The sensitivity (gain) and frequency bandwidth (B.W) response characteristics of the
oscilloscope are mainly determine by the vertical amplifier .Since the Vertical amplifier and Horizontal deflecting system in oscilloscope gain-B.W. product is constant, to obtain a greater sensitivity the B.W. is narrowed, or vice-versa.
Some oscilloscopes give two alternatives, switching to a wide bandwidth position, and switching
to a high sensitivity position.
Block Diagram of a Vertical Amplifier-
The block diagram of a vertical amplifier is a shown fig
Vertical amplifier consists of the following stages.
1. Pre-amplifier.
2. Phase inverter.
3. Driver amplifier.
4. Output amplifier.
The vertical amplifier consists of several stages, with fixed overall sensitivity gain expressed in
V/div. The advantage of fixed gain is that the amplifier can be more easily designed to meet the
requirements of stability and B.W.
The vertical amplifier is kept within its signal handling
capability by proper selection the input attenuator switch. The first element of the pre-amplifier
is the input stage, often consisting of a FET source follower whose high input impedance isolates
the amplifier from the attenuator.
This FET input stage is followed by a BJT emitter follower, to match the medium impedance of
FET output with the low impedance input of the phase inverter.
This phase inverter provides two anti phase output signals which are required operate the pushpull output amplifier. The push-pull output stage delivers equal signal voltages of opposite polarity to the vertical plates of the CRT. The advantages of push-pull operation in CRO are
similar to those obtained from push-pull operation in other applications; better voltage
cancellation ran the source or power supply (i.e. dc), even harmonic suppression, especially large
2nd harmonic is cancelled out, and greater power output per tube as a suit of even harmonic
cancellation. In addition, a number of defocusing and non linear effects are reduced, because
neither plate is at ground potential.
HORIZONTAL DEFLETING SYSTEM-
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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Advantages of General Purpose Oscilloscope
A general purpose oscilloscope consists of following parts.
- Cathode ray tube
- Vertical amplifier
- Delay line
- Time base circuit
- Horizontal amplifier
- Trigger circuit
- Power supply.
- It provides a graphical display of the amplitude of a signal as a function of time. Hence, it is
used to measure various electrical parameters. - Amplitude of signals like voltage, current, power etc., can be measured by the oscilloscope.
- Apart from amplitude measurement, it can measure frequency, phase angle, time delay of the
signal, time between two events, and relative timing of two related signals. - It has an advantage over electro-mechanical measuring devices that it can respond very well
to high frequency signals because it is completely an electronic device. - General purpose CRO’s are used for maintenance of electronic equipment and laboratory
work. - It can also be used to measure capacitance, inductance, etc.
Advantages of Dual Beam for Multiple Trace Oscilloscopes
- A multiple trace oscilloscope making use of dual beam provides a simultaneous display of the
two input waveforms on the CRO screen. Hence dual beam CRO is used to compare one signal
with another signal. - It can capture two fast transient events.
- It also provides a continuous display of the signals, whereas the display of the two signals
provided by a dual trace oscilloscope consists of small gaps in the trace. - It has two separate vertical channels for two input signals.
- It can also have two separate time base circuits (i.e. horizontal deflection systems). Hence, in
dual beam CRO two input signals can be swept horizontally at different rates. Due to this feature,
a fast signal can be graphically compared with a slow signal simultaneously on the CRO screen.
