On 7/28/2014 9:36 AM, Kim Elmore wrote:
I've been thinking, too. A square wave has an infinite number I'd odd
harmonics, not all harmonics. Regardless, it's an excellent way to
look at IMD, far better than two tones.
Yup.
What if we modulated the TX with pink noise? That really does have
everything in it. Make the pink noise much wider than the expected
audio bandwidth. Ideally, the spectrum should be limited to the TX
filtered BW. Wouldn't this be nearly the most severe test?
Actually, it's a near ideal test for performance of the rig on SSB. One
of the major virtues of pink noise as a test signal is that it roughly
approximates the spectrum of both music and speech. In pro audio, we've
used "shaped" pink noise for years to specify the performance of
loudspeakers, where the shaping is simply bandpass filtering of pink
noise for the region where the loudspeakers are intended to operate. I
strongly encourage the use of pink noise shaped to a bandwidth of 100 Hz
to 5 kHz for this purpose.
I've discussed this with Rob Sherwood several times, and taken him to
task for using white noise for that purpose because it does NOT
approximate speech. His response is that his intent is to represent
clipped and distorted audio. My problem with that is that good audio
signal processing should be tailored to the spectra of speech, so using
white noise as a test signal will cause that signal processing to
misbehave (or behave differently from how it would work with speech).
For those who don't know, pink noise is random noise with equal power
per PERCENTAGE bandwidth -- per octave, per third-octave, per
sixth-octave, etc. It can be generated by applying a 3dB/octave rolloff
to white noise.
73, Jim K9YC
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