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Working Principle of Sampling Oscilloscope

  • A sampling oscilloscope is a modified oscilloscope. It is meant for avoiding the difficulties of bandwidth requirements. The problem of reduction in the intensity of the image on the screen of the C.R.O, while working with high frequency signals is also averted in the sampling oscilloscope. It improves the high frequency performance of the oscilloscope.

    These are used for continuous display for frequencies above 300 MHz. Sampling techniques are to be used to obtain good display of the signal. The display is made up of dots of as many as 1000 in number. Vertical deflection for each dot is obtained from the later points progressively in each successive cycle of input.

    The horizontal deflection of the beam is obtained by supplying the stair case wave to the horizontal deflecting plates.

    Principle of Sampling Oscilloscope:


    In a sampling oscilloscope as the name itself indicates the input signal is not directly applied to the vertical amplifier. The waveform is reconstructed taking several samples from the recurrent cycles of the input signal. Therefore the beam will not have the writing speed of the input signal. Instead it will be moving step by step, or point by point utilising number of samples that may be around 1000 or more. This not only relaxes the bandwidth requirement of the vertical amplifier but also improves the intensity of image formation.

    Explanation of Block Diagram of Sampling Circuitry of a Sampling Oscilloscope:


    Description of the block diagram: The sampling gate is the first stage that receives the input signal. It receives the sampling pulse from the voltage comparator. The output of the sampling gate drives the vertical amplifier. The vertical signal out of the vertical amplifier is given to the vertical deflecting plates as usual. The block diagram is shown in Figure.
    Sampling Oscilloscope Block Diagram


    The trigger input is given to the blocking oscillator. The output of the blocking oscillator is given to the ramp generator. The ramp generators frequency is controlled by the time scale switch. The output of the ramp generator is given to the voltage comparator. The voltage comparator receives the input from the attenuator also which is connected the output of the range selector. The stair case generator receives its input from the comparator. The output of the stair case generator goes to the horizontal deflecting circuit.

    Working of Sampling Oscilloscope: 


    The input signal is received and is reconstructed in the sampling oscilloscope. The input signals voltage will be measured at very short intervals of time when the sample pulses occur. The sample pulses will turn on the sampling circuit only during the short intervals. The spot on the screen of the cathode ray tube will be positioned vertically to those voltage levels only. As the second and subsequent samples are taken during the next cycles of the input voltage the position of the spot will represent the different amplitudes of signal, over the different sampling periods. At the same time the horizontal plates will be applied with increasing voltage corresponding to the sample instants. This results in the formation of the pattern following point by point movement of the spot on the screen of the C.R tube.



    The sampling gate is connected to the input terminals. The input signal forward biases the diodes of the gate. Thus the gate input capacitance is offered to the input terminals. So the gate input capacitor gets charged to the input voltage. This voltage is amplified by the vertical amplifier and is applied to the vertical deflecting plates. This occurs only when the sampling gate is permitted by the sampling pulse. The sampling is done in synchronisation with the input signal. Therefore a delay circuit is provided in the vertical amplifier. The sweep circuit is triggered by the input signal.

    On receiving the trigger pulse the blocking oscillator starts a perfectly linear ramp voltage. This goes to the comparator. The comparator compares the output of the staircase generator with the amplitude of the ramp voltage. When the ramp voltage amplitude is equal to the amplitude of the staircase voltage, the staircase voltage generator is permitted to advance one step. At this instant a sampling pulse is applied to the sampling gate. Therefore only at this instant the sampling input is taken by the vertical amplifier. It is amplified and is applied to the vertical deflecting plates.

    It is to be noted here that the displacement of the beam horizontally is synchronised with the trigger pulses. As explained earlier the trigger pulses determine the instants of sampling. The final image on the screen of the C.R. tube is determined by the size of the steps of the staircase generator. The larger the step the more will be distance between the consequent horizontal spots that make up the image.

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    Storage Oscilloscope Notes

  • The storage oscilloscope finds many applications in the field of mechanical engineering and bio-medical science. It can represent very slow sweep signals. The persistence of the phosphor in conventional CR tubes ranges from few micro-seconds to a few seconds. In applications where the persistence of the screen is smaller than the rate at which the signal sweeps across the screen, the start of the display will disappear before the end of the display is written. The Lecture Notes of Storage Oscilloscope are explained below.



    The storage oscilloscope or variable persistence CRO has persistence times greater than few seconds extended over hours. In these types the persistence of the screen can be adjusted to match the sweep time. This storage oscilloscope has all the electrodes commonly available in a conventional CRO. In addition to the above there are some special electrodes. The constructional details of the storage type CR Tube are shown below in Figure.
    Storage Oscilloscope block diagram explanation

    The storage mesh or storage target, mounted just behind the phosphor screen is a conductive mesh covered with a highly resistive coating of magnesium fluoride. A narrow focused beam of high energy electron gun called the 'write gun' is used to write the information to be stored. The write gun etches a positively charged pattern on the storage target by knocking of secondary emission electrons. Because of the excellent insulation properties of the magnesium fluoride coating this positively charged pattern remains exactly in the same position out the storage target where it was first deposited. An electron beam deflected in the conventional manner both in the horizontal and vertical directions traces the waveform pattern on the storage target.

