Amplifier (Diagram 2 ); Input Coupling Control Bit States - Tektronix 2246 1Y Service Manual

Theory of Operation—2246 1Y and 2246 Mod A Service
Table 3-2

Input Coupling Control Bit States

Coupling
CH 1 GND or
CH 2 GND
GND/CAL
AC
DC
The probe coding signal (CH 1 PRB) is applied to a
multiplexer (U500, Diagram 7) where it is selected to
be digitized in turn with the other probe-code signals
and the front panel potentiometers. The Measure­
ment Processor determines, from the digitized value
of the voltage, the attenuation factor of any attached
coded probe (Tektronix coded probes). The scale
factor of the VOLTS/DIV readout is then switched to
reflect the correct scaling of the displayed signal.
Uncoded probes and coaxial cables are interpreted
as having no attenuation for setting the readout
scale factors.
High-Impedance Attenuator
Switching relays K102 and K103 control the signal
path through the high-impedance
AT117. Signal attenuation is done by two 10X
attenuator sections; for 100X attenuation, the two
sections are cascaded (see Table 3-3 for switching
logic). The 1 M il termination resistance at the out­
put of the attenuator is divided into two parts:
750 k fl and 250 k fl. An output taken across the total
resistance is applied to the buffer amplifier fast-path
input; another output taken across the 250 k fl
section is applied to the slow-path input. Low-
frequency compensation for the hybrid attenuators
is adjusted by CIO and C11 (parts are part of the
hybrid circuit on the ceramic carrier); input C is
adjusted using C l 14.
Input Buffer Amplifier and IX , 2X, 5X Attenuators
Input Buffer Amplifier U112 (for CH 1) is a hybrid
device. The amplifier portion of the circuitry is a
fast-path/slow-path buffer having unity voltage gain
that presents a high-resistance, low-capacitance
load to the signal from
attenuator and a low output impedance to the
3-8
CH 1 AC or
CH 2 AC
0 1
1 1
1
0
attenuator,
the high-impedance
low-impedance attenuator at the output of the
amplifier. The switchable low-impedance, voltage
divider network of U112 provides 1, 2, and 5 times
attenuation of the output signal for application to the
Vertical Preamplifier.
The input signal is applied to pin 2 (fast-path input)
and pin 4 (slow-path input) of U112 from the 1 M fl
divider at the output
attenuator. Internal circuitry of U112 isolates the
signal from loading of the low-impedance attenuator
and provides the slow-path and fast-path signal
amplification. The fast amplifier path quickly passes
the fast leading and falling edges of an input signal
with the slow path catching up to complete the signal
transfer. The output of the buffer sees a 300 f l input
impedance to the low-impedance attenuator, and
the preamplifier sees a 75 f t output impedance at
pin 8 of U112 for all VOLTS/DIV switch settings. At
2 mV/div, additional capacitive loading is applied to
the output of the input buffer amplifier by Cl 85 and
Q181. This increases the high-gain bandwidth of the
amplifier.
CH 3 and CH 4 Input Amplifiers
The CH 3 and CH 4 input buffer amplifiers are identi­
cal discrete FET amplifiers. Input coupling for these
two vertical inputs is always DC; there is no coupling
switch. The 1 M fl input is formed by a series voltage
divider that attenuates the input signal by five times
for application to the gate of the input FETs. The
VOLTS/DIV setting (either 0.1 V or 0.5 V) is made in
the Preamplifier stage of the channel. Operation of
CH 3 is described; like components in CH 4 do the
same job.
From the gate of Q131A, diode CR131 provides pro­
tection from negative overvoltages exceeding about
-8 V. Input C is adjusted by C l34 for low-frequency
compensation. High-frequency response is com­
pensated by C l38 across load resistor R137. Step
balance is adjusted by R141 in the source lead of
Q131B. The single-ended output of U131A is applied
via R139 (a 75 f l resistor) to the CH 3 Preamplifier.
The impedance seen by the other differential input
of the Preamplifier (U230, pin 8, Diagram 2) is
matched by the parallel combination of R158 and
C l59 in series with R160.
The probe-coding signal, CH 3 PRB, is read the
same way as the CH 1 and CH 2 probe-coding
signals. The VOLTS/DIV readout for Channel 3 is
switched to correctly match the probe attenuation
factor (when properly coded probes are used).
of the high-impedance