# What would you call this modulation?

• posted

I am experimenting with a 500 Kc/s carrier system with audio modulation. For various reasons I found it convenient to modulate the frequency of the carrier by controlling the duration of each cycle.

After some consideration I realised that this cannot correctly be called frequency modulation because it is the reciprocal of the frequency which is proportional to the modulation waveform, not the frequency itself. At high carrier frequencies with low deviation this may not matter, because the error due between the deviation and its reciprocal is very small, but the deviation on this system is more than 20% of the carrier frequency.

I then considered that "pulse duration modulation" might be a more appropriate name; but this is misleading because the system is not using a pulsed carrier, the information is contained in the duration between zero-axis crossings of a sinewave.

The term "time modulation" is probably the least inaccurate, but it is vague and doesn't really explain what is going on (it also suggest the surreal possibility that time can somehow be modulated). Can anyone come up with a better suggestion?

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• posted

On a sunny day (Wed, 3 May 2017 09:55:27 +0100) it happened snipped-for-privacy@poppyrecords.invalid.invalid (Adrian Tuddenham) wrote in :

Frequency modulation refers to a change in frequency. It does not specify what the 'correction' looks like. For example for FM broadcast there is pre-emphasis, more deviation for higher modulating frequencies, we still call that frequency modulation. What's is a name? There is also PM (phase modulation), but to change phase you need to change frequency... Better just specify the math I'd think.

But do not listen to my extreme ideas, at your own risk. :-)

• posted

"Period modulation".

Cheers

Phil Hobbs

• posted

Frequency Modulation. I believe this is common enough that there is a shorthand name, "FM".

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Rick C```
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** Which is precisely what FM is - after amplitude imiting.

Assuming the modulation frequency is

• posted

Well, it's still FM, you're modulating the frequency, but it's not a linear amplitude to frequency relationship, so maybe just 'non-linear FM' if you need a special phrase. Period modulation has already been suggested, and that seems to be a good description too.

So I'd go for 'non-linear frequency modulation (period modulation)'.

Cheers

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Clive```
• posted

• posted

I have seen PDM used to describe such modulation.

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John Larkin         Highland Technology, Inc

lunatic fringe electronics```
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On a sunny day (Wed, 3 May 2017 06:12:27 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

hehe, that is actually a fun thing. Imagine a round racetrack.

2 cars, one just a few meters behind the other (or degrees in a circular track). If the one behind wants to catch up, then he must speed up. There is no such thing as 'instantaneous' in that sense, it will take him part of, or one or more rounds to catch up (zero the phase difference). During those times his round lapse time will be shorter. So 1 / laps time will be higher, is the frequency. Pedantic? I dunno.
• posted

The analogy doesn't hold. I can vary the phase with no change in frequency at all. signal = sin (wt + phi) I can change phi all day without changing w. So yes, there is instantaneous phase change limited only by the channel bandwidth.

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Rick C```
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You seem to be multiplying a sine with PWM resulting in FM , PM, AM plus spewing sidebands and harmonics as the carrier zero cross shifts and the average cycle amplitude shifts with PWM.

I would call it dirty FM modulation or DFM or non-linear FM (NLFM)

• posted

But you can rewrite that as sin ((w+dphi/dt)*t), so the frequency

*is* different while the phase is changing. Frequency is just the rate of change of phase. Jan is right.

Jeroen Belleman

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Perhaps I'm reading it wrong, but if you're controlling the duration of a constant RF frequency, I would simply call it PWM (pulse width modulation). However, if you're controlling only the frequency of each cycle, and keeping the duty cycle constant, then it's simply FM.

If you look at what it produces on a spectrum analyzer, I would guess(tm) that it would look much like FM, complete with Bessel function peaks and following Carson's rule for occupied bandwidth. Therefore, methinks it would be FM.

The ITU and FCC have a menu of emission designators for the purpose. These define the method of modulation (which is the current question), the occupied bandwidth, and some optional details such as the data type. I suggest you try to cram your modulation system into their format and see if anything fits.

FCC Emissions Designator:

Emission types:

FCC 2.201 subpart C

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Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com ```
• posted

PDM generally refers to "Pulse Density Modulation", which is quite different.

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I don't agree the two equations are the same. What values do you plug in to get a 90 degree phase shift or any other phase shift?

Phase can be shifted instantaneously. If I invert the sine wave that corresponds to a 180 degree phase shift. A(t) * sin(wt) lets me do that with no phase variable and no change in frequency.

Which of the three forms is "correct"?

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Rick C```
• posted

TLAs are often ambiguous.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
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I'd never heard of "pulse duration modulation" (not sure how it's different than PWM) but evidently the Internet has.

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Maybe I get out more than you do.

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John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  ```
• posted

I'd call it "FM" unless someone really needs the details. Then I'd call it variable period modulation.

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Tim Wescott
Wescott Design Services ```
• posted

I think the test for it being FM or not is, can you tell the difference at the receiving end without knowing the input

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