Oscilloscopes & recorders

 

1.                  Oscilloscope

a)                  A hot cathode produces an electron beam that passes through two deflecting devices (eg simple plates), one which deflects the beam horizontally on the X-axis and the other that deflects the beam vertically on the Y-axis

b)                 The plates deflect the beam depending on the potential applied to them

c)                  The beam hits a fluorescent screen where it produces a tracing

d)                 The advantage of the cathode ray tube is that the electron beam has negligible inertia, thus leading to a very high frequency response

e)                 


 

2.                  Galvanometer

a)                  When an electric current flows in a wire, an associated electric field exists around the wire

b)                 If this wire carrying an electric field is placed in a magnetic field there is a force which tends to move it in a direction perpendicular to both the electric field and the magnetic current

c)                  The interaction of an electric current and a magnetic field is the working principle of the galvanometer

d)                 Operation:

i)                    Coil of wire suspended on a jewelled bearing in a magnetic field

ii)                  The current to be measured passes through the coil and the interaction between the electric current and the magnetic field cause the coil to rotate

iii)                 The deflection of the coil is proportional to the electric current passing through it and is indicated by a pointer moving over a scale

iv)                Can be modified to provide a continuous record eg using heated stylus

v)                  Limitations:

a)                  Inertia of device limits its maximum frequency response to 80 Hz — just adequate for ECG recording

b)                 Narrow tracings

e)                 


 

3.                  Potentiometric Recorder

a)                  Consists of a resistor with a sliding contact, the position of which can be varied

b)                 High sensitivity

c)                  Low frequency response

d)                 Used for slowly changing signal eg temperature

e)                  Can provide a wide tracing

f)                   Operation:

i)                    A power supply supplies a constant current to a slide wire A, B

ii)                  The wires resistance per unit length is constant

iii)                 The input voltage is applied between one end of the slide wire and the sliding contact C

iv)                The sliding contact, C, is moved along the wire by a servo-amplifier and servo-motor which is activated when a potential difference is detected at G

v)                  The motor thus moves the sliding contact in such a way that the potential difference across B-C always matches the input voltage

vi)                Since a pen is linked to the sliding contact it produces a record of the input voltage

g)