Tektronix DC 502, Instruction Manual

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Operating Instructions — DC 502
Set the MEASUREMENT INTERVAL switch to
MANUAL, apply the signal, and push the START button.
The GATE indicator will light and the progressing count
will be displayed. Adjust the ATTEN and TRIGGER
LEVEL controls as necessary for a steady count. To stop
the counting, release the START button. The GATE light
will go out and the displayed count will be held. The
displayed count can continue when the START button is
depressed again. The counter can be cleared to zero at any
time by pressing the RESET button or by moving the
MEASUREMENT INTERVAL switch to another position.
4-10 PRE-SCALE INPUT.
In the MANUAL mode related
to this input, the displayed count advances one count for
every ten incoming events. The incoming events must have
transition times and periods suitable for the 50 Ω pre-scale
input triggering requirements.
USING THE COUNTER
DIRECT INPUT Attenuation and Trigger Level
Adjustment
Signals to be counted in the DIRECT INPUT channel
may have a wide variety of shapes and amplitudes, many of
which are unsuitable to drive the counting circuits. Because
of this, the signal is first passed through an attenuator, then
applied to a signal-shaping circuit which converts it to
rectangular pulses of uniform amplitude. This circuit
includes a reference level adjustable b and — 2
volts to which the incoming signal is compared, allowing
the 300-millivolt sensitivity window of the signal-shaping
circuit to be adjusted to a convenient amplitude on the
incoming waveform (see Fig. 1-2). Obtaining a steady,
reliable reading is dependent upon the proper selection of
input attenuation and proper adjustment of the TRIGGER
LEVEL control.
Noise Impulses 200-millivolt
Fig. 1-2. Two examples of triggering circuit output showing how
proper adjustment of TRIGGER LEVEL control can avoid an
erroneous count.
Generally, the best point on a waveform for triggering
the counter is where the slope is steep and therefore usually
free of noise. On a sine-wave signal, for example, the
steepest slope occurs at the zero-crossing point. Noise
pulses or other signal components of sufficient amplitude
to produce unwanted trigger pulses will cause an erratic or
incorrect count. Fig. 1-2 shows the TRIGGER LEVEL
control adjusted to avoid error. In critical measurement
applications, monitor the incoming signal with a test
oscilloscope.
Signal Connection
Coaxial cables and probes offer very convenient means of
connecting the signals to the front-panel input BNC
connectors. These devices are shielded to prevent pickup of
electrostatic interference which can cause erroneous trig­
gering and a faulty count. For the DIRECT INPUT, a X10
probe not only reduces the size of the signal, but also
presents a high input impedance to allow the circuit under
test to perform very close to normal operating conditions.
For the 4-10 PRE-SCALE INPUT, the 50 Ω input requires
careful impedance matching. If the signal must be atten­
uated to avoid exceeding the maximum input limit of 10 V,
use 50 Ω attenuator pads terminated by the 50 Ω input
impedance of the DC 502.
Measurement Interval and Display Time Controls
The MEASUREMENT INTERVAL switch selects the
time interval (also called gate time) during which the
DC 502 counts. The internal time-base circuit derives gate
times from an accurate 1-MHz reference signal to make
frequency measurements. These gate times are 0.01 s, 0.1 s,
1 s, or 10 s. The measurement interval selected determines
the measurement range and resolution. Also, the displayed
decimal point is positioned correctly and the correct
measurement units (MHz or kHz) are indicated for the
corresponding switch position.
The DISPLAY TIME control sets the length of time a
measurement can be held in the counter and displayed. The
HOLD detent position allows a measurement to be held
indefinitely, or until the counter is reset to zero by the
front-panel RESET button.
1-3