O'Reilly logo

Oscilloscopes, 5th Edition by Ian Hickman

Stay ahead with the world's most comprehensive technology and business learning platform.

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, tutorials, and more.

Start Free Trial

No credit card required

10
How oscilloscopes work
(2): circuitry
Figure 10.1 shows the block diagram of a typical dual trace, high-
performance oscilloscope. Two identical input channels A and B
are switched alternately to a common amplifier, which drives a
delay line. This is shown diagrammatically as composed of
discrete inductors and capacitors, although in a modern instru-
ment it would usually consist of a length of delay cable. This is
similar to coaxial cable, except that it has a centre conductor
wound in the form of a spiral and hence provides much greater
delay per unit length. As the drive to the trigger circuit is picked
off before the delay line, the delay introduced by the latter
permits the whole of the leading edge from which the scan was
Y
go,n
sh~ft control
TT
final Y
l
tr,gger
rt
c,rcu,t
~ [
1
A A
and B and
chonnel
ext
~ shop,ng I _ O+
om p network I"o~ ~
_. I.y-y- - J'l.
free running 9
]
t~mebase
t r, ggered
9
},
preomp
eHt -1
power /
supplyj
I molns
J~.
input
L
-I '~ J
";" ~"
r
_ 9 ~ e.e~pon ded
sh=ft normol
Figure 10.1 Block diagram of dual trace, mains-operated oscilloscope (courtesy
Enertec Instrumentation Ltd)
How oscilloscopes work (2): circuitry 189
triggered to be observed. This assumes of course that the risetime
of the leading edge and the 'wake-up time' of the trigger circuit
are together less than the delay introduced by the delay line,
which is generally tens of nanoseconds.
The final Y amplifier produces the push-pull voltages that
drive the Y plates, and in a higher-performance instrument the
peak-to-peak output swings required might be little more than a
few tens of volts or less, especially if using a tube with a high
p.d.a, ratio and a scan-expansion lens. The X amplifier has to
provide several times as much voltage swing as the Y amplifier, as
the X-plate sensitivity is less than that of the Y plates. Fortunately,
a substantially smaller bandwidth suffices for the X amplifier,
easing the circuit design problems: the c.r.t, designer takes
advantage of this to maximize the Y-plate sensitivity at the
expense of the X-plate sensitivity.
The X deflection amplifier is driven with a sawtooth waveform
produced by a 'sweep' or 'timebase' generator, which itself is
triggered by a pulse from the trigger circuit. The trigger circuit
produces a pulse each time the Y input voltage crosses a given
threshold voltage, which is usually adjustable by the front-panel
trigger level control. Thus the sweep always starts at the same
point on the waveform, the sweep generator thereafter being
insensitive to further trigger pulses until it has completed both
the trace and the following (blanked) 'retrace' or 'flyback'.
Circuit elements
Traditionally, oscilloscope designers made use mainly of discrete
components, especially in critical stages such as the Y amplifier
output stage driving the c.r.t, deflector plates. However, integrated
circuits are being used to an increasing degree, especially in high-
performance oscilloscopes, and this trend will doubtless continue
and accelerate. Few if any integrated circuits are produced by the
major semiconductor manufacturers specifically for oscilloscopes
in the way that i.c.s are mass produced specially for TV sets. The
largest oscilloscope manufacturers have their own in-house i.c.
facilities, often producing i.c.s in hybrid form, since in scope
applications one is always seeking to wring the last ounce of
performance out of every circuit. The same consideration is likely
190 Oscilloscopes
to ensure that certain sections of oscilloscopes will continue to be
designed using mainly discrete components.
Figure 10.2 shows two of the basic circuit 'building blocks' used
in oscilloscopes. The long-tailed pair is widely used in both forms
shown, the second being especially common in analogue inte-
grated circuits. It provides balanced push-pull outputs, even if
only one input terminal is driven; i.e. it converts from unbalanced
R L R L
~JTR 3
__ __j,_
(a)
k IRL
TR2
T R1
w
(b)
Figure 10.2 Basic" circuit 'building blocks' commonly used in oscilloscopes: (a)
long-tailed pairs, (b) casc()de circuit

With Safari, you learn the way you learn best. Get unlimited access to videos, live online training, learning paths, books, interactive tutorials, and more.

Start Free Trial

No credit card required