    The stored pattern may be made available for observation at a later time using two special electron guns, called flood guns. The flood guns are located inside the CRT in a position between the deflecting plates and the storage target. They emit low-velocity electrons over a large area towards the entire screen. When the flood guns are switched on in the viewing mode, low energy electrons are sprayed on the screen. The electron trajectories are adjusted by the -collimation electrodes' which constitute a low voltage electrostatic lens system. Hence the flood electrons cross over the screen area. The collector mesh collects most of the flood electrons. Therefore they never reach the phosphor screen. In the area near the stored positive charge on the storage target, the positive field pulls some of the flood electrons through the storage mesh and these electrons continue to bombard the phosphor. The display of the CRT will be an exact copy of the pattern which was initially stored on the target. The display will be visible as long as the flood guns continue emission of low energy electrons. To erase the pattern which is etched on the storage mesh, a negative voltage is applied to the storage target. This neutralises the stored positive charge.

    In order to obtain variable persistence, the erase voltage is applied in the form of train of pulses instead of a steady d.c. voltage. The rate of erasure can be varied by varying the width of the pulses in the train. The variable persistence control is made available on the front panel of the CRO. It is the width control of the erase pulse generator.


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    Dual Beam Oscilloscope Block Diagram

  • Dual Beam Oscilloscope Block Diagram Explanation:

    The Dual Beam cathode ray oscilloscope is a specially made CRO. Two types of CR Tubes are used. In one tube the electron gun assembly produces a single beam which is mechanically split into two separate beams. In the other two completely separate electron beams are produced. There are two vertical deflecting plates (two sets of plates, one for each beam). The horizontal plates are common for the two beams.
    block diagram of dual beam cro

    It can be seen from the block diagram shown above in Figure that the trigger circuit, sweep generator and horizontal amplifier are common for the two beams. Two vertical pre-amplifiers drive two main vertical amplifiers. Individual delay lines are provided. The outputs of the main vertical amplifiers are connected to the vertical deflecting plates. Internal triggering can be done taking the signal from either of the vertical pre-amplifiers. External and Line triggering modes are as usual present.

    Dual beam oscilloscopes are used whenever patterns of interrelated waveforms are to be displayed. It is suitable for the display of different input signals. This is an ideal oscilloscope. However its cost is more than the single beam or dual trace types.


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    Dual Trace CRO Working and Block Diagram

  • SPECIAL PURPOSE CRO:

    The single trace oscilloscope can be modified to offer dual triple and four traces using electron switching. Such CROs can be called multi-trace scopes. Dual beam CROs can be constructed with two guns that give two electron beams. Storage oscilloscopes, Sampling oscilloscopes, and Digital read out oscilloscopes are the other special types.

    Dual Trace CRO Working and Block Diagram: 

    A block diagram illustrating the principle of the dual trace oscilloscope is given in Figure. It consists of a CR Tube, the usual deflecting system consisting of the sweep generator, vertical amplifier, horizontal amplifier. There are two vertical amplifier stages. An additional stage is an "eIectron switch” in this CRO. As the vertical amplifiers are two in number the delay lines are also two.
    With the above arrangement the dual trace oscilloscope is nothing but a single beam oscilloscope, with the input switched alternately to give the impression of two patterns on the screen. The beam produced by the cathode ray follows the signals of channel A, once and channel B, the other time. The sweep is common for both the channels. The following display modes are possible with the dual trace oscilloscope.
    Block Diagram of Dual Trace CRO

    1. A only.
    2. B only.
    3. A and B chopped.
    4. A and B alternate.
    5. A and B added.
    6. A vs B (X-Y mode).

    A mode selector on the front panel to select the required mode of operation. In the alternate mode the electron switch alternately connects the main vertical amplifier to channel A and channel B alternately. Each of the vertical preamplifiers have a calibrated attenuator, and position control. Therefore the individual positions of the pattern and also the amplitude can be individually controlled. The electron switch switches the two channels in synchronization with the sweep. One channel is covered in one sweep cycle. This results in the two images being stable on the screen. This mode is suited for fast sweep rates, when the two images appear as one simultaneous and stable display.

    The sweep trigger signal is obtained from either of the channels before the electron switch. This gives the correct phase relation between the A and B channels.

    In the chopped mode the sweep generator is made free running at the rate of 100 to 500 kHz. If the choping rate is faster than the sweep rate the individual little segments fed to the main vertical amplifier form original channel A and B waveforms without the interruptions. If the sweep rate is close to the chopping rate the continuity in the waveform will be lost.

    In the mode 'add' the signals of A and B channels are added algebraically. The sum signal is displayed on the screen. Using the polarity inversion switches in both channels we can have the following displays. A + B, A — B, B — A and A — B.

