Measurement Examples - Tektronix DC 5010 Anleitung

Operating Instructions-DC 501 0
Triggering tlhe Counter
Reducing Measurement Errors
The dc triggering level is determined by the SLOPE and
I /
LEVEL selecti~on, or by the AUTO TRIG button.
The LEVEL CH A and CH B buttons, in conjunction with
the increment
(t),
and decrement (J) buttons, are used to
move the triggering hysteresis window continuously up or
down through1 a k2.0 V range in 4 mV steps. The hyster-
esis window is typically 50 mV peak-to-peak. To determine
the exact trigger level settings, push LEVEL CH A (or LEV-
EL CH B); the respective levels will be displayed. To return
to the measurlement cycle, press the LEVEL CH A or LEVEL
CH B button again (whichever is lighted); pressing any func-
tion button will1 also return the instrument to the measure-
ment mode.
When the AUTO TRlG button is activated, the micro-
processor performs a software routine to determine the
maximum and minimum limits of the Channel A and Channel
B input voltage swings. Then the routine automatically sets
the triggering levels of each channel to 50% (+24 mV for
+
slope, -24 rniV for
-
slope) of its respective measured
minimum and maximum values when making frequency, pe-
riod, and totahze measurements. AUTO TRlG is also useful
for pulse width measurements (WIDTH A mode) and TIME
A-B measurements. Successful use of the Auto Trig here
requires signall amplitudes of at least twice the effective hys-
teresis. Thus, signals with amplitudes greater than 140 mV
-
_,
peak-to-peak are typically necessary. This is because the
actual trip level of the hysteresis window is set exactly at
the 50% point for Width and Time A 4 B .
Figure 2-4 illustrates typical trigger level settings and
shows the imiportance of setting trigger levels properly in
order to avoid errors due to input signal risetimes (falltimes),
or where the transition times of the start and stop pulses
are different (,or just slow). Observation of the SHAPED
OUT signals on an oscilloscope, while setting the trigger
levels on slow but complex waveforms, aids in reducing trig-
ger setting difficulties.
The use of the Auto Trig, though very convenient, does
not reduce the need to consider input noise amplitudes, cou-
pling, impedarlce matching, and attenuation factors. Large
amounts of overshoot and ringing of the input signal may
cause erroneous counts due to an undesirable level setting.
The median vdue of the input signal may be displayed. For
mid-point settings, the low frequency limit for the Auto Trig
mode is 10 Hz:. Below 10 Hz, the automatic triggering level
will still be set between the signals maximum and minimum,
but not necessarily at the 50% point. For dc inputs, the level
determination provided by auto trigger once again becomes
correct.
As an aid in reducing measurement errors, keep in mind
the following factors.
Use the ATTEN controls and high impedance,
attenuator type probes when measuring signals from
high impedance circuits.
Use the 50 Q TERMination control for low imped-
ance, high frequency 50 Q systems.
Consider trigger errors caused by input signals with
slow rise or fall times.
Use the 20 MHz FILTER to reduce high frequency
noise.
Average the measurement over a larger number of
cycles of the input signal (greater number of AVGS)
Maintain the counter environment at a constant
temperature.
For greater stability, allow extra instrument warm-up
time (> 112 hour).
Substitute the standard time base with the optional,
higher stability time base.
Apply a 1 MHz, 5 MHz, or 10 MHz external time ref-
erence standard (NBS) to the rear interface inputs.
Recalibrate, if necessary.
MEASUREMENT
Frequency A and Period A
When the counter is in either
EXAMPLES
the FREQ A or PERIOD A
modes, it always measures the period of the Channel A in-
put signal. For FREQ A, the microprocessor computes the
frequency as:
1
f
=
-
(T
=
period)
P
and displays the answer in frequency units. For PERIOD A,
the answer is displayed in units of time. The 320 MHz inter-
nal clock insures very high resolution in both frequency and
period. For period measurements of fast signals with 10'
Averages, this resolution is k31.25 attosecs (31.25
x
sec).
REV OCT 1981