7
Digital storage
oscilloscopes
Since they were first introduced in 1971, the design and
performance of digital storage oscilloscopes - DSOs - has
advanced immeasurably. Furthermore, the pace of development
has quickened perceptibly in recent years. So in the fourth
edition of this book a chapter was devoted entirely to them.
However, even so it is only possible to cover their design and uses
in a fairly brief way. Readers requiring a book covering the
subject in greater depth should consult
Digital Storage Oscilloscopes,
Butterworth-Heinemann, ISBN 0 7506 2856 1, by the same
author.
The circuitry of conventional real-time oscilloscopes - the 'how
they work' - together with the construction of the c.r.t.s they use
are covered in Chapters 9 to 11, but in this chapter I have
followed the same plan as the preceding one on sampling scopes:
details on the measurement methods available with DSOs and on
how they are implemented by the internal circuitry of the scope
are all covered in one chapter. Figure 7.1, then, is a simplified
block diagram of a basic DSO. Comparing it with the block
diagram of a real-time analogue scope, see Figure 2.1, will show
considerable similarities: the major difference is that the vertical
signal, after passing through the input attenuator, Y preamplifier
and trigger pick-off stage, is not routed directly to the Y deflection
stage. Instead it is sampled at intervals and the samples fed to an
ADC to be 'digitized', i.e. converted to a string of numbers: each
number represents the voltage of the input signal at the instant
the corresponding sample was taken. The digitized data is stored
in a 'channel store', i.e. that part of the total digital memory
which is allocated to the particular Y input channel, of which
there are usually at least two and often more. The digital memory
consists of a bank of RAM (random access memory) ICs
(integrated circuits).
116 Oscilloscopes
I
"
input
and
i
trig pick off
amplifiers
digitezer
' 'h I" (analogue
I
0 O tO dig;t|l
9 sample and buffer
Jvotts/d,v I
9 m t'f'r converter)
sto, e
J m-~ J hold circumt a p
,
le input
,~ il ~..To.,~ P" I l~~,c, .... .,,o,.
I attenultor I -J-- Jr-- ' "
...... ! h ///// ~ ~ digital
to
I
I i t o!.~ Illfl--- UIIIi
enllogue from
ch
1
tr" capac to ~ ~
converter
9 =g , r ~ r---'--1
Y2
1~Impl,ng -'~
store I nA~
L
channel
~wtch - I .... [--I J
Y2 9 trig
control read out switch
~-- ]r~ v r I
,~ ~-"~ .~o,,,,.,. ~, ~ .~o,. r~'-~ ,:,";cTo,
shaper
(,nd t t sw'tch y.
= Y1 controls deflection
from count-down)
F~_ vertical V
amplif;er
r-" cons,o=
ext Y
! 1 L
retrace blanking CRT
J (fly back
SuDPres,tton)
~"' {,
j~. ..... ,o,., I~ >
=nput
trig source (sawtooth or
I.~.~ X EHT
selector time base staircase) deflection
and control amphfier
circuits
Figure 7.1 Simplified outline block diagram of a [ypical DSO (digital storage
oscilloscope)
For display purposes, the data currently in the store is read out
sequentially and the samples passed to a DAC - a digital-to-
analogue converter. There they are reconstituted into a series of
discrete voltage levels forming a stepwise approximation to the
original waveform. This is fed, along with the reconstituted
waveforms(s) from the other Y channel(s), to the vertical
deflection amplifier for the usual dual or four trace display. Note
that the readout and display of samples constituting the stored
waveform need not occur at the same sample rate that was used
to 'acquire' the waveform in the first place. It is sufficient to use
a display sample rate adequate to ensure that each and every
trace displayed is rewritten fifty or more times a second; this will
prevent flicker of the display. This means that in principle, as we
saw with sampling scopes in Chapter 6, one could use a Y
deflection amplifier and c.r.t. (or LCD display panel) with very
modest bandwidth as the display in a DSO, even though the
instrument as a whole is capable of displaying signals with a
bandwidth of tens or even hundreds of megahertz. In practice,
Digital storage oscilloscopes 117
however, some DSOs are also capable of being operated as
conventional real-time oscilloscopes, with a bandwidth in this
mode equal to their bandwidth in digital mode. A good example
is the Fluke PM3370, with a real-time analogue bandwidth of
>60MHz, and a maximum digitizing rate of 200Ms/s (mega-
samples per second) single shot, 10Gs/s in equivalent time
repetitive mode, see Figure 1.5.
There are very real advantages to such a 'dual purpose'
instrument, as will become apparent later in the chapter. But
there is another approach. A manufacturer may elect not to equip
a DSO with a real-time analogue capability at all- in which case
all signals displayed are reconstituted from the stored data. In
such instruments the display tube is often a raster scanned,
magnetically deflected c.r.t., either monochrome or colour- the
technology of a TV display, or maybe an LCD type, either
monochrome or colour.
