Q. Describe in detail the construction and working of analog type storage
oscilloscope. Explain the principle of secondary emission.
Analog Storage Oscilloscopes: Storage oscilloscopes, capable of high persistence
phosphors, have many applications. Example of these is the capture and storage
of transients and the steady display of a very low frequency signal. Two techniques
are used to store signals in an oscilloscope and these are called analog and digital
storage. Analog storage is capable of higher speeds, but is less versatile than digital
Principle of Secondary Emission: shows a simple electrode arrangement to
illustrate this principle. When a beam of electrons from the electron gun strikes
the target it emits secondary electrons, which are gathered by the collector. The
collector is at positive potential of the target is a at a potential of which can be
varied and the electron gun is at ground potential.
The value of the current in the primary electron beam, coming from the electron
gun and is the electron current emitted from the target and collected by the
collector, the ratio is called the secondary emission ratio.
The value of this ratio depends on the primary electron velocity and intensity
and on the chemical composition of the target.
(b) will show the variation of the secondary emission ratio with target voltage, for
the arrangement of when the target voltage is much grater than the collector
voltage all secondary electrons emitted from the target are attracted back to it.
Therefore the collectorcurrent or secondary current is zero and so also is the
secondary emission ratio. The operating point is now well to the right of In.
If the target voltage is slightly negative, as at point f, then al the electrons from
the gun are deflected on to the collector before they reach the target. Therefore
although there is no secondary emission the collector current equals the beam
current and the secondary emission ratio is unity. This point is known as the lower
stable point. As the target volute increases from these point electrons are attracted
away from the collector, but they do not have enough energy to release secondary
electrons from the largest. Therefore the secondary emission ratio falls to a
minimum at A. Beyond the minimum point secondary emission from the target
starts to occur and these electrons are accumulated by the collector, so increasing
the secondary emission ratio.
The secondary emission ratio increase though the crossover point E until it
reaches a peak at C. Beyond this point secondary electrons emitted from the target
are attracted back in numbers greater than those which reach the collector, so that
the secondary emission ratio decreases sharply. The curve reaches the upper stable
point at G, where the primary and secondary currents are equal and then
decreases to zero. The lower and upper stable points represent the erased and
written conditions of the CRT screen and in the absence of a target voltage the can
remain in one of these two stages only.
Variable Persistence Storage: It is an analog storage oscilloscope is also known as
halftone storage or mesh storage.
Construction: Given in illustrates the construction of a CRT using this storage
technique. There are two screens, a storage mesh which retains the image traced
on it by the wiring gun and the phosphorscreen, which is very similar to that used
in a conventional CRT. Dielectric material, consisting of a thin layer of material
such as magnesium fluoride, is deposited on the storage mesh and this acts as the
storage target. The writing gun is at a high negative voltage, the flood gun at a few
volts negative, the collector mesh at about 100 V positive and the storage mesh at
ground potential or a few volts negative. The collimator consists of a conductive
coating on the inside surface of the CRT. It is biased so as to distribute the flood
gun electrons evenly over the surface of the target and causes the electron to be
perpendicular to the storage mesh.
Working: When the writing gun is aimed at the storage target it causes areas
where it strikes to be charged to a positive potential, due to secondary emission.
These areas are maintained at their upper stable point, even after the writing gun
is switched off, due to the action of the flood gun. Electrons from the flood gun also
pass through those areas which are positively charged causing the phosphor
beyond to glow and displaying the original signal traced by the writing gun.
The pattern stored on the storage target lasts forabout one hour, but it can display
a bright image for about one minute. The stored pattern fades due to electrons
from the flood gun charging other parts of the storage surface, giving an
impression that the whole screen has been written. This is known as fading
positive. To erase an image which has been stored the storage mesh is momentarily
raised to the same positive potential as the collector mesh.
Application: Such as the storage of an entire waveform of a slow moving signal,
which then fades the next trace is written.
It can also be used to store several traces before the first one fades, so as to see how
the signal changes with time.
