Q. what is oscilloscope probe compensation and how is it adjusted? What effects are noted when the compensation is not correctly adjustment?
Direct Probe: This probe is simplest of all the probe and uses a shielded coaxial cable. It avoids stray pick up which may create problems when low level signals are being measured. It is usually used for low frequency or low impedance circuit. In using the shielded probe, the shunt capacitance of the probe and cable is added to the input impedance and capacity of the scope and acts to lower the response of the oscilloscope to high impedance and high frequency circuit.
External high impedanc34 probes are used to increase the input resistance and reduce the effective input capacitance of the oscilloscope. A resistor and capacitor combination can be added to an oscilloscope as shown. In effect, of the probe, supposing it is intended to attenuate the signal by a factor of 10. By using equation, where k=10, the values of can be computed as under.
The input resistance has been increased by a factor of 10 while at the same time =, the input capacitance has been decreased by a factor of 10. The combination of and is called probe.
Capacitor is usually adjustable to compensate for difference in input capacitance between oscilloscopes. If the probe capacitance is adjusted the wrong value the oscilloscope will exhibit a factor frequency response. The adjustment of probe is usually checked by displaying a square wave on the CRT screen. If the probe is not properly compensated the display of square waveform will be adversely affected. If the value of is too small, the leading edge of the square wave is rounded off but if the value of is too large the leading edge of square wave overshoots.