    In the X-Y mode the 13 channel is connected to the horizontal amplifier, disconnecting the sweep signal. As the calibration of the two preamplifiers is the same and they have the same delay time accurate measurements can be made.

    In addition to the above modes of operation this CRO can be used as single trace one using either of the channels individually.

    When cost is the problem in acquiring a dual beam CRO this dual trace oscilloscope is advantageous.

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    Different Types of CRO Probes

  • Probes for CRO: 


    CRO probe performs the important function of connecting the circuit under study to the vertical input terminals of the CRO. It helps in not loading the circuit under test. For the different types of measurements using different types of CRO probes are available. They are:

    1. Passive probes.
    2. Active probes.
    3. Current probes.
    4. High voltage probes.

    (1) Passive Probes:


    The passive probe is made of a length of coaxial cable with a tip at one end and a BNC connector at the other. Though the probe directly connects the tip to the CRO, the shunt capacitance of the cable is to be considered. Approximately the 50 Ω cable has a capacitance of 30 pF/0.3 m length. As this would mean a 150 pF capacitance of length of 1.5m of cable, its impedance at high frequencies will be low. Hence such a probe is useful at low frequencies only.

    Compensated passive probes are used that have an attenuation of 10 to 1 over greater frequency range. Such a probe consists of an attenuating resistor shunted by a small variable capacitor of small value. 3.5



    (2) Active Probes:


    Active probes use FETs or BJTs. Mineature vacuum tubes were used earlier. The active probe consisting of FET is shown in block diagram below.
    Active Probe using FET
    The probe contains three parts. The probe head, the coaxial cable and the termination. The probe head contains the FET in a source follower form. It is followed by an emitter follower stage. This emitter follower drives the coaxial cable which has an impedance of 50 Ω and is terminated in 50 Ω active device which offers the characteristic impedance to it. Input impedance of FET circuit is 10 MΩ shunted by 5 pF capacitance.

    (3) Current Probes:


    This probe presents an inductive coupling of the signal to the CRO. The current probe consists of a sensor, a coaxial cable and a termination circuit.

    The split core current probe arrangement is shown below in Fig.
    Split core current probe
    The split core passive probe can be clipped around a conductor whose current is to be measured. The current transformer is the sensing device in this probe. It consists of a stationary U-piece and a movable flat piece. A coil of several turns (around 25 turns) is wound on the leg of the ferrite core that forms the secondary of the transformer. The single turn primary is the conductor under test. Due to the flow of current through the conductor under test a voltage will be produced at the secondary of the current transformer. This output is coupled through a coaxial cable to the input of the CRO, through the termination. The termination circuit can be passive or active that provides the characteristic impedance of the cable. The current probe can sense changes in current only. Hence it is useful only to measure a.c signals. The probe sensitivity may be around 10 mA/mV.

    (4) High voltage probes:


    This is used to apply high voltage of the order of kilo volts to CRO. A voltage division of 1000 to 1 may be used. The probe head is made of high impact strength thermoplastic material and is of special design to protect the user from shock.

    It consists of a 100 Ω resistor in the probe head that is 10 cm long. It has distributed capacitance as shown in figure. This is connected to the termination box through a special probe cable. The attenuation ratio is obtained by adjusting resistor R5, in series with R4 = 100 kΩ, and with CRO input resistance of 1 MΩ as shown. The probe compensation is done by adjusting the network consisting of R1 R2 C1 C2 and C3. The probe cable terminates in its characteristic impedance by resistors R3 and R6.

    Above 100 kHz the shunting capacitance of the circuit is noticeable. At high temperature also the high voltage probe is not efficient.

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    Horizontal Amplifier Block Diagram

  • The block diagram of the horizontal amplifier is shown below in figure.

    Block Diagram of Horizontal Amplifier
    The horizontal amplifier in CRO deals only with the sweep frequency signals. Hence its band width requirements are not critical as that of the vertical amplifier. The horizontal amplifier handles signals of considerable amplitudes. However the gain of this amplifier must be large. The reason is the low deflection sensitivity of the horizontal deflecting plates. The gain of this amplifier must be sufficient that it produces the required sweep over the screen.

    The horizontal amplifier consists of an input signal amplifier which is a single ended amplifier. This receives its input form the mode switch. The mode switch presents either the output of the internal sweep generator, or the external horizontal input signal. The CRO can be used with external signal in X-Y mode instead of the usual Y-t mode, in the external position of the mode switch. The output of the input signal amplifier is given to a paraphase amplifier which drives the push pull output amplifier. The push pull output amplifier presents the positive and negative going ramp voltages of the required amplitude to the two deflecting plates. (Horizontal). This ensures simultaneous application of voltages to the deflecting plates. Along with the sweep voltage an offset voltage also is presented to the input signal amplifier. This D.C offset voltage positions the spot on the screen of the cathode ray tube.


