Interesting, thanks Jim.
Eddie
On 07/04/2011 19:33, James Moritz wrote:
Dear Roger, LF Group,
These vertical stripes are indeed due to harmonics of the mains
frequency, specifically the 179th harmonic at nominally 8950Hz. If you
watch these harmonics with, say, "QRSS3" spectrogram settings, you
will see wandering "carriers" at multiples of 50Hz, that drift around
in a range of a few 10s of Hz near 9kHz, and may change frequency at a
rate of several Hz per minute. However, with a millihertz-resolution
spectrogram, this drift rate is fast enough that some energy appears
in most of the displayed FFT bins on the screen during one FFT window
period, then drifts completely out of the displayed range. Hence the
result is a vertical streak of noise. 8970Hz/179 is apparently close
to the limit of how far the mains frequency is allowed to drift; I
usually see these bands of noise a few times a day around 8970Hz, but
it rarely reaches 8976Hz
Cheers, Jim Moritz
73 de M0BMU
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