There are broadly two types of oscilloscope named as analogue oscilloscope and digital
oscilloscope where the difference between both is well ill-starred by their name itself. Analogue
oscilloscope involves only analogue signal during the processing of the signal in its various
section like amplification, attenuation, filtration, deflection of any other operation some of its
examples are sampling oscilloscope, conventional low frequency oscilloscope. But it is to
mention here that it does not mean that it is not able to display digital signal as any wave of input
signal can be studied by them irrespective of the types of oscilloscope used with some
exceptions. On the other hand digital oscilloscope uses digital processing techniques in its
various sections of processing like storage, transition between different sections. It is the
availability of electronic circuits at relatively low cost that many digital features can be added to
the analogue oscilloscopes like their storage. Some of the examples of such digital features
involve a trigger after an elapse time or it may also be after a fixed number of pulses have
passed. Digital display have some parameters like
• Integrated digital voltmeter
• Digital counter
• Remote control
In spite of the digital features embedded in it the basic system still remains an analogue
oscilloscope on the basis of its working with some exceptions. Like an analogue oscilloscope
CRT is used in it. The difference lies in the fact that it digitizes the analogue signal and as result
all the substantial signals occur to be in digital form. As also has been told above that a
conventional CRT is used here while storage of signal takes place in digital memory cells. The
following figure well describes the working of basic oscilloscope with the help of a block
diagram. This is basically a complete digital oscilloscope. The signal that is encountered by the
system generally occurs in analogue form which is first digitized at input section and then it is
stored in memory in the same form. Being in digital form various types of analysis can be done
upon it and many other type of information can be built with this system and this all is possible
because of the digital form of the given signal because in digital form an information signal can
be easily fed into a computer and analysed by conventional methods.
As conventional CRTs work upon analogue signal only so in order to use CRT the signal must
be converted back to its analogue form before feeding it to CRT. Therefore, the digital signal is
first retrieved from memory cell then converted back to analogue form by using a conventional
digital to analogue converter then is applied to CRT for its reproduction as a display.
Block diagram of digital storage type oscilloscope
The process of conversion from analogue to digital form is called Digitizing. Digitizing of the
analogue signal takes place by use of sampling form. According to sampling theorem the
samples of the input wave forms are taken out at fixed intervals of time which is further
quantized to give a series of digital number equivalent to the magnitude of those samples. To
make sure that no portion of the information signal is going to be lost the sampling rate should be
greater than the frequency of the signal.
What is sampling frequency?
Sampling frequent is the rate at which the samples are taken out from input analogue signal in
order to digitize the given information.
Sampled input signal and quantized levels
And according to Nyquist rate the sampling frequency should be equal to double of that of the
input analogue signal. This rate ensures that no information is lost during digitizing process. This
requirement for higher frequency rate implies the digitizer, which is an analogue to digital
converter, must have a fast conversion rate. A diagram can illustrate the working of sampling
technique in which samples of analogue wave form are being taken at fixed intervals of time
denoted by points a,b,c etc. the higher sampling needs expensive flash analogue to digital
converter and resolution of such converters falls with increase in sampling rate. This is the basic
reason that band width and resolution of the system is kept limited to by the converter section.
There is absolution to overcome the need of high performance converter which is the use of an
analogue store. The following figure shows one of such a kind. The sampling of input signal is
performed and these samples are then stored in a shift register.
Block diagram of analogue storage type oscilloscope
This shift register is of analogue nature. When it is required to display the data on CRT it can be
directly fed to CRT without having need of conversion from analogue to digital form and this
saves the time of conversion and this result in fast processing. The speed of this process is about
100 mega samples per second and this has an advantage that now a cheap analogue to digital
converter can be utilized. Resolution of the converter does not fall with increase in sampling rate.
What are the Limitations of digital storage type
As, all the techniques have its own advantages and disadvantage it also has some limitation that
oscilloscope does not take the input signal when the digitizing process is going on. This results in
a blind spot in this time period. When sweep speed is less then it is switched out analogue
memory and deeds the analogue signal to convector the same time of operation. Many input
channels can be used with digital storage oscilloscope. If all the channels are made to share the
same store them memory allotted to each channel gets reduced. Sharing of many channel
simultaneously is performed with the help of a multiplexer which a sequential circuit that makes
it possible to uses same device by many user.
Input channel of such an oscilloscope can reach up to 40 channels per system which is very
easily available in commercial market. This type of instrument is not used in small level
laboratories but at commercial level laboratories and large research centres. The storage capacity
of such a device ranges up tp 25000 dots. A floppy disc can be used in many such advanced
oscilloscope as a storage device that leads to increment to the capability of the system as a
whole. Floppy disc plays a role of external storage device and the memory stored upon it can be
saved by the user.