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    CRO Synchronization

  • In order to obtain a steady pattern on the screen of the CRO, the sweep generator has to commence its charging and discharging in step with the input signal. In other words the frequency of the time base generator must be equal to a whole multiple of the input signal frequency. If this condition is not satisfied the pattern on the screen is not stable and moves along the time axis. The circuit of the UJT relaxation oscillator explained above is of the free running type. There is no control over the generation of each new sweep. Such a sweep voltage will not produce stable pattern.

    Synchronization is the process of interlocking the input signal and the time base generators signal. When the input signal and the sweep voltage are synchronized the pattern on the screen will be stable.

    Synchronization can be done in several ways. The type of synchronization depends on le circuit used in the CRO.

    The time base voltage produced by the relaxation oscillator using UJT as explained above an be synchronized with the input signal using the base two (B2) terminal as the synchronizing terminal. This is explained as follows.

    What we do in synchronizing is simple. The run up ramp of the sweep voltage will be nade to close prematurely. This can be done by applying a train of negative pulses to the base two (B2) of the UJT, or by applying sinusoidal voltage to the base of the UJT (B2). When negative pulses are applied the run up ramp ends prematurely.



    Application of the negative pulse voltage to base two (B2) will cause reduction in peak voltage Ep of the UJT at the instants where the pulse is occurring. The sweep generator produces sweep voltage that has its ramp amplitude determined by the peak voltage of the UJT. For the first few cycles of the sync pulses the time base ramp follows the free running mode. At one instant when the negative pulse reducing the peak voltage of the UJT is able to discharge the capacitor earlier to reaching the amplitude of the previous cycle, the synchronizing signal and the sweep signal are locked together. That is after this instant the capacitor discharges only at the peaks of the sync signal. This means that the sweep frequency has assumed the frequency of the sync signal, which is lower slightly than its original frequency. In order to obtain this action the following conditions are to be satisfied.

    1. The period of the sync signal must be shorter than the period of the time base voltage. 

    2. The amplitude of the sync signal must be sufficiently large for proper operation of the circuit.

    Alternating voltage can also be used to synchronize the sweep generators. When a sync voltage of alternating nature is applied to the base two (2) of the UJT, the peak voltage of the UJT, Ep is varied between a maximum and minimum. These limits depend on the amplitude of the sync voltage. When such a voltage is used for synchronization the period of ramp of the sweep generator may either be enhanced or reduced.

    In the above figure we find that two sweep voltages of different frequency are shown. The sync. voltage is same for both. The sweep waveform with dotted line has its time period longer than the sync. period. The period to this sweep signal is shortened by the sync. signal. The solid line representing a sweep signal whose frequency is greater than the sync signal is modified to have a reduced frequency in synchronization with the sync. signal frequency. This is done by premature discharge in the first case and extended charging in the second case. In both the cases the sweep signal assumes the frequency of the sync. signal after passing through some cycles of the sync. signal.

    In the cathode ray oscilloscopes the sync. signal can be obtained from the output of the vertical amplifier using the same signal for synchronizing the sweep, with the input signal. A low voltage derived from the supply mains can also be used. External sync. voltages can also be applied. Hence there will be a selector switch that connects the sweep generators sync. input terminal to either vertical amplifier, low voltage a.c or to external sync. voltage. These functions are termed as internal, line and external synchronization, and are obtained by the sync. selector switch. A diagram showing this sync. selector is shown below in figure.
    Sync Selector Circuit
    The sweep circuits used in CRO must have excellent linearity to give accurate results of the measurements made. To improve the linearity of the sweep circuits the following methods can be used.

    1. Constant current charging, using a constant current source.
    2. The Miller sweep circuit.
    3. The phantastron circuit which is a variation of Miller circuit.
    4. The boot-strap circuit.
    5. Compensating networks which are used to improve the linearity of Miller and boot-strap circuits.

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    Triggered Sweep in CRO

  • Normal sweep generators apply their sweep voltage through the horizontal amplifier to the deflecting plates. This does not required any start signal for the sweep to generate its ramp voltage.

    An advantage can be derived in displaying signals of short duration, stretched over a large area of the screen, by controlling the sweep generators output to be present only at our command. This is done by applying what is called a TRIGGER PULSE that triggers or starts the sweep voltage. The trigger pulse can be derived from the waveform under observation. A modified UJT sweep generator circuit is shown below in Figure. 
    Triggered Sweep Circuit
    From the figure it can be observed that this is a modification of the UJT sweep generator. A voltage divider is formed by the two resistors R3 and R4, across the supply VBB. The values of the resistances R3 and R4 are so selected that the voltage on the cathode of the diode is less than the peak voltage of the UJT, VP. The timing circuit consists of Rt and Ct which is connected to the supply. The UJT is connected as shown in the circuit, with R2 in base 2 and R1 in base 1. The diode is placed between the mid point of the RTCT combination for timing and the potential divider formed by R3 and R4.



