Tektronix DC503a, Instruction Manual

Page: 202 / 300

TM 9-6625-474-14&P-3
Table 2-5
GATE TIME
VS
MEASUREMENT RESOLUTION
Measurement Rate. Once a stable measurement is
obtained, the rate at which measurements are made can
be controlled by the DISPLAY TIME control. Turning the
control clockwise holds off the gate and stores the display
for a longer time before a new measurement is made and
displayed. Display time and gate time together complete a
measurement-display cycle.
The DISPLAY TIME control is uncalibrated and
variable from about 0.1 second (in the MIN position) to
about 5 seconds. At the extreme clockwise end of the
control is a detent position called HOLD. In HOLD, the last
count taken will be stored and displayed for an indefinite
period. A new count and display can be initiated by
pressing the RESET button, moving the DISPLAY TIME
control out of the detent, or changing the gate time.
PERIOD MODES
The period and period average modes allow single
period measurements or multiple period averages to be
made with input frequencies into CH B. These modes are
useful for making low frequency measurements where
maximum resolution is desired without waiting for long
measurement time. Simply stated, the PERIOD B mode
reverses the functions of signal and clock as compared to
FREQUENCY A mode. Refer to Fig. 2-4A.
Averaging. Resolution and accuracy is improved by
averaging the signal value over a large number of signal
events. This increases the total time to take a measure-
ment, i.e., similar to selecting a longer gate time in
FREQUENCY A mode. Refer to Fig. 2-4B.
Low Frequencies. Period Measurements of signals
below 10 Hz, and particularly in the lowest decade from
0.1 Hz to 1.0 Hz, become rather sensitive to wave shape
and amplitude. Since it is desirable for the signals to pass
through the trigger hysteresis abruptly, square waves are
preferred. Other wave shapes can be accurately measured
if the amplitude is kept high.
TIME INTERVAL MODES
Two modes of time interval measurement can be
selected: WIDTH B, and TIME A - B. The WIDTH B mode
measures the time between two points on a waveform,
These two points are selected by the CH B triggering
controls such that the counter main gate turns on at the
point selected by the CH B SLOPE and LEVEL controls,
and turns off at the same level but the opposite slope,
Refer to Fig. 2-4C.
The TIME A - B mode measures the time between two
points on two waveforms. These two points are controlled
such that the CH A triggering controls select the point at
which the main gate turns on, andthe CH B controls select
the point at which the main gate turns off. Refer to Fig. 2-
4D.
Triggering. The voltage levels necessary to establish
the triggering points on any selected slope are monitored
and set with digital voltmeter readings at the CH A/CH B
TRIG LEVEL pin jacks on the front panel or rear interface
connections. Fig. 2-5 illustrates typical TRIG LEVEL
voltage settings for various time interval measurements.
When making these measurements, each channel must be
dc coupled and coaxial cables must be properly ter-
minated in order to maintain signal fidelity.
WIDTH B Mode. In order to measure pulse duration
(Fig. 2-5, waveform 3), the 50% level must be determined.
Set the FUNCTION switch to WIDTH B and the CH B
LEVEL control fully counterclockwise. Apply the input
signal to the CH B input connector. The GATE indicator
must be off.