Theory of Operation
1710J-Series Waveform Monitors
5–5
The internal sync current source, for the other side of the common emitter Source
Switch (U795D), is Q792. It provides signal current through pin 5 of U795D
which also forward biases CR696 when the switching signal (EXT) is high.
CR698 keeps CR696 from conducting when external sync input is selected.
When external sync is selected, EXT (from Diagram 5) goes low, turning on
U795 (pins 1, 2, & 3) so that signal current from Q798 (the external sync current
source) forward biases CR697. The 0.5 mA of signal current from Q798
(external) or Q792 (internal) drives into a common base stage, Q799, which
develops a 1 V video signal across R797.
The sync stripper removes the active video portions of the signal to generate the
sync required for timing signals. The circuit detects the sync tip, stretches it
(amplifies that portion of the signal), and generates a clean sync signal. The
circuit responds well to pulses up to 1 MHz, then rolls off to eliminate any effect
from subcarrier or high frequency noise at the sync level.
The sync stripper circuit consist of a two-stage amplifier and a clamp (or dc
restorer). Figure 5–2 shows a simplified schematic of the circuit. Both amplifier
stages feed back sync level information to the clamp.
COMP
SYNC
OUT
U892B
Q992
&
U892C
Clamp
R992
Video In
(Source
switch)
R993
Figure 5–2: Simplified block diagram of the sync stripper
The first stage of the amplifier inverts the video signal and clips it near the sync
tip. (The bandwidth of the sync stripper keeps the circuit from clamping to high
frequency components of the video signal.) This operational amplifier stage is
made up of Q992 and U892C. The gain setting resistors R993 (R
i
) and R992 (R
f
)
let the amplifier provide high gain to the sync tip portion of the signal, but clip
any signal components slightly above the sync level.
Sync Stripper
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