Theory of Operation—2246 1Y and 2246 Mod A Service
POSITION VOLTAGE SWITCHES. Selection of the
channel or readout position signals to be supplied to
the paraphase amplifier summing node is controlled
by several sources. The vertical channel enable sig
nals (CH 1 EN through CH 4 EN) from Display Control
ler U600 (Diagram 4) turn on the appropriate channel
position signal for the enabled Vertical Preamplifier
when displaying waveforms. The nominal position
range of the vertical signal is ±12 divisions.
When position-dependent readout (labeled cursors
that follow the vertical channel position controls) is
displayed, the RO CH 1 POS EN through RO CH 4 POS
EN signals from tri-sta te latch U2403 (shown on Dia
gram 9) enable the appropriate vertical position sig
nal into the summing node at the input to the
paraphase amplifier. The Readout Position Enable
signal lines are tri-stated (open) during display of the
channel signals so that the Vertical Channel Enable
signals have control of the position enable lines. Also,
the Readout Position signals cannot override the Ver
tical Channel Enable signal levels to turn on a Vertical
Channel Preamplifier with the series resistors (R212,
R222, R232, and R242) in the signal path. The vertical
position of the enabled vertical channel is added to
the position of the readout so that the cursors appear
at the correct vertical position in the display.
When non-position tracking readout is displayed
(i.e., menus and scale-factor readouts), the vertical
screen position of the readout is conveyed by the RO
VERT signal only. The RO VERT signal is enabled into
the summing node input of the paraphase amplifier
by the RO VERT EN signal for both readout types (po
sition-tracking or fixe d). Extra noise bypassing pro
vided by decoupling components R205, R207, and
C268 on the RO VERT signal line reduces jitter of the
readout display.
During vertical SELF CAL, the RO CH 1 POS EN
through RO CH 4 POS EN signals enable the appropri
ate vertical position signal into the summing node at
the input to the paraphase amplifier without turning on
a channel Vertical Preamplifier. The gain and offset of
the voltage followers and position switches may then
be calculated independently from the vertical channel
signal. The computed offsets are then used by the
Measurement Processor to correctly place the posi
tion-tracking readouts (cursors) on the display rela
tive to the vertical position of the waveform.
The TRACE SEP EN and RO TR SEP EN signals operate
the same as described for the channel enable and
readout position enable signals. A slight difference
between the channel vertical position signals and the
TRACE SEP signal is that TRACE SEP is attenuated
more. The higher value of R206 on pin 13 of U201
reduces the TRACE SEP range to ±4 divisions as
compared to ±12 divisions for the vertical signals.
POSITION PARAPHASE AMPLIFIER. The Position
Paraphase amplifier circuitry is form ed by a transistor
array (U280) driving two discrete transistors (Q284
and Q285). The circuit is configured as two negative-
feedback amplifiers that produce a differential output
current from the summed single-ended input cur
rent. Transistors U280B and U280E are constant-cur
rent sources for their associated amplifier pairs in the
array. The feedback path for the U280A-U280F am
plifier combination is via R286 from the emitter of
Q284. The no-signal feedback current through R286
is 1 mA. Feedback for the U280C-U280D combina
tion is via R289 from the em itter of Q285. Feedback
current in R289 is 100 p.A. Both Q284 and Q285 are
high beta transistors requiring little base-drive cur
rent. The overall vertical displacement response from
the input (at the base of U280F) to the output is
200 p.A per division of vertical screen displacement.
The signal applied to the base of U280C is the
inverted position signal developed across R290 in the
em itter of Q284. The signal is again inverted by
U280C to drive the base of Q285 in the opposite
direction from the signal at the base of Q284. The
standing dc current (no signal input) output current
into the delay line input summing node is 11 mA; the
same as the output of the vertical preamplifiers. Ver
tical centering of the menu and readout displays
within the graticule area is done using VERTICAL
READOUT CENTERING potentiometer R260.
Vertical Output Amplifier
W A R N IN G
Vertical Output 1C U701 runs hot and can burn
you if touched. The metal tab on top of the de
vice is NOT ground; it is the - 5 V supply to the
1C.
Vertical Output 1C U701 buffers the signal output of
the delay line and provides the circuitry for the BW
LIMIT and BEAM FIND functions and for the vertical
signal gain adjustment. The inputs to the Vertical Am
plifier are terminated in 75 n by external resistors
R706 and R707. External filter components C707,
L701, and L702 produce the bandwidth limiting of the
vertical signal when internally switched into the output
amplifier circuitry of U701.
REV OCT 1991
3-11
Summary of Contents for 2246 1Y
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