    When the supply is switched on the capacitor starts charging through the resistor Rt towards VBB till that point where the diode becomes forward biased and conducts. As the diode is conducting the capacitor CT is clamped at a voltage VD the voltage at the cathode of the capacitor. It cannot reach the voltage VP required for conduction. This is shown in the waveform. If now a negative voltage of sufficient amplitude is applied to base 2 of UJT. The peak voltage decreases momentarily and the UJT fires. Therefore the capacitor discharges through the UJT till the maintaining voltage Vmin as shown in the waveform is reached. Now the UJT switches off. Hence the capacitor again charges towards the VBB till VD where it is clamped. This process repeats as long as the supply voltage and the trigger pulses are present. The sweep waveform is as shown in the Figure. It must be noted here that the trigger pulse initiates the retrace before the sweep could be generated. Hence the part of the wave form corresponding to the retrace period will be lost. This is why we use the delay line in the vertical section of the CRO.


    The block diagram of the Triggered sweep generator is shown below in figure.
    Block Diagram of Triggered Sweep Circuit
    The trigger selector connects the comparator to either the output of the vertical amplifier. external trigger, or to the secondary of the low voltage transformer line. The reference level of the comparator is set by the trigger level control. The comparator gives out its output when the trigger input signal exceeds a particular value as set by the trigger level control. A schmitt trigger that follows the comparator generates negative pulses each time the comparator's output crosses the trigger level. This triggers the sweep generator to start the new sweep.

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    Horizontal Deflection System in CRO

  • The horizontal deflecting system of the cathode ray oscilloscope (CRO) consists of the following stages.

    1. Sweep generator or time base generator.
    2. The horizontal amplifier,
    3. The trigger circuit.

    For an oscilloscope to display the waveform under study a voltage which is linearly increasing with time is required. Also the voltage has to fall in its amplitudes at time periods that are equal or multiples of the input signal time periods. This voltage which is of the saw tooth waveform is applied to the horizontal deflecting plates to sweep the beam horizontally. Hence the name sweep voltage or time base voltage.

    A sweep generator is used to produce this sweep voltage. The sweep voltage will be employing an R.C. charging circuit. The capacitor after acquiring a predetermined value of charge will be discharged using a voltage operated switch. Charging of the capacitor is done through a constant current generator. The sweep voltage waveform is shown below Figure. 
    The linearly increasing portion of the voltage is called 'ramp' voltage, The beam is swept from left of the screen to the right during the rise period of the sweep voltage Ts. The time taken by the voltage to fall to the initial value from the maximum, Tr is called the retrace period of fly back period. During this retrace period the electron beam is cutoff, in order to blank the display of the retrace on screen.

    The sweep generators may employ vacuum tube circuits, transistors or ICs. Thyratrons, gas diodes were used as the voltage operated switches in vacuum tube type circuits. Transistor circuits use UJT for this purpose. IC packages are available that generate the ramp voltage. 555 IC can be used for generating the sweep voltage.



    (a) Sweep Generator Using UJT :


    The circuit of a sweep generator using a UJT is shown below in Figure.

    The RC circuit consists of the resistor RT and the capacitor CT. The UJT is connected as a voltage operated switch across the capacitor. When supply is switched on the capacitor gets charged through the resistance RT. As the emitter and base one (1) of the UJT are across the capacitor CT the emitter voltage goes on increasing with the charge of the capacitor. When the voltage across the capacitor that is the emitter voltage on the UJT reaches a value slightly greater than the peak voltage of UJT, the emitter base one (1) diode conducts heavily. This discharges the capacitor. As the voltage at the emitter is lost the UJT ceases to conduct. Again the capacitor gets charged.
    UJT SWEEP GENERATOR
    This process of charging and discharging repeats itself as long as the supply is present. The voltage across the capacitor gives the required sweep voltage. These circuits that generate non-sinusoidal waveforms are called the relaxation oscillators. The frequency of this sweep generator can be varied in steps by using number of capacitors (CT) connected as required using a switch. This gives coarse frequency variation and provides required number of ranges marked as TIME/DIV on the selector switch. Varying the value of the resistor also varies the frequency. This makes the fine frequency control of the sweep generator. By changing the value of the capacitor and the resistor we are changing only the time constant of the RTCT the timing resistor and timing capacitor of the circuit. Two supplies can be used for the above circuit to improve the linearity of the sweep circuit. One a high voltage supply for the timing resistor, capacitor and the other for the UJT.

    The other stages of Horizontal Deflection System in CRO are as follows.




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    Lumped Parameter Delay Line in CRO

  • Lumped Parameter Delay Line in CRO:


    The lumped parameter delay line consists of symmetrical L.C. networks connected in cascade. When such a section (T section for example) is terminated in its characteristic impedance, the looking back impedance will also be the characteristic impedance. This section now behaves as a low pass filter whose attenuation and phase shift are functions of frequency.

    The pass band of such a filter is defined as the frequency range over which the attenuation is zero. The cutoff frequency is  1/ π(LC).

