Tektronix 1730 Series, Инструкция по эксплуатации

Страница: 91 / 258

1730–Series Theory of Operation
4–7
COMP
SYNC
OUT
U892B
Q992
&
U892C
CLAMP
R992
R993
VIDEO
IN
(SOURCE
SWITCH)
Figure 4-2: Simplified block diagram of the Sync Stripper.
An inverting amplifier, U892B, is the second amplifier stage. It provides
negative-going sync and cleans up any remaining noise or active video on the
signal. Output of the second stage is also fed to the clamp.
The clamp circuit is formed around U892E and U892A. U892E and CR990
form a current switch. When the first stage output level is at sync tip, current
flows through U892E, which charges C887. At the same time U892B pulls
down on CR887 to provide a discharge path for C887. The result of these
opposing actions is to establish an equilibrium voltage on C887. At the end of
sync time U892C saturates and pulls down on CR990 to shut off U892E.
The three filters are driven from current source Q791 through one of the analog
switch sections of U786. Only one switch section will be closed at a time, as
dictated by its enable, from the Microprocessor (Diagram 5) going low.
Chrominance filters are clamped to ground when low pass filtering or flat is
selected. In this condition Q777 and Q776 are turned on clamping the chromi-
nance filter outputs to ground.
When the chrominance filter is turned on an additional bias current for the ac-
coupled filters (3.58 and 4.43 MHz) is required. It is supplied by pulling the
emitters of either CR671 or CR670 low with the Microprocessor-generated
enable signal, which turns on Q775 to saturate Q774. When Q774 saturates its
collector goes to +11.8 V. Signal current from the enabled filter drives the
emitter of a common base amplifier input to the Gain Cell (Diagram 2). At a
0 Vdc level 2 mA of bias current is added to 1 mA of signal current that drives
the input of the Gain Cell.
When dual filter or dual input display is selected a blanking signal is required to
mask any potential switching transit that might occur. Whenever CH B or FLAT
goes active an RC circuit consisting of C94 (CH B) or C871 (FLAT) and R878
Filter Selection