Tektronix 7D14, Instruction Manual

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Operating lnstructions-7D14
but where the signal is sufficiently larger than the P-P
VOLTS setting to ensure a steady count. Generally, begin
with the largest P-P VOLTS setting. Then decrease the
switch setting if necessary to obtain a steady count.
The COUPLING switch determines the coupling be­
tween the CH A INPUT connector and the trigger circuit.
Generally, use AC coupling to block the DC component of
a signal. Otherwise, the DC component might cause the
signal level to be beyond the range of the LEVEL/SLOPE
control. DC coupling can be used for low-frequency signals,
or for pulses with known DC levels. See Table 1-1 in the
Specification section for the frequency range and amplitude
limits for each coupling method. The COUPLING switch
does not affect signals selected via the TRIG SOURCE posi­
tion of the INPUT SENS switch.
The BW switch provides a means of filtering out high-
frequency noise pulses which may cause erratic readings.
Generally, use the 5 MHz setting unless the signal to be
counted is above this frequency. The BW switch affects
signals connected through both the internal TRIG SOURCE
and the CH A INPUT connector.
The trigger circuit has a hysteresis (basic sensitivity) of
approximately 100 millivolts. A typical signal waveform at
the input to the trigger is shown in Fig. 2-3A. The
hysteresis of the trigger is represented by the shaded
area.
Each time the signal level (including DC level) rises above
the upper hysteresis limit e 2 , and falls below the lower
hysteresis limit e1, the trigger circuit produces a rectangular
waveform as shown in Fig. 2-3B. This rectangular waveform
is the signal actually counted by the counting circuits.
Generally, the best point on a waveform for triggering
the counter is where the slope is steep, and therefore
usually free of noise Assuming a sine-wave signal wave ­
form, the steepest slope occurs at the zero-crossing point.
Fig. 2-3. (A) Signal input to trigger; shaded area represents trigger
hysteresis with PRESET button depressed. (B) Trigger output.
This is the point on the positive-going slope selected for
triggering when the PRESET pushbutton is pressed, and is
the condition represented in Fig. 2-3A and B.
Noise pulses or other signal components may be pre­
sented which are of sufficient amplitude to produce un­
wanted trigger pulses. A typical condition is represented in
Fig. 2-4A. The resulting trigger pulses shown in Fig. 2-4B
will cause an erratic or incorrect count.
Pressing the VAR pushbutton enables the LEVEL/
SLOPE control to be used to move the hysteresis above or
below the 0-volt level. The condition represented in Fig.
2-4A and B can be corrected by using the LEVEL/SLOPE
control to move the hysteresis to a level above or below the
Fig. 2-4. (A) Input signal with noise in PRESET hysteresis level; and
(B) resultant trigger output.
Fig. 2-5. (A) Signal input to trigger; shaded area represents hysteresis
set above noise pulses. (Bl Resultant Trigger output.
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