To set up a time domain reading
can be a bit cumbersome. This is mainly due to the fact that the setup
is often done using FFT parameters such as Fmax and lines of resolution.
This section will first explain how a time domain is set up and then provide
some easy to use examples. The time domain equivalent to Fmax and lines
of resolution are:
-
Fmax = Period
(length of time sample being collected)
-
Lines of Resolution
= Bytes (how many pieces of data are collected to create the sample)
-
512 Bits is equivalent
to 200 lines of resolution
-
1024 bits is equivalent
to 400 lines of resolution
-
2048 bits is equivalent
to 800 lines of resolution
-
4096 bits is equivalent
to 1600 lines of resolution
As you may already know, a time domain
plot is just as susceptible to resolution limitations as an FFT is. Figures
1, 2 and 3 are each from the same time domain plot - the latter two are
zoomed in on. |
Figure 1
|
Figure 2
|
Figure 3
|
The plot shown in Figure 1 - an actual,
real-life time domain plot - was collected with 2048 amplitude values (the
time domain equivalent of "lines of resolution"). The length of the time
sample is 0.114 secs. |
| Figure 2 shows a portion of the
same time domain plot with the sample reduced to 0.04 seconds (by zooming
in). This is done in the same manner as one would zoom in a an FFT. It
still looks pretty good but just as with a spectrum, zooming in has done
nothing to improve the accuracy of the data. Once collected, you
can never improve or in any way change the accuracy of any plot
- time domain or spectrum. The resolution is dictated by the parameters
set up and cannot be altered after the fact. |
| Figure 3 shows the same plot with
the sample reduced to only 0.01 seconds. It is now quite clear that the
time domain plot is generated by compiling a series of amplitude values
and connecting them with lines - the same way an FFT is generated. This
is JUST AS IMPORTANT and must be stressed just as much as with the
spectrum. Zooming in to this level does nothing to improve the resolution
and is about as helpful in viewing the big picture as looking at a forest
with your face 2 inches from a particular tree would be - in other words,
not helpful at all. |
| So how do we set up a time domain
reading using FFT parameters from this information ? |
