Some Concepts from the Labs

RC Filter and LabVIEW labFunction generator and internal

resistanceRoot-Mean-Square (rms) value of a

sinusoidal voltageOscilloscope 10:1 probe as a

voltage divider

5 Oct. 2004

High-pass first-order RC filter

F i g u r e 1 . R C f i l t e r w i t h s e r i e s c a p a c i t o r a n d o u t p u t r e s i s t o r R .

F i g u r e 2 . T w o - p o r t c i r c u i t .

+

-

V o u tV in

+

-

RC

v in v o u t

you can show that the magnitude of the transfer function is

2)ωRC(1

ωRC

in

out

V

V

An approximate log-log plot of transfer function magnitude vs. frequency is shown in Figure 3:

1

|Vo u t /Vi n |

RC

1

LabVIEW front panel

LabVIEW Block Diagram

Function Generator and Internal Resistance

The function generators in the lab have an internalresistance (think “Thevenin resistance) of 50 ohms.Their output voltage indication is calibratedassuming that you have a 50-ohm load connected.So if the indication of amplitude says 1 volt peak-to-peak, what is the actual open-circuit zero-to-peakoutput voltage?

Root-Mean-Square (rms)value of a sinusoidal voltage

It is usual in discussing sinusoidal voltages to refer to the effectivevalue of the voltage – the voltage of a DC source that wouldproduce that same power dissipation in a resistor R as theSinusoidal voltage would. This effective voltage is called the root-mean-square (rms) voltage, Vrms.

Your text (Hambley, 3rd ed.) goes through the analysis on pp. 191-3to show that applying this condition on average power dissipationyields the result

 Vrms Vpeak

1

2-------=

For example, if Vrms for the 60 Hz voltage at an outlet in a houseis 120 volts, the peak value of the sinusoidal voltage is 170 volts.

Oscilloscope Probe as a Voltage Divider

Your 10:1 oscilloscope probe is designed to protect the ‘scopefrom having too high a voltage presented at its input. Theprobe contains a 10:1 voltage divider that reduces the voltage atthe probe tip to 1/10th of the voltage at the tip, which then appears at the input to the ‘scope.

(Note: This means that the 10:1 probe can’t be used as if it were an ordinary cable to connect pieces of equipment.)