig gen "LAG-27", not meant to be used as an RF sig gen. So that might explain if you had issues using this one, also this unit (likely) was made in Japan, and only looks like the unstable China made RF sig gen.
Seems like they copied this case style look for the "new" China made product.

Taken from the web:
LAG-27 Dimensions (WHD) 9.4 x 5.9 x 5.1 inch
China RF Gen Dimensions L12 x W8.5 x H6 inch

The A.F. sig gen LAG-27 even looks a lot like the R.F. sig gen: "Lodestar".

On a side note I think I've figured out what the "infinite time domain" is, lol