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

6
Sampling oscilloscopes
In addition to the ART (analogue real-time) oscilloscopes at
which this book has looked so far, there are other types of great
importance; in particular sampling oscilloscopes and DSOs-
digital storage oscilloscopes. The latter have gained wide accept-
ance as the limitations of the early models have been overcome,
new techniques to extend their capabilities being introduced with
almost bewildering speed. Chapter 7, then, is devoted entirely to
DSOs. But we will look first, in the rest of this chapter, at
sampling oscilloscopes. There are four reasons for doing things in
this order.
First, historically speaking, sampling oscilloscopes predate
DSOs by the best part of two decades. Second, an important class
of DSO - the digital sampling oscilloscope - uses exactly the same
technique for capturing a repetitive, very high frequency wave-
form as that used in the traditional sampling oscilloscope
described in the remainder of this chapter. This
technique
is thus of
extreme contemporary importance, even though the type of
sampling oscilloscope described below is no longer in current
manufacture.
Third, it will enable us to clear up one phenomenon - aliasing
-
before tackling the increasingly complex digital storage scene.
And fourth, although they no longer feature in oscilloscope
manufacturers' catalogues, there are of course many
analogue
sampling scopes, as distinct from the later digital sampling
oscilloscopes, still in use.
Both sampling scopes and DSOs look at an input signal at
discrete 'sampling' instants, rather than continuously like an
analogue real-time scope. They are therefore only aware of the
state of the signal at these instants and are completely ignorant of
what happens in between the samples. This ignorance is the basic
cause of aliasing, as will become apparent shortly.
Analogue sampling oscilloscopes, which I shall call simply
sampling oscilloscopes from here on, offer certain advantages
over ordinary real-time scopes but, as is always the case in
Sampling oscilloscopes 89
electronics (as indeed in life itself), these advantages are not
obtained without some accompanying limitations. Sampling
scopes were introduced in the late 1950s and offered unheard-of
bandwidth compared with real-time oscilloscopes of the day. In
the latter, by using a 'distributed amplifier' consisting of many
valves effectively harnessed in parallel, and restricting the c.r.t.'s
Y deflection range to just four divisions against the eight provided
as standard nowadays, a bandwidth of 85 MHz was achieved. In
contrast, the Hewlett-Packard model 180 sampling oscilloscope
boasted a bandwidth of no less than 2 GHz (2000MHz), more
than twenty times that of the best real-time scopes of the day.
Subsequently, following great advances in the design of
cathode ray tubes and using advanced solid state circuit tech-
niques, real-time oscilloscopes with a bandwidth of 500MHz
became available from a small number of manufacturers. The
state of the art was represented by the now discontinued
Tektronix 7104 oscilloscope, with a bandwidth (via the u
amplifiers) of 1000MHz, or in excess of 2000MHz for signals
connected directly to the Y plates of the cathode ray tube.
Corresponding advances in sampling oscilloscopes led to
instruments with bandwidths of 14 GHz in the early 1970s, and
latterly to the Tektronix l1801B DSO. This digital
sampling
oscilloscope (as distinct from an ordinary digital storage oscillo-
scope) has pushed the bandwidth of such instruments of 50 GHz.
Thus there is much the same ratio between the maximum
bandwidths of real-time and sampling oscilloscopes as prevailed
in the 1950s.
So how do sampling scopes achieve their notably superior
bandwidth? And what are the limitations which were mentioned
earlier? Clues can be gained from the block diagram of a basic
real-time scope, see Figure 2.1. The bandwidth limiting factors
there are the input attenuator, Y amplifier, Y deflection stage and
of course the c.r.t, itself. The techniques used to maximize the
bandwidth of the attenuator and amplifiers are discussed in
Chapter 10 whilst the corresponding techniques in the case of the
c.r.t, are covered in Chapter 9. The sampling oscilloscope avoids
all these limitations at one fell swoop, by simply not attempting to
deal with the whole signal in real time. Instead, it takes samples

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