In this case, the display may be 'bit mapped', which requires
more memory than other types of DSOs, but which greatly
expands the range of display possibilities. The DSOs in the
Hewlett-Packard range are good examples of this type of
instrument; see, for example, Figure 7.3. Note that with both
the dual purpose and the digital-only instruments, however
high the sampling rate (and allowing for 'equivalent time' time
sampling, of which more later) the Y bandwidth can never
exceed that of the input attenuator and Y preamplifier. Like-
wise, however great the vertical resolution (however many bits
the ADC outputs per sample), the vertical measurement accur-
acy will be limited by the linearity (freedom from distortion) of
the Y preamplifier and the ADC. Furthermore, when a dual
purpose instrument is used in the analogue mode, the hori-
zontal accuracy will be limited by the timebase, X amplifier and
c.r.t, linearity to around 2 per cent. By contrast, in digital
storage mode, the
measurement
(as distinct from the
display)
accuracy in the X direction will usually be 0.01 per cent or
better.
So much by way of introduction; now let us look at the various
operating modes of DSOs, how they work and the implications
for the user.
118
Oscilloscopes
Roll
mode
We
will
sr.art.
wi~h
roll
rnc.)de,
no1
becairse
iI
is
1.11~
most
useful
modc
but
because
it
has
been
availablc
on
DSOs from
an
early
stage,
1)rt.a
use
i
I
is
III
nda
riieii
rally
di
fl'err
ri
I
from
a
(-(.In
ve
11
I
iona
I
scope display and because it will lead in nicely tu the other
operating modes
of
DSOs.
For simplicity, consider
a
DSO
with
1024
points
of
memory per input channel, typical
of
the lower
to
middle range of jnstruments. Some
DSOs
display the 1024 points
across the usual ten horizontal graticule divisions, while others
overscan
by
2.4 per cent, giving exactly 100 points per graticule
division
-
to simplify the numbers in the following explanations
we will assume the latter.
Roll
mode operation is rather like
a
chart recorder, where a
trace is written on a strip of paper being drawn
at
a
steady rate
from
a
roll
of
chartpaper. Imagine the paper moving
from
right to
left
and
you
have an analogy
of
roll mode. The trace
on
the
scrwri
of
lhe oscilloscope appears
lo
be
written by
a
1~11
hidden
just
to
the right
of
the screen and the display
scrolls
across
disappearing
off
thc
lcft
of
tlic.
SC~C'C'II.
111
fact,
inlorrnatioii
or1
tlic
part
of
the
rvavcforni
off
the screen
to
the
left
is
losi:
it
docs
not
pilr
UII
011
rht.
rloor like
lie
paper lrorri
a
charl recorder
would.
Figurc
7.2
shows
an indctcrminatc wavclorm which could
corresporid
to
arty pliysical variable
-
it
ntight,
lor
example, be
the
outpul
volldge
of
a load-bearing transdiicer
measuring
the
stress at one
point
of
a bridge
as
traffic passes over.
Let
us assume
that
the
DSO
is set up
to
take
100 samples per second, then after
(just over) ten seconds
it
will have filled up the 1024 memory
locations
-
which are numbered
0
to 1023
-as
at
A
in Figure
7.2.
Ten milliseconds later
it
will
be
time
to
take another sample.
But
before doing
so.
the
digital representations
of
the
samples
currently in
store
in locations
0
to
1023 are read
out
one after the
other
and
passed in turn
to
the
DAC
which turns
them
back into
voltage levels.
These are displayed sequentially
across
the screen
from lrft
to
right, thus displaying the first tcn second segment
of
the
waveform.
Another
sample
is
now taken
-
but locations
0
to
1023
are
already
full
arid
there
is
110
storage location
1024.
50
Ihis
new
sample
is
stored
in
locat.ion
0,
overwriting
the
digit.al value
Digital storage oscilloscopes I19
previously stored there. This new 'sample 0' corresponds to a
point in time about ten seconds later than the previous sample 0,
as at B in Figure 7.2. The channel memory is thus cyclic; like a
loop of recording tape, earlier information is replaced con-
tinuously by later, as indicated in Figure 7.2. As soon as the new
sample is stored in location 0, all the stored sample values are
cycled through the DAC and displayed again, this time starting
with location 1 at the left of the screen and continuing through to
location 1023, finishing up with location 0 (the last sample
acquired) at the right of the screen. The trace displayed is thus the
voltage
trigger-'
level
I
I
10 20 30
seconds
c 0
O
.m
o 1 2
~1~3
r - .....
m 1
t...
O I
6
A~
1
B ~
C ~t
I
D1
0 1 0
1023=~ 2_etc ....... I-
1023
I
0
I
1
I
512 511
I !
100%
i~ tl. ~t-t#-,i
X
vre,r,uuer ~ j
E information stored I
I
F 50% pretrigger L__ t'
I
G 100% post
trigger I
greater than 100% post trigger j
H
(equivalent to delayed sweep I
I
in a conventional 'scope)
Note:
sampling ceases at point X
write ~ read
J
=_ time
0
..... time
sampling ceases as
soon as trigger detected
cessation of
sampling
~ delayed
•
I
•
I i
Figure 7.2 Roll mode
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