Q. Explain the different types of probes used in cathode Ray Oscilloscope (CRO).

Sol. probes: The probe performs the very important function of connecting the test circuit to the oscilloscope without altering, loading or otherwise disturbing the test circuit.

   The probes are of three different types:

(i)                               Direct reading probe

(ii)                             Circuit isolation probe

(iii)                            Detector probe

(i)                               Direct Reading Probe: Refer to Prob. 6(a)

(ii)                             Circuit Isolation Probe: Used in order to avoid the undesirable circuit loading effect of the shielded probe. The isolation of the probe, which is used along with a capacitive voltage divider, decreases the input capacitance and increases the input resistance of the oscilloscope. This way the loading effects are drastically reduced.

(iii)                            Detector Probe: When analyzing the response to modulated signals used in communication equipments like AM, FM AND TV receivers, the erector probe functions to separate the low frequency modulation component from the high frequency carrier. The amplitude of the modulator carrier is displayed on the oscilloscope by rectifying and by passing action. This permits an oscilloscope capable of audio-frequency response to perform signal tracing tests on communication signals in the range of hundreds of MHz a range, which is beyond the capabilities of all oscilloscopes except the highly specialized ones.

Prob.9. Write short note on storage oscilloscopes.

Sol. Storage Oscilloscope: The bistable storage tube is between two to ten times slower than a comparable variable persistence tube. However it is capable of much longer storage times, measured in hours rather in minutes as for variable persistence. The bistable tube is also capable of operating split screen mode, where half the screen has storage capability and the other half is a conventional and the other half is conventional phosphor tube.

    Unlike the variable persistence tube the same phosphor screen is used for both storage and display. The screen consists of p, phosphor, doped to have good secondary commission characteristics and deposited on a conductive backplane made from a transparent meta film. The phosphor layer consists of a thin coating of scattered particles, so as to give a discontinuo9us surface. This stops the boundary migration of stored charge. The thin phosphor coating also has a short life since it suffers from light output reduction with time.

   The conductive film is held at a low positive potential, so as to attract a cloud of low energy electrons from the flood gun. These electrons have insufficient energy to penetrate the phosphor and are gathered by the collimator. When the write gun is switched on, its high energy electrons result in secondary emission from areas traced on the screen, moving these to the upper stable point. The trace is therefore at a high positive potential and this is maintained due to the low leakage of the phosphor. The low energy electrons from the flood gun are now attracted to the positive areas of the screen and go through the phosphor to reach the metal film at the back. In passing through the phosphor they cause it to glow displaying the area traced out by the writing gun. The screen can be erased by setting the metal film to a negative voltage repelling the electrons back into the storage area and returning the phosphor to the low stable point.