That's not really an accurate description. One can directly change the phase electrically, without going by any frequency change method. Of course, changing the phase, will automatically lead to a changing frequency spectrum.
-- Kevin Aylward
Den onsdag den 3. maj 2017 kl. 22.16.03 UTC+2 skrev Kevin Aylward:
but could you tell the difference between inverting the phase electrically and changing to a very high frequency for just long enough to end up at the opposite phase?
** Saying that is not helpful, cos it is the generic name for frequency and phase modulation.
He claims to be heavily modulating ( >20% )the period of the carrier with audio, so the frequency changes are large.
Linear FM implies deviations around a centre frequency proportional to signal amplitude. Negative and positive modulation peaks of the same amplitude producing equal deviations in Hz.
What the OP seems to have is period modulation, so frequency changes are not symmetrical about the unmodulated value.
Very simply, if a positive modulation peak causes the carrier to double in frequency, a similar negative one will cause it to halve.
.... Phil
Since one is the derivative of the other, they kinda have to change together.
OK, how is it different than PWM?
Or, for that matter, PCM.
PWM is usually considered to be variable duty cycle at a constant frequency. The kind of modulation suggested here varies the period of each cycle proportional to the baseband modulation input, as I understand it. It's like FM but sort of reciprocal. For small modulation amplitudes, it becomes indistinguishable from FM.
Generating and detecting it are sort of interesting.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
PCM is transmission of digitized, coded values, in words or frames. That's very different from PWM or PDM or FM.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
So we're talking an infinite bandwidth channel?
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
the inverse of frequency is period, so what you're doing is modulating the period, but the result is still indistinguishable from frequency modulation with the input being the reciprocal for your input.
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with PDM the duration of the pulse is significant - eg the servos used by radio-control hobbyists.
with PWM the duty cycle is significant - eg: dimmers.
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PCM is binary numbers as a serial stream
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On a sunny day (Wed, 3 May 2017 21:15:54 +0100) it happened "Kevin Aylward" wrote in :
Exactly, and that was my point: PM can be seen as a special case of FM :-), where you change frequency to get a specific phase relationship.
Provided that you keep = 0 then you can have some phase jitter at constant nominal frequency averaged over the longer term, but the times between successive zero crossings each cycle are still varying slightly.
A slightly higher or lower frequency for sufficient time that the phase drifts by 90 degrees relative to a fixed frequency reference signal.
Even allowing that you can invert "instantly" the only time when the frequency actually remains the same is when the inversion is exactly synchronised with the zero crossings (and even then there are a nasty high frequency components from the sharp discontinuity in gradient).
If you switch phase at any other time then there is a significant component of 2f generated and an abrupt waveform discontinuity. It is most obvious if you invert the signal at the peak of the sine waveform.
Jeroen's description is the most accurate mathematically but Jan's encapsulates the same concept in a more accessible way. (BTW good to see you back Jan)
Manchester coding is an example of a signal using ideal phase shift invert but it is nothing like a constant frequency (data dependent).
-- Regards, Martin Brown
Is that not called PPM (pulse position modulation)?
Three letters don't define things very well. A timing diagram and some equations would better specify a modulation scheme.
"Position" could mean various things.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The point is phase is not addressed by the equation. So it is a pointless argument that the phase can only be changed by modifying the frequency.
Timing of the phase change is not really relevant. The phase can be modified at any value of w. The two are independent.
Sure, there are many artifacts generated by modulation in general. Still not relevant to the issue.
You are saying sin (wt + phi) is the "least" appropriate form?
-- Rick C
Pulse position modulation is just as the name indicates. Pulses are varied in timing but not amplitude or width. It uses digital pulses that are not sine waves.
-- Rick C
It would be useful to categorize this kind of modulation, if only because it mimics the easy-to-do pulse swallowing that is employed in PLLs that have a dual-modulo divider. That means that its detection could use 'digital PLL' logic, much as constellation modulation/detection in the old 56k modems did.
A completely suitable name, however, escapes me. It's NOT FM, because there is no implication of a center frequency; the permissible symbols can be several discrete frequencies, none preferred. Adjacent cycles can jump from N to N+5 as easily as from N to N+1 (periods of the master clock).
You are using the identity cos(pi)= -1 implicitly.
Relative timing and frequency of the phase changes is *ABSOLUTELY* relevant to how closely the resulting waveform looks like a sine wave.
It is critical.
Yes. Unless you have an infinite time domain to play with any finite length of sin(wt) always has sidebands in the frequency domain.
-- Regards, Martin Brown
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