Keithley 194 Instruction Manual page 47

OPERATION
2. Press: SAMPLES, FREQiTIME (if necessary to enter
data as time duration), 6, 0, 2, ms.
3. Note that the 602msec programmed value has chang-
ed to 600msec, which is the nearest integer multiple of
the l2msec sampling interval.
Example 4: Setting the sampling duration smaller than
the sampling interval.
The sampling duration cannot be smaller than the sam-
pling interval, as discussed previously. The instrument will
round off the value if you attempt to program the instru-
ment in this manner, as in the example below.
1. Press RATE, FREQiTIME (if necessary to place the
display in the time interval mode), 1, 0, 0, ms, ENTER.
This keystroke sequence programs a 10011~ sampling
interval.
2. Press SAMPLES, FREQiTIME (if necessary to display
and enter time units), 2, 0, ms.
3. Note that the measurement duration is changed to Opsec
(1 sample) because it is rounded down.
Example 5: Automatic recalculation of number of samples.
If the sampling rate is changed after programming
measurement duration, the instrument will automatical-
ly recalculate the number of samples to keep the measure-
ment duration the same, as in the example below.
1. Press RATE, FREQ/TIME (if necessary to enter time in-
terval information), 1, ms. At this point a lmsec sampl-
ing interval has been programmed.
2. Press SAMPLES, FREQiTIME (if necessary to enter
duration as time information), 5, 0, 0, ms, ENTER. This
sequence enters a 500msec sampling duration.
3. Press SAMPLES and FREQiTIMlZ to display the number
of samples. The display should indicate that the pro-
grammed number of samples is 501 because the sampl-
ing duration is 500msec, and the sampling interval is
1lllSeC.
4. Change the sampling interval to 5msec as follows.Press:
RATE, 5, ms.
5. Press SAMPLES and FREQiTIME to display the pro-
grammed number of samples. Note that the number
of samples has been changed to 101 because of the
change in sampling interval, although the sampling
duration remains at 500msec, as previously programm-
ed (you can verify these values by pressing the
FREQiTlME key while in the samples mode).
3.5.3 Samples and Rate Selection
Considerations
Because the Model 194 can sample input signals at rates
as high as lMHz, it is ideal for many applications involv-
ing the analysis of many time-varying signals, both of the
periodic and transient variety. To ensure optimum accuracy
when measuring such signals, care must be taken when
selecting both the sampling rate and the number of
samples. In the following paragraphs, we will discuss some
of these considerations that should be taken into account
when choosing sampling rates and measurement
durations.
Input signals to the Model 194 are in analog form. Inter-
nally, however, the Model 194 operates in the digital world.
Thus, the analog signal must be converted into digital in-
formation by the A/D (Analog-to-Digital) converter of the
instrument. This conversion process is not continuous, but
rather is done at discrete intervals, determined by the
entered rate parameter. If the sampling rate is too slow,
considerable information about the original analog signal
will be lost, and errors can creep into the resulting data.
For example, assume that a sinusoidal waveform is being
sampled at regular intervals, and the result digitized and
stored internally.
Once the data is sampled and stored, we can attempt to
reconstruct the original waveform from the data. However,
the result is no longer a smooth, continuous waveform,
but is instead made up of discrete steps. Thus, as the result
of this digitization process, we may have lost much im-
portant information about the original signal.
To make the steps in the reconstructed data smaller, we
can increase the sampling rate. At the same time, we will
have to increase the number of samples per measurement
if we still wish to measure at least one cycle of the applied
signal.
It is clear then, that we should make the sampling rate suf-
ficiently high so as not to loose important information pre-
sent in the original input signal. Information theory states
that, for sinusoidal waveforms, the sampling frequency
must be at least twice as high as the highest frequency
component in the measured sienal. Thus. if a 1OOkHz
signal is to be sampled, the sampling frequency must be
at least 200kHz.
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