Selecting Gate And Delay Time; Setting Trigger Levels - Keithley 776 Instruction Manual

Opernting Instnrctions
3-9. Selecting Gate And Delay Time
The Model 776 may be operated in its preset gate or
delay times of one second. or with one of its 500 built-in
increments. When selecting the gate time. the instru-
ment will increment up or down. one gate time. each
time the [fi] or [u] push-button is pressed. The present
gate time may be noted oo the display by pressing the
[GT] push-button.
The gate times may only be selected in conjunction
with frequency. time averaged. ratio FRQ/B_ and Phase
A to B functions.
The gate time may be selected as follows:
1. Select a frequency measurement function.
2. Press the [GT] push-button. Obsewe that the GT light
turns on and that the instrument displays the following
message:
1.0
This reading indicates the selected gate time in sec-
onds.
3. To change the gate time press the [VERNIER fl] or
[VERNIER 111 push-buttons. Pressing the [I?] push-but-
ton increases the value of the gate time. Conversely.
pressing the [DOWN] push-button decreases the value
of the gate time. Depressing the [l?] or [u] push-buttons
for more than 1 second. causes the instrument to incre-
ment or decrement contirwously.
4. Pressing the [fil push-button and the instrument was
at a gate time of 100 seconds forces the instrument into
a USER GATE mode. This mode requires an external
gating signal. Operating the counter with an external
gate is explained later in this section.
5. Pressing the [l?l and [u] simultaneously changes the
gate time to a preset value of 1.0 second.
6. To resume normal display operation. press the [GT]
push-button. The GT light turns off and the instrument
is now ready to perform measurements with the newly
selected gate time.
NOTE
To prevent operator error. there is an
internal audible alarm that sounds
whenever a limit is reached. (e.g low
limit of 100 ps).
The sequence of selecting the delay time is similar to
the procedure for selecting the gate time. Delay times
may only be selected and applied to single shot (non-av-
3-8
eraged) time measurement. To select a delay time value
proceed as follows:
1. Select a single shot time measurement function such
as Period. Pulse Width. or Time Interval A to B.
2. Press the IDLY] push-button. The DLY light turns on
and the instrument displays the following mcssagc:
1.0
This reading indicates the selected delay time in
seconds.
3. To change the delay time press the [VERNIER 01 or
[VERNIER u] push-buttons. Pressing the [fi] push-but-
ton increases the delay time. Conversely. pressing the
[u] push-button decreases the delay time. Depressing
the [I?] or [u] push-buttons for more than 1 second.
causes the instrument to increment or decrement con-
tinuously.
4. Pressing the [fl] push-button and the instrument was
at a delay time of 100 seconds forces the instrument into
a USER DELAY mode. This mode requires an external
delay signal. Operating the counter with an external
delay is explained later in this section.
5. Pressing the [fl] and [u] simultaneously changes the
delay time to a preset value of 1.0 seconds.
6. To resume normal display operation. press the IDLY]
push-button The DLY light turns off and the instrument
is now ready to perform measurements with the newly
selected delay time.
3-10. Setting Trigger Levels
There are two trigger level push-buttons associatedwith
each Channel A (TL A) and Channel B (TL B). The
[VERNIER fi] and [VERNIER u] push-buttons. when
operated in conjunction with TL A or TL B. set
the
voltage level that will trigger the instrument. To set the
trigger levels proceed as follows:
NOTE
The procedure for setting the trigger
level is identical for Channels A and
B. Trigger level A (TL A) can only be
modified when functions that are re-
lated to input A are selected. e.g. FRQ
Al TIME A to B etc. Selecting a func-
tion which involves Channel B en-
ables access to the TL B parameter.
1. Using the procedure which is described above. set the
instrument to Frequency A function. If other front panel