    If we pass signals through such a section whose frequencies are far lower than the cut off frequency of the section, the output will be faithful reproduction of the input. The important point here is that though the output is a faithful reproduction of the input, the output appears only after a delay. This is what exactly we want from a delay line. The delay time is given by :

    td = 1/fcπ = (LC) approximately.

    When number of such sections are cascaded in to a lumped parameter delay line. The total delay time will be multiplied by 'n' where n is the number of T sections in the delay line. Hence Td = n.td, where 'n' is the number of cascaded T sections.

    The lumped parameter delay line suffers from phase distortion at high frequencies of the input signal. The response of the delay line for step input has over shoot and ringing which is called the transient response distortion. Use of 'm' derived filters improves the system. In any case the impedance matching is very important. The sections must be terminated in the characteristic impedance, that requires complex termination circuitory. A practical push pull driven delay line is shown in the following Figure.

    PRACTICAL DELAY LINE

    Distributed Parameter Delay Line :


    A specially manufactured coaxial cable with large value of inductance per unit length makes a delay line of this type. The straight centre conductor is replaced by a continuous coil of wire, wound on a flexible inner core in the form of a helix. Eddy currents are minimised by the use of braided insulated wire, electrically connected at the ends of the cable. The construction of such a delay line is shown explained below. 

    The inductance of the delay line is offered by the inner coil. The inductance can be increased by winding the helical coil over a ferromagnetic core. This increases the delay time and also the characteristic impedance. The capacitance of the delay line is that of the two coaxial cylinders separated by a polyethylene dielectric. Using a thinner dielectric the capacitance can be increased. The parameters of a helical high impedance delay line are typically 1000 Ω of Z0. and 180 ns/m delay time td.

    The coaxial delay line is advantageous compared to the lumped parameter delay line in that, it requires a smaller space and does not require critical adjustments.

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    Delay Line in CRO

  • Delay Line in CRO : 


    In the circuit of the CRO we have several stages like the attenuator, amplifier, pulse shaper, sweep generator etc. When an external signal is applied to the vertical input terminals, it reaches the vertical deflecting plates and starts deflecting the beam in the vertical direction (up and down). As explained earlier, if the time base voltage also appears at the characteristic instant, of commencement, of the cycle of the applied voltage, on the horizontal deflecting plates, the wave form can be viewed completely.

    The horizontal deflection section of the CRO has the sweep generator, the trigger circuit, the horizontal amplifier stages in which the sweep signal is generated, shaped, amplified and presented to the horizontal deflecting plates. A delay is caused by this circuity in transmitting the sweep signal to the horizontal deflecting plates. The delay that is caused by the horizontal deflecting circuits is about 80 ns. In order to present the leading edge of the signal under observation, the signal to the vertical deflecting plates must be delayed by the same time atleast. The delay line provides the required delay to the vertical deflecting voltage.

    Delay line can theoretically be inserted at any place in the vertical deflecting system. One point to be noted here is that the trigger pick-off must precede the delay line. (the trigger circuit is explained in horizontal section)

    There are two types of delay lines used with CROs. The lumped parameter delay line and the distributed parameter delay line, are the two types.

    A block diagram showing the position of the delay line is given below. Fig. 3.16.
    Position of Delay Line
    In the above block diagram the delay line is placed after the main amplifier. The delay line adds a time delay of 200 ns to the vertical signal. Trigger pick-off is obtained from the output of the main amplifier. The delay produced by the horizontal section is 80 ns. Hence the time base signal precede the vertical signal in appearing at the horizontal deflecting plates than the vertical signal appearing at the vertical deflecting plates thus serving the purpose of delaying the vertical signal to the vertical deflecting plates. 

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    Vertical Amplifier with Block Diagram

  • The Vertical Amplifier : The vertical amplifier has to amplify the signal it receives from attenuator. Its requirements are critical. The following are its requirements :

    1. It must be designed to have sufficient gain to drive the deflecting plates.

    2. Its gain must be constant for any frequency of the input signal with in the limits of the frequency range specified for the oscilloscope. In other words its band width must be large.

    3. Keeping in view of the required stability and band width, it must be an amplifier with fixed gain. The signal handling ability can be improved to the requirements by proper design of the attenuator.

    4. The vertical amplifier should not introduce any phase shift, in the signal it is handling.

    5. It should not produce distortion of the signal under study.

    6. It must be free from spurious responses.

    Vertical amplifier consists of the following stages.

    1. Pre-amplifier.
    2. Phase inverter.
    3. Driver amplifier.
    4. Output amplifier.

    With all the above stages, its fixed overall sensitivity or gain is expressed in terms of the deflection factor, V/div. Fixed gain amplifier will only be used as the design is easier to offer the required standard of stability.

    When the attenuator is kept in the most sensitive position, the gain of the vertical amplifier corresponds to the lowest reading of the VOLTS/DIV scale of the attenuator. The preamplifier and the phase invertor can be of the plug in type, to cater to the multiplex modes in which a CRO can be operated. The driver amplifier and the output amplifier will be provided in the main frame of the cathode ray oscilloscope. Oscilloscopes with different plug in type units designed for specific measurements are available. They extend the scope of the instrument.

