Tektronix FG 5010 User Manual

Theory
of
Operation -
FG
5010
plifier
receives the
current drive
from the
Multiplier
circuits
on
Diagram
24 ,
and sends the
amplified signal to
the Step
Attenuator Circuit . The amplifier is of complementary archi
tecture ,
so
only the
upper half
is discussed
here ,
since the
lower half
operates in the
same way .
The signal current from Q1107 in the Multiplier is fed
through
R1311 to the
base of
Q1213 , an
emitter
follower
stage
that shifts
the input
level . At
the input of
Q1213
,
C1321 , R1322 , and R1321 form a damping network ; tran
sient
response
of the circuit
is set by
adjusting R1321
.
From the emitter of Q1213 , the signal is applied to
Q1211 ,
which is a gain
stage
that
steers current into
the
output . The
signal is
fed
through Q1211
to the
emitter of
Q1212 . The base voltage of Q1212 is set by Q1221 , R1223 ,
and CR1212 . This forms a current source through Q1212 ,
CR1311 , R1312 , and the lower half circuits that in turn cre
ate
the bias for
the output stages
, Q1312 and
Q1311 . De
pending on
the
applied signal
, Q1211
steers the
current
coming through R1222 to ground or through Q1212 . Output
transistors Q1312 and Q1311 work in parallel to drive the
output
back
termination
network , which
provides a 50 £ re
verse
termination
for the
generator . Potentiometer
R1521
sets the resistance for precisely 50 22 , including the amplifier
resistance . Capacitor C1421 and potentiometer R1422 set
the reactive part of the impedance to allow the generator to
drive an open circuit at the end of a coaxial cable with rea
sonably good transient response . Feedback for the amplifier
is provided by R1313 , ensuring a virtual ground at that
node . The signal from the output termination is fed to the
Step
Attenuator
for output
attenuation .
Step
Attenuator
This circuit consists of three thick - film hybrid attenuators ,
six magnetic latch relays , and eight relay driver stages . Two
of the attenuators , AT1531 and AT1432 , each provide
20 dB attenuation . The third , AT1332 , provides 10 dB of at
tenuation . The circuit thus provides from to 50 dB of
coarse
attenuation in 10 dB
steps . Each side
of
AT1531 is
switchable , so the output amplifier can be completely dis
connected from the attenuator string . This permits a good
back termination with approximately V amplitude , while
keeping the Offset Generator connected to the instrument
output .
The network formed by R1236 , L1231 and R1235 cou
ples the offset voltage from the Offset Generator ( Diagram
26 ) to the output while maintaining the desirable output im
pedance , 50 2.
This
permits the offset
to be at full
value
even when the attenuator is set for minimum signal .
4-34
Output
Switch
This
circuit permits the
CPU to have
control over
whether
the
output
signal is
connected to
the
output connector
. The
circuit consists of latch U1651 , inverter - buffers U1541E , F ,
and
G , plus relay
K1331 .
The RESET signal from the Display Driver circuits enters
the Output Board at P1310-8A , is inverted through U1541G ,
then
applied to pin 10 , the
Reset input
to latch
U1651 . Thus ,
during
power up
or
following a
power
interruption
, the
RESET signal holds K1331 de - energized and the output is
disconnected until the CPU directs otherwise . The relay is
closed by
loading a zero
at the
Q1 output
of U1551
,
which
permits the
CPU
direct control
by way of
inverter
U1541E .
The CPU must first set U1651 by loading a one at the Q5
output
of U1551
. This clocks U1651
, which allows
the signal
from U1541F to close the relay , connecting the output
signal .
OFFSET
GENERATOR
Introduction
The Offset Generator receives the digital offset instruc
tions from the CPU into the two latches , which store the
data . The circuits then respond by producing the offset cur
rent
required ,
and applying
it to the output
network
depicted
on
Diagram
25 .
The Offset Generator consists of the latches , which are
described in the introductory material at the beginning of the
Output Section description ; the DAC , which produces the
amount of current that is proportional to the applied data ;
the Offset Current Source , which supplies the current to
both
of the offset
generators ; the
Positive Offset
Generator ,
which
produces
the positive offset
current , and
the
Negative
Offset Generator , which produces the Negative Offset
Current .
DAC
The DAC consists of DAC U1641 , amplifier U1751 ,
Zener diode VR1751 , and related components . Feedback
resistance for U1751
is
provided via pin 16 of U1641
to the
internal feedback resistor . Potentiometer R1743 nulls any
offset difference between this and the next stage . Resistor
R1637 and Zener diode VR1751 provide a 5 V reference for
the DAC . The analog voltage from U1751 is applied through
R1633 to U1631
, part of
the Offset
Current
Source .
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Tektronix FG 5010 Specifications

General

Brand	Tektronix
Model	FG 5010
Category	Portable Generator
Language	English