Tektronix 7D11 DIGITAL DELAY Service Instructions Manual page 11

Operating Information-7D11 Service
A signal can also be routed to the 7 D 11 through an
amplifier unit via the internal trigger circuitry of the
mainframe (7D 11 installed in a horizontal compartment).
This method of signal connection minimizes circuit loading,
especially when triggering a time-base unit in parallel with
the 7D 11.
NOTE
Only external signals can be used with the Events
Start Trigger.
The front-panel output signals, DL Y'D TR IG OUT and
DLY INTERVAL OUT, should be connected to other
equipment with 50-ohm coaxial cables. The cables should
be terminated in 50 ohms to maintain the rise and falltime
characteristics of these signals.
Count Mode
General. Two basic count modes, Time and Events, can
be performed by the 7D 11, as selected by the COUNT
MODE switch. The delay interval in both modes is selected
by the DELAY TIME OR EVENTS control and is read out
on the CRT.
TIME INT CLOCK. The 7D 11 counts precise incre-
ments of time after the receipt of a trigger. The TRIGGER
controls select and condition the signal to start the time
delay. Accuracy in this mode is determined by an internal,
crystal-controlled oscillator.
TIME EXT 1 MHz. This count mode is the same as
TIME INT CLOCK except the accuracy is derived from an
external, one-megahertz standard.
EVENTS. The 7D 11 counts events, periodic or aperi-
odic, at count rates to 50 megahertz. The EVENTS START
TRIGGER provides a means of discriminating between the
event that starts the delay and the events to be counted.
The events to be counted are selected and conditioned by
the TRIGGER controls.
Trigger Controls
The input signal may have a wide variety of shapes and
amplitudes, many of which are unsuitable as delay-initiating
triggers. For this reason, these signals are first applied to a
trigger circuit where they are converted to pulses of
uniform amplitude and shape. This makes it possible to
start the delay with a pulse that has a constant size,
eliminating variations of the delay circuit operation caused
by changing input signals. The TRIGGER controls provide
a means to select the signal source, filter unwanted
frequencies, and start the delay at any voltage level on
either slope of the waveform.
1-4
Triggered Light
The TRIG'D light provides a convenient indication of
the Trigger circuit condition. If the TRIGGER controls are
correctly set and an adequate signal is applied, the TRIG'D
light is on. If the TR IG'D light is off, no delay interval is
started. The cause might be an incorrectly set TRIGGER
control, low signal amplitude, or a signal repetition rate
outside the usable frequency range. This feature can be
used as a general indication of correct triggering when there
is no display on the CRT. The Delay Interval Pedestal and
Z-Axis Blanking displays also aid in obtaining correct
TRIGGER control settings. See the discussion of these
features under Output Signals to Mainframe for further
information.
NOTE
When the 7D11 is used in the EVENTS count mode,
the EVENTS START TRIGGER affects the output
of the Trigger circuit but has no effect on the
TRIG'D light.
Trigger Coupling
The TRIGGER pushbuttons located below the
COUPLING title select the method in which the input
signal is connected to the Trigger circuit. Each position
permits selection or rejection of various frequency compo-
nents of the signal used to trigger the delay start.
AC. In this position of the COUPLING switch, the DC
component of the input signal is blocked. Signals with
low-frequency components below about 30 hertz are
attenuated. In general, AC COUPLING can be used for
most applications. However, if the signal contains unwanted
frequency components or if the delay is to be triggered at a
low repetition rate or DC level, one of the other switch
positions will provide better results.
The triggering point in the AC position depends upon
the average voltage level of the input signal. I f the input
signal occurs randomly, the average voltage level will vary,
causing the triggering point to vary also. This shift of the
triggering point may be enough so it is impossible to
maintain a stable delay start; in such cases, use DC
coupling.
AC LF REJ. In this position, DC is rejected and
low-frequency input signals below about 30 kilohertz are
attenuated. Therefore, the delay is triggered only by the
higher-frequency components of the input signal. The AC
LF REJ position is particularly useful for providing stable
triggering if the input signal contains line-frequency compo-
nents.
AC HF REJ. This COUPLING switch position passes all
low-frequency signals between about 30 hertz and 50
kilohertz. DC is rejected and signals above 50 kilohertz are
attenuated. This position is useful to trigger the delay from
the low-frequency components of a complex waveform.
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