LF: Re: 3 carrier peak volts

[Markus Vester <markusvester@aol.com>]

Hi Mark,

the reason while it works is that the carriers are equispaced, so f2-f1 = f3-f2 = 40 Hz. So the whole pattern is repeating every 25 ms. Every time when f1 and f3 are in phase, f2 is now in quadrature. 

This three carrier scheme is the simplest form of a quadratic phase, forming a rudimantary chirp. For a large number of equispaced carriers, the waveform will asymptotically approach a perfect sweep, like in chirp radar. I have also employed it to reduce PEP in my "chirped hell" multitone picture transmission mode.   

Unfortunately it usually won't work for OFDM because each subcarrier is independently modulated, typically in both amplitude and phase. This will destroy the beneficial phasing and lead to the usual quasi-Gaussian amplitude statistics.

Best 73,
Markus

-----Ursprüngliche Mitteilung-----
Von: gm4ism <gm4ism@blueyonder.co.uk>
An: markusvester <markusvester@aol.com>
Verschickt: Sa, 28. Okt 2017 13:40
Betreff: 3 carrier peak volts

Marcus.

I don’t seem to be able to post, so my comment below direct to you

The peak voltage is a vector addition  of the magnitude and phase angle of each carrier but there is one thing I think overlooked.

Each carrier has a different frequency and therefor the vectors rotate a different rates.

It doesn’t matter if that all start with a fixed relative phase, they cant maintain  that phase relationship and therefor  there will still be times when the peak voltage is the sum of the  individual voltages, as they will appear in phase  at some time.

How often and for how long is a function of the difference in frequency and number of carriers.

This principal is what lies behind the Peak to Mean ratio of multi-carrier systems such as DAB and other OFDM modulation schemes

The  hundreds of carriers can in theory all line up giving peak voltages  that are ridiculous.  Actually these  peaks are statistically so rare and short lived that the accepted  peak to mean power of a large number of carriers is usually take as 10dB for the purpose of determining  component ratings.

 

Mark GM4ISM