Tektronix 455, Instruction Manual, Page: 68 / 246

C ircuit Description—4 5 5 /A2/B2
When both pins 13 and 14 o f U2790 are LO, the Sweep
Start Comparator (inside U2790) minus input is pulled LO.
This causes pin 4 of the 1C to become a high impedance,
allowing Ct to begin charging through R t. As Ct begins
to charge toward the voltage applied to Rt, the gate of
Q2784 (connected to the junction o f Ct - Rt), goes posi­
tive by the same amount. This increases the current
through Q2784 and decreases the current through Q2782.
The decrease in the Q2782 drain current through R2787
produces a positive-going voltage at the base o f Q2786.
The voltage at the collector of Q2786 and the negative side
of Ct is pulled down. This results in a negative-going voltage
applied across Ct, maintaining a constant charging current
and giving a linear rate o f fall to the sawtooth output
signal.
The sawtooth o utput continues to fall in this manner until
it reaches —2.4 volts and initiates a H oldoff Start pulse at
pin 10. The H old off Start pulse starts the holdo ff ramp in
U2750 and resets the sweep by causing pin 13 o f U2790
to go HI.
The B sweep is dependent on the signal at the delayed gate
input (pin 14) and resets only when the A sweep ends.
The Delay Start and B Start controls, R2782, and R2682,
adjust the quiescent level of the A and B sweeps, thus de­
termining the correct starting points fo r the sweep outputs.
Sweep Control 1C
The Sweep Control circuit is contained w ith in a single 1C
U2750. Fig. 4-7 shows a block diagram o f the circuitry
in this 1C.
The follow ing is a brief description of the function asso­
ciated w ith each pin o f U2750.
Pin 1. Single sweep mode. When this pin is LO (ground)
and pin 4 is HI, the sweep is in the single sweep triggering
mode.
Pin 2. Single sweep reset. Pushing the SGL SWP push­
button resets the sweep triggering circuit, making it
ready fo r another single sweep to be triggered. The
READY indicator lights when the single sweep circuit
is reset and any sweep in progress is terminated.
Pin 3. Autom atic tim ing. In the automatic triggering
mode, when no trigger pulse occurs w ith in about 100 ms
follow ing h oldo ff, C2757 charges sufficiently to allow
pin 3 to be H I. This causes pin 6 to produce a sweep
gating pulse to trigger the sweep and produce a baseline
trace even when no trigger is available.
Pin 4. Autom atic mode. Grounding the pin enables auto­
matic sweep operation when pin 1 is HI.
Pin 5. Logic Gate. This is the gate input from the A
Trigger Generator that is used to initiate several sweep
control functions.
Pin 6. Autom atic Gate. Provides a gate outp ut, in the
automatic mode, to trigger the sweep when no trigger
signal occurs w ith in about 100 ms follow ing the end o f
the holdoff.
Pin 7. A Gate. Provides a gate pulse to trigger the alter­
nate trace circu itry in the Vertical Module.
Pin 8. Ground.
Pin 9. H o ld o ff output. This pin switches from LO to
HI to reset the sweep and trigger circuits.
Pin 10. H old off Timing. RC networks are connected
between this pin and pin 11 to vary the h o ld o ff time,
depending on the setting o f the A TRIGGER HOLDOFF
control and the A T IM E /D IV switch.
Pin 11. H oldoff Ramp. A negative-going ramp is pre­
sent here that determines the h o ld o ff time.
Pin 12. Hold O ff Start. A positive-going pulse, which
occurs at the end o f the sweep, is applied to this pin to
terminate any automatic gate present, start the h o ld o ff
ramp, and initiate the reset pulse to the A trigger circuit.
4-16 REV. A, O C T 1975