Using a function generator as shown in Figure 3(b) (square wave output – 0 to 5

Question

Using a function generator as shown in Figure 3(b) (square wave output – 0 to 5 V at 50 kHz) and an oscilloscope, measure and record the propagation delay times (tPHL and tPLH) and the rise and fall times (tr and tf). Perform these measurements for a VCC setting of 5 V and capacitive loads of 0 pF, 100 pF, and 400 pF. Use a BNC cable instead of the scope probe and observe the difference in delay measurements. Why is there a difference in values when using the BNC cable?

Explanation / Answer

BNC is just like 1X probe. The cable capacitance of the BNC and the input capacitance of scope has an RC effect on circuits with faster edges. The scope can present a slower edge times than actual one. But this is not the case with the oscilloscope probes those come with a compensation circuit. The scope probe provides a capacitance divider with the scope capacitance in series. Therefore the actual edge times can be obtained.

BNC can measure the signal without any attenuation i.e; 1:1 signal can be given to scope. A scope probe has variable attenuation ranges from 1X to 10:1 (10X probes) and some even upto 1000:1 (1000X probes). 1000X probes are used to measure higher voltage ranges. 1X probes have input impedance of 1M which is suitable for low frequency applications. BNC has an inner conductor with plastic dielectric arround it and shield also. At higher frequencies the dielectric causes losses.

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