Vibration Testing - Keithley 194 Instruction Manual

3.22.7 Noise Analysis
In s"me cases, it may be necessary to analyze noise, rather
than attempt to eliminate it. Peak-to-peak or RMS noise
values are the quantities most often required when analyz-
ing noise. The Model 194 can be used to perform such
noise analysis in a manner similar to that described in the
last paragraph for noise reduction.
Once again, let us consider the AC noise signal riding on
a DC level (Figure 3-30). The first thing we must do is
separate the noise signal from the DC level for proper
analysis. By using AC coupling on the input, the DC signal
will be effectively eliminated.
Once the DC signal has been eliminated, the next con-
siderations are the sampling duration and rate. The dura-
tion, of course, will depend on the length of the time span
required for the analysis in question. The sampling rate
depends on the expected noise spectrum. Again, a good
yardstick is to choose a sampling frequency at least twice
as high as the noise frequency, assuming that the noise
signal is basically sinusoidal in nature.
Once a measurement sequence has been initiated, peak-
to-peak or RMS values of the noise can be obtained by us-
ing the appropriate mathematical function. The instrument
will then calculate and display the corresponding noise
characteristic.
3.22.6 Mechanical Vibration Testing
Most equipment manufacturers perform some sort of
vibration tests on their equipment in order to get some idea
as to how well it will hold up in the real word. As with
any test procedure, data is meaningful only if the test con-
ditions are precisely controlled. The parameters of note
with vibration testing are displacement, instantaneous
velocity, and acceleration. The Model 194 can be used with
external equipment to perform such tests with relative
simplicity.
OPERATION
The basic instrument configuration for performing these
tests is shown in Figure 3-31. Additional equipment re-
quired besides the Model 194 include the vibration table
itself, an accelerometer, and the charge amplifier. The
charge amplifier is necessary to convert the output "f the
piezoelectric accelerometer into a DC voltage that can be
measured by the Model 194.
When the equipment is being tested, the xc&ration
measured at the table will be converted into rl voltage and
measured by the Model 194. The resulting data is then
stored in the buffer of the instrument f"r further .malvsis.
Data can be recalled as required to determine the acceicra-
tion at any instant of time.
The velocity can be found by integrating the accelcrati"n
as follows:
v = \ cl dt
The integral function of the Model lY4 cwld be used t"
perform the necessary conversion from acceleration t"
velocity. Specific transient waveforms could also be pl"t-
ted using the analog output, if required.
Figure 3-31. Vibration Testing
3-5113-52