Tektronix FG 504

Introduction
T HE O
R Y O F O
PER ATIO N
The following is α description of
the electrical circuits
in the F G 504 . R efer to
the block diagram and
the
schematic diagrams
on the foldout p ages at
the back of
the manual to
aid in the understanding of
this description .
Triangle
Generator or M ain L oop
(Diagram 1)
T his circuitry consists of α
d iode bri d ge, timing
capacitors, p ositive and
n egative current sources,
and the
semiconductor
switches n ecessary to generate the b
asic
triangle waveform .
The current sources are
controlled b y the FRE-
QUENCY Hz (START)
and STOP controls, as well as the
sweep, when operating
in t he swept frequency
mode .
The calibration
voltage for the FREQUENCY
Hz
(START) d ial is
sup p lie d via operational amplifier U513 and
potentiometer
R 145 . On the h ighest
MULTIPLIER setting
(10 6 ),
R 128 is switched i nto the
circuit to im p rove the
linearity of the FREQUENCY
Hz (START) d ial
. This
frequency
control voltage is fed i nto operational
amplifier
U 30A
. When the F G 504 operates in the
swept frequency
mode, the sweep ramp isfed to
operational amplifier U 25B
t hrough STOPFREQ
control R 5 an d b uffer U 5A
. This
control sets the
maximum ramp voltage ap p
lied to the
in p ut of U 25B
. Thisaction setsthemaximum
or upperlimit
in the swept
frequency mode . The voltage
controlled
frequency
input voltage, phase lock loop
correction
voltage, an d t he sweep ram
p voltage, through t he STOP
FREQ d ial, are
summed at p in 6 of U 25B .
Operational
amplifier U
30B serves as α level
shifter . These voltages
and the voltage from the
FREQUENCY Hz (START) d
ial
are summed at p ins 3 of U
45 and U 155 . The voltage at
pins
3 of U45 and U155
increases for α d ecrease in
frequency
and decreases
for an increase in frequency .
The output
from U
155 is fed to the base of Q165 .
The output from U 45
controls p ositive current source
transistor Q55 .
The SYMMETRY control
adjusts the current balance
between the positive and
negative current sources . The
collector of Q165 connects
to the base of Q175 . Α
current
mirror is formedby
transistors Q170A, Β , and
Q175 . The
matched p air of
transistors, 0170 Α and Β ,
h ave equal
emitter-to-base
voltages . Therefore,
their collector cur-
rents
are equal . This ensures that
the collector current of
0165 exactly matches the collector
current of Q175 .
Section 3-FG 504(SN Β 040000
& UP)
To un derstand the
operation of the triangle
generator
loop, assume
that switch driver Q100 is
conducting an d
090 is
off . Current flows from
the negative current source
through C R180 . Diodes
CR182 and CR184 are
back-
biased with no current
flowing . No current
flows through
C R190 . Current
from the p ositive current
source flows
through d iodes
CR188 an d C R186,
charging the timing
capacitors . In the slowest
five MULTIPLIER switch set-
tings, α cap acitance
multiplier re p laces the
timing
cap acitors ; switches
S240-3 and 9 close
and S240-16
opens .
This capacitance
multi p lier consists of U
215,
Q230,
Q225 and their associated
resistors and capacitors
.
W ith switches S240-5,
6, and 7 closed, the p ositive
timing
current d ivides . M ost of
the current flows through
R202,
and t he remaining current
flows through R200,
R 204, and
C210 . One-tenth
of the current flows through
R200 and
R204 to
charge C210 because the
combine d resistance of
R 200 and R204 is tentimes
the resistance of R 202 . This h as
the effect of
multiplying the value of C210 b y
α factor of
ten . The
multiplying factor is increase
d as a d ditional
resistances
are added in series
with C210 . F or this
d iscussion, the voltage at
pin 3 of U 215 ramps up .
The
voltage between p ins
2 and 3 of U 215 remains at zero
and
the action of U 215,
Q225, an d Q230 causes
the current
through R202
to remain constant .
The
positive-going ramp is applied to
the gate of fet
Q255A
. The other h alf of the fet
buffer, Q255B, supplies
constant current to
Q255A . Theram p at the source
of
Q255A drives α set of
two complementary
Darlington
am p lifiers . Transistors
Q270 and Q280compose
one set ;
Q285 and Q295
the other . The ramp output is
taken from
the emitters of
Q280 and Q295 .
The ramp
is also fed to the base of
Q310to the n egative
input of
comparator amplifier U 375
through R322, and to
the
positive input of U 375 through
R336 . The waveform at
the emitter of Q310
serves to compensate the
drive
currents for h igh fre q
uency com p onents at the
inputs of
U 375 .
W ith t he ramp rising in the positive
d irection, the
voltage at p in 3 of U 375 is about
0 .4 V more positive than
the voltage at p in 9 .
This makes U 375 inoperative . As
the
ramp rises in
the positive direction, p in 6 of U
375 goes
positive an
d
pin
7 n egative . Pin 7 of U 375
connects to the
base
of
switching transistor Q65 and pin 6
connectstothe
base of Q80 . As the ramp
continues in t he positive
direction, Q80 decreases
conduction and Q65 increases