What is a Spectrogram?
A spectrogram displays the power of
a signal as a function of both time and frequency simultaneously. Many engineers
are familiar with time domain (power vs. time) and frequency domain (power vs.
frequency) plots. While somewhat useful, each plot only tells half the story. A
time domain plot will tell you "when" something happened, but it won't tell you
"what" happened. A frequency domain plot will tell you "what" the signal looks
like, but it won't tell you when it occurred. In order to see the full picture,
what is happening and when it is happening, it is necessary to view the signal
in joint time-frequency space, i.e., a spectrogram.
Time Domain Plot
This is a time domain plot of an
EDGE signal showing eight time slot. The different time slots can be clearly
seen ("when"), but it is difficult to see what is going on in each time slot.
We know that the first time slot has much worse EVM than the other time slots,
but we can't see it by looking at the time domain alone. A time domain
plot folds together all frequency data at each point in time.
Frequency Domain Plot
This is a frequency domain plot of
the same signal. Now we can see what the signal looks like, but we lost the
"when". A frequency domain plot folds together all time domain data for
each frequency.
Spectrogram
This is a spectrogram of the first
time slot. Frequency is displayed on the X-axis and time is displayed on
the Y-axis. We can now see both the time and frequency information in the
signal simultaneously. The broadband noise burst from 220 microseconds to
320 microseconds, which was obscured in the other plots, is obvious when viewed
in the joint time-frequency domain. This allows engineers to quickly identify
problems.
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