    The functional block diagram of a vertical amplifier is shown in Figure.

    Vertical Amplifier Block Diagram
    The preamplifier uses its first element a FET. The attenuator is isolated from the input of the amplifier by the high input resistance of the FET. An impedance transformer action is done by the FET acting as the first element in the source follower form. Its output impedance is moderate. In some cases to match the source follower to the input of the paraphase amplifier (phase invertor) a BJT may be used as a emitter follower. This again acts as an impedance transformer, to couple the output of the source follower to the input of the paraphase amplifier. A paraphase amplifier is required to drive the output stage which mostly is a push pull stage. Such push pull amplifiers require .equal amplitudes of out of phase voltages as inputs to the two base circuits. To accomplish the. required, signal voltage at the input of the output amplifier the output of the paraphase amplifier must be sufficiently amplified. This is done by the driver amplifier which by virtue of its gain provides the required input voltage to the output amplifier.

    The push-pull output stage supplies the deflecting voltage to the vertical deflecting plates. The reason for using the push-pull stage is in that it aids the linearity of the deflection of the CR Tube. The vertical amplifier section may use vacuum tubes or transistors. Vacuum tube versions of CROs are out dated now. Excepting the cathode ray tube, now a days the cathode ray oscilloscopes use either transistor version or integrated circuit version. A complete CRO may consist of both transistorised sub-systems as well as ICs. With the introduction of the semi_ conducting devices, the size, weight of the oscilloscope have been reduced considerably. Portable oscilloscopes are now available with smaller screen diameters.

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    Input Attenuator in CRO

  • Input Attenuator in Cathode Ray Oscilloscope


    The input attenuator consists of number of RC potential dividers controlled on the CRO front panel. This control is done by the VOLTS/DIV selector. This selector will be calibrated in terms of the deflection factor (V/div). The sequence of attenuation commonly used with the CROs is 1-2-5. For example the range of the attenuator setting can be 0.1. 02, 0.5, 1, 2, 5, 10, 20 and 50 volts per division with a maximum attenuation of 50 v/div setting. There are attenuators that have 12 settings.

    To ensure linear operation of the CRO over its specified frequency range the attenuator must be designed to work independently for any frequency uneffected by the frequency of the input signal. To obtain independent operation of the attenuator compensation technique is used. The input attenuator is shown in Figure along with the amplifiers input circuit. From the figure showing only two positions of the attenuator, it can be seen that in the first position of the attenuator the input signal is directly connected to the input of the vertical amplifier. The input signals appears directly without attenuation. In this position it corresponds to the minimum attenuation and from the above example will be 0.1 V/div setting. This discloses the maximum sensitivity of the vertical deflecting system. In the second position of the switch it can be observed that the resistor R1, Cv, the resistor R, and capacitor C form a voltage divider consisting of R1 Cv Ri Ci parallel combination in them. The magnitude of the input voltage now will be depending on the values of resistances R1 and Ri. The input to the amplifier is given by

    Va = (Ri/R1 + Ri)Vs

    Input Attenuator in CRO

    It can also be seen that two capacitors and the two resistors form a bridge as shown in Figure. At balance the product of the resistance and capacitance of RiCi and R1. Cv will he equal. Also at balance there will be no current in branch PQ. Therefore the PQ connection can be omitted from the circuit. Thus the voltage at the amplifiers input is va as given above. As no reactive terms are there in the above expression for the input voltage to the amplifier, the voltage is independent of frequency of the signal. However this is valid only if the bridge is balanced. In other words the attenuator works as a compensated attenuator only when the balance condition is satisfied.

    To balance the bridge and hence to compensate the attenuator the following procedure is employed. A square wave test signal will be applied to the attenuator input. The waveform obtained on the screen will be continuously observed adjusting the value of Cv. The value of the capacitor will be adjusted until the true waveform of the applied signal is observed on the CRO. If too large a value of Cv is offered over compensation results giving a waveform with over shoot. Too small a value will give under compensation rounding off the corners of the waveform observed. These two effects are noticed when high frequency signal is observed. With overdamping a pure sine wave appears with enhanced amplitude of positive half cycle. Under underdamped conditions the sine wave appears nearer to a rectified voltage waveform with inversion. Therefore the attenuator must be correctly compensated to prevent distortion of the waveform. For the different attenuator settings different RC combinations with the value of C adjusted for proper compensation are used.

    The selection of the resistances and capacitances of the attenuator are such that the CRO vertical input always presents the same impedance to the circuit under consideration irrespective of the V/div setting. Cathode ray oscilloscopes have input parameters typically of 1 MΩ shunted by capacitance ranging from 20 pF to 35 pF.

