On 11/05/2012 02:15, Bruce Perens wrote:
David, Mel, and I have been having a discussion of PAPR: peak vs.
average power ratio, off of the list. This is also called "crest factor".
When multiple carriers in our FDM modem are close to a peak of their
waveform simultaneously, we can get a momentary output power that is a
large multiple of the average. For our FDMDV-derived modem, the ratio
is 13.5 dB
We don't actually operate our SSB amplifiers in a way that avoids
clipping, because we can tolerate /some/ clipping. Clipping increases
the bit error rate but increasing power reduces the bit error rate
generally. In the 925 mile test, the peak power spikes that should be
visible aren't, they're either buried in noise or were clipped, and
the bit error rate is still /zero/ for much of the transmission. It
sounds to me like the amplifier was operated with about 6 dB headroom,
but we don't have a measurement and there is much unknown about how
the amplifier clips.
Hi, for measuring my hifi, I exclusively use log sine sweeps. If you
stick with the standard log-sine then you get interesting features which
is that you can do all the mathematics very simply in the time domain
and additionally the harmonic distortion vs frequency drops out for free
I forget the details now, I think you can construct arbitrary crest
factors by adjusting the group delay when you build your sweep. The
benefit there is you can reduce power arbitrarily at different
frequencies, eg to work around known ringing, peaks, etc. You need to
invert using an FFT rather than the simple
reverse-the-input-and-multiply trick, but I guess that should hold no
fear..?
The point being to blat your radios with some measurement tones and have
a look at the results... The sweeps will show you not only whether they
clip, but also clipping vs freq.
Oh, lovely thing about sine sweeps is they are robust against many types
of distortion, so great for deliberately measuring into the distortion
range.
Apologies if I'm teaching you to suck eggs..
Regards
Ed W
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