Hardware Reference
In-Depth Information
Figure 11.10 Frequency spectrum of a 1 kHz square wave generated by a
general-purpose waveform generator. The instrument is set to capture 256
samples per second. Note that the noise floor appears to be only about 40 dB
below the fundamental. Note also the preponderance of odd harmonics at 3, 5,
7 kHz, and so on
noise floor is at a level of 20 dB (or 40 dB down on the level of the fundamental
which is + 20 dB).
Figure 11.11 shows the effect of increasing the sampling rate to 512 samples
per second. The increased sampling rate makes it possible to make a more
accurate assessment of the levels of the individual harmonic components but
clearly there is more present in this spectrum than just the expected harmonics!
Figure 11.12 shows the effect of a further increase in sampling rate. At 1024
samples per second, it is possible to see the noise floor a little more accurately
and also to detect the presence of multiple signal components between each of
the harmonics of the 1 kHz fundamental.
At 2048 and 4096 samples per second we obtain a rather different view
of what's going on (see Figures 11.13 and 11.14). In fact, the noise floor is
nearer 60 dB and what we are looking at is a series of harmonics each with
a set of side-frequency components that result from modulation by unwanted
signal components with a spacing of about 250 Hz. This display very effectively
shows the advantages of using a high sampling rate and having sufficient buffer
memory available to actually store the captured data!
Finally, and in contrast with the previous examples, Figure 11.15 shows a
much purer signal. This signal is a 1 kHz sine wave derived from a low-distortion
AF signal generator. Here the DSO has been set to capture samples at a rate
of 4096 per second within a frequency range of DC to 12.2 kHz. The display
clearly shows the second harmonic (at a level of 50 or 70 dB relative to the
fundamental), plus further harmonics at 3, 5, and 7 kHz (all of which are greater
than 75 dB down on the fundamental).
When cursors are added to a frequency spectrum display, it is possible to
make extremely accurate measurements. Figure 11.16 shows the frequency
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