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    Vertical Deflection System in CRO

  • Vertical Deflection System with Block diagram


    The vertical deflecting system of a cathode ray oscilloscope has critical requirements. The functions and requirements are listed below: The vertical deflecting system has to provide the following facilities : 

    1. Amplify and reproduce the input signal. The amplifier must have good fidelity That is it must amplify the input signal within the limits of its bandwidth, without effecting the amplitude, frequency and phase. 

    2. It has to isolate the cathode ray tube from the input signal. That is it has to act as a buffer. 

    3. It must have provisions for the different modes of operation. 

    The functional block diagram of the vertical deflecting system of a general purpose oscilloscope is shown in figure.

    Block diagram of vertical deflection system

    The vertical deflecting system consists of the following elements: 

    1. Probe. 
    2. Selector for input signal. 
    3. Attenuator to adjust the gain of amplifier. 
    4. The vertical amplifier that amplifies the signal.

    (a) The Probe : 

    it connects the vertical amplifier to the circuit in which the waveform is to be observed on CRO. There are many types of probes. The general purpose probe shown in the block diagram above ;as ire probe. This passive probe has a resistor which acts as the attenuator for the signal. The capacitor is to act as the compensator. These two components are used in the probe Tip of the probe connects the circuit. Ground connection is established through a clip connector. The other end of the probe will be connected to the vertical input terminals of the cathode ray oscilloscope using a BNC connector or the like.

    (b) Input Selector : 

    The input selector is nothing but a single pole three way switch. It's pole is connected to the input terminals of the vertical amplifier. The two extreme positions namely way one and way three are connected to the attenuator through the capacitor and directly, respectively. The middle position of the 'switch that is the second way is connected to earth. In the first position of the switch the input is directly connected to the input attenuator. This position is marked as DC. The d.c input signals will directly appear at the attenuator. In this position both and d.c and a.c components are available after due attenuation at the input of the amplifier. This mode is convenient for measurement of total instantaneous valued of signal voltages.

    In the third position the input signal passes through a capacitor before it reaches the attenuator. Hence any D.C is blocked. Only the alternating component of the signal will appear at the attenuator. In this mode we can measure alternating voltages superimposed over d.c.

    While changing the switch from the a.c position to d.c was two connects the attenuator to ground. This discharges any stored charge in the attenuator.






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    Fluorescent Screen in CRT


  • The Fluorescent Screen in CRT


    Fluorescence is the property of some crystalline materials like phosphor or zinc oxide to emit light when stimulated by light radiation. 

    Phosphorescence is the property of fluorescent materials to continue light emission even after the source that exited is withdrawn. 

    Persistence of the phosphor is the time duration for which the after glow or phosphorescence occurs.

    The screen of the cathode ray tube is coated with material called phosphor that will emit light when bombarded by electrons. There are many possible phosphors that differ in colour, persistence, efficiency etc. One of the commonest is willemite. This is a composition of zinc orthosilicate, ZnO + SiO2, with traces of manganese as an "activator". This produces the greenish trace of small general purpose cathode ray tubes. Compounds of zinc, cadmium, magnesium, and silicon are the other useful materials for the screens. Presence of metals like silver, manganese, copper and chromium in proportions as small as 1 part in 105 will increase the light output of the screen by a factor 10 to 100 and will also affect the colour. When used these materials are called activators.

    Phosphors are prepared by grinding, crystallizing, regrinding etc. and are then deposited on the end of the cathode ray tube by settling over of a liquid suspension.

    The light output of a fluorescent screen is proportional to the number of bombarding electrons that is the beam current and increases approximately as the square of the anode voltage. The spectral distribution of the light depends upon the base material and upon the activator. A great variety of colours is available.

    The luminance, (light intensity from the screen) depends on the following factors.

    1. The number of bombarding electrons, (i.e., beam current) per second.
    2. The energy with which the bombarding electron strikes the screen. This is determined by the accelerating potential.
    3. It is a function of the time the beam strikes a given area of phosphor. Hence the sweep speed will affect the luminance.
    4. It is a function of the physical characteristics of the phosphor itself. 

    A table showing the characteristics of some of the commonly used phosphors is given below Table.

    The choice of phosphor for particular application depends on number of factors. p31 phosphor, has high luminance and medium persistence. It is used in general purpose oscilloscopes.

    PHOSPHOR DATA CHART

    Phosphor Type
    Fluorescence
    Phosphorescence
    Relative Luminance
    Decay to 0.1 %(ms)
    Comments
    P1
    Yellow - Green
    Yellow - Green
    50%
    95
    General Purpose; replaced by P 31 in most applications.
    P2
    Blue - Green
    Yellow - Green
    50%
    120
    Good compromise for high  and low - speed applications
    P4
    White
    White
    50%
    20
    Television displays
    P7
    Blue
    Yellow - Green
    35%
    1.500
    Long decay : observation of low – speed phenomena.
    P11
    Purple - Blue
    Purple - Blue
    15%
    20
    Photographic applications
    P31
    Yellow - Green
    Yellow - Green
    100%
    32
    General – Purpose; Brightest available phosphor.



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