How oscilloscopes work (2): circuitry 203
a radio transmitter, should not be applied to an oscilloscope on the
more sensitive ranges, as damage may result. It is also worth
noting that the input impedance of an oscilloscope is not constant.
At d.c. it is 1 M~, and virtually 1 M~ up to a few hundred hertz.
Thereafter, it becomes predominantly a capacitive reactance
falling with increasing frequency, being typically only 4kO at
1 MHz.
The circuit of Figure 10.10 is reasonably simple, but it will only
perform satisfactorily if the layout is suitable, a comment that
applies to the Y amplifier and indeed every section of an
oscilloscope. Poor layout or construction in the Y input attenu-
ator can result in partial shunting of the series elements of one
pad by the unused components of other ranges. This will result in
a non-constant frequency response, which will result in its being
impossible to obtain a true squarewave response, except on the
most sensitive range where no attenuation is in circuit. Needless
to say, the attenuator shown in Figure 10.10 and incorporated in
the 4S6 oscilloscope is designed with intersection screens, to
avoid such problems.
Trigger, timebase and X deflection
circuitry
Figure 10.11 is the circuit diagram of the trigger-processing
circuits, timebase and X deflection amplifier of a dual-trace
15 MHz oscilloscope, manufactured by Gould (formerly Advance
Ltd). It is a good example of the tendency noted earlier for
modern oscilloscope designs increasingly to incorporate inte-
grated circuits while retaining discrete components for those
circuit functions where they are more appropriate. The various
sections of the circuit are labelled (e.g. ramp generator, X output
amplifier, etc.) and detailed operation is described below, as it is
typical of modern oscilloscope practice, even though this partic-
ular model is no longer current.
The trigger source switches, $502 and $503, connect the
required trigger signal via the trigger coupling switches, S 504 and
S 505, to the trigger buffer amplifier formed by TR601 and TR602.
$502 selects the differential CH1 signal via R313 and R314 from
IC301. $503 selects the equivalent CH2 signal via R363 and R364
from IC351. Where both $502 and $503 are selected, both of the
204 Oscilloscopes
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How oscilloscopes work (2): circuitry 20~
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Figure 10.11 X circuits of Gould OS255 oscilloscope, showing timebase
generator, trigger circuits and X-deflection amplifier (courtesy Gould Electronics
Ltd). The OS255 has been discontinued but the circuitry shown above is quite
typical
206
Uscjlloscopes
above signals are disconnected and the single-sided inpiit
from
the EXT
TRIG
input socket
SKC
is selected.
When the a.c. coupling switch, S504, is out, the trigger signals
are directly coupled-through, but when this switch
is
in, a.c.
coupling is introduced via
C603
and
C604 (C601
on external).
TR601
and
TR602
form a differential buffer amplifier with
the
d.c. balance controlled by the trigger level control,
R602.
The
differential output from this stage is applied to the comparator,
IC602,
which
has
positive feedback applied by
R623
to
form
a
Schmitt trigger circuit. The changeover switch,
S506,
reverses the
output frvrii
TR601
and
TR602 to
determine tht. trigger
slupe.
When
hot.11
S504
and
S505
are 'in'
(a.c.
and
d.c.
in
forTV
ruode),
the junction
of
R603
and
CGlO
is connected
to
~lic
-1
1
V
supply.
T)601
and
D608
are
t~roughl irilo cx)ridiiction,
while
D602
arid
D604
are rcvcrsc
biased.
This
diverts
the
output
of
thc
Trigger
amplifier away
frorn
IC602
into
TR605,
which amplilies the
posilive lips
of
llic
waveform orily. TK605
is
prevented from
saturation by feeding back
the
peak detected sync pulses via
TR607
and
TR606
to
thc
emitter
of
TR605. These
pulscs
are amplified
hy
IC60lb
and
applied
via
R617
and
D603
to
the Schinitt trigger.
1C602. 1C601a
is
used
in
conjunction with S504 and S505 to
disable the
sync
separator
when
a.c.
or
d.c. is selected.
At
the fast timchase sweep speeds,
S262a
is
open
and TR603
is
cut oft. However,
a1
speeds
of
100
pdcm and
slower,
R608
is
connected
to
+I
I
V
and
TR603
is
switched
on.
This effectively
grounds
C609
to
inlroducc an
RC
integrating time constant into
Ihe sync pulse signal path
in
thc
TV modc
to
separate
out
the
fra
m
c'
t
r
igger.
The squarewave Iriggc'r output from
IC602
is applied (with
d.c.
bias
of
zener
diode,
D605)
as
thc
clock
to
the
D-type
TTL
flip-flop,
IC501
a.
A
positivc.-going triggcr
chdgch
will
clock
thc kistahlc,
driving
Cj
Iow.
111
ttic waiting state,
Cj
was IiigIi
(+4.5
v),
turning
on
TI7261
via
R507
arid
R2h2,
holding Ihe iriput (arid
hence
Ihc
oiitpiit)
of
thc.
operational
amplifier,
IC261,
at
0
V.
This timchase
;IrnpliIier
is
cunnectcd
as
a
direct voltage follower.
When the tr-igger signal sends
6
of
1C50
1
a
low, the timebase
clamp transistor,
TR26
1,
is turned
off.
Part
of
the
constant current
generated
by
TR264
flows through the resistor network,
R272,
to
How oscilloscopes work (2): circuitry 207
charge C263 at a constant rate. The resultant positive-going
linear ramp voltage generated at the input of IC261 is buffered by
the amplifier to generate the low-impedance ramp output.
The timebase range switch, $262, selects the tap point on the
network, R272, to vary the ramp slope in the 1, 2, 5 sequence
over a range of three decades. On all fast sweep ranges TR262
is biased off, but on ramps 0.5ms/cm and slower $262c
connects R263 to +llV. TR262 is turned on and C264 is
effectively connected in parallel with C263 to slow the sweep
rate 1000 times.
The constant current in the ramp generator is derived from the
current mirror circuit formed by TR263 and TR264. The variable
gain control, R269, provides an approximate 3:1 range of
variation in this current; R506 provides a preset calibration
control on the slow sweep rates, only when S262 is closed.
When the ramp reaches its maximum level, the negative bias
introduced by R521 and R519 is overcome and TR503 turns on,
driving the reset input of the timebase bistable low. As the
bistable switches, Q returns high, and TR261 conducts to
discharge the timing capacitor(s) and the sweep is complete.
However, a hold-off action takes place to inhibit trigger signals
during the sweep; this remains for a short period after a sweep
to ensure that the ramp potential is fully reset before the next
sweep can be triggered. As the ramp goes positive, D506
conducts to charge C502, reverse biasing D503 and turning on
TR502. At the end of the sweep when the timebase is reset, Q
goes low and the D input follows via the action of D508 and
R511. The ramp output returns rapidly towards 0 V, but TR502
remains in conduction for a period determined by C502 and
R518. Only when TR502 turns off can R516 and D507 take the
D input high for the bistable to respond to the next clock
input.
TR501 acts in a way similar to TR262 (described above) to
introduce additional hold-off time through C501 on the slower
half of the timebase ranges.
The brightline facility causes the timebase to free-run in the
absence of trigger signals. The squarewave output from the
Schmitt trigger, IC602, is coupled via C615 into the peak detector
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