RC Analog circuit Questions

[Catalyst]

Hi all,

I have a series R-C circuit with a connection to a function generator set to a 10MHz Sine wave via a BNC cable.

If I want to take voltage drop measurements across the R and the C individually do I have to consider the length of the BNC cable feeding the circuit?

The cable I have is 2 Metres, does it have to be comparable to 1/4 or 1/2 of the wavelength?


How important is the cable length of the feeding signal?



Added after 1 hours 57 minutes:

Just to add to the previous question.....


The little series RC circuit has the following parameters:

Voltage Source Input = 2V pk-pk, 60Hz, Sine Wave
R = 1 MegaOhm
C = 180 pF

Measured Voltage Drop Across 'R' = 1.76V
Measured Voltage Drop Across 'C' = 1.88V


The total phasor voltage = (1.76² + 1,88²)½ = 2.5V (Approximately)

Why isn't the total voltage 2 Volts?

Im sure I'm missing something very simple here.


Any guidance is much appreciated


Cat

 

[rtarbell]

Considering that your R is very large, the resistnace of the BNC cable would be negligable. Also, even though you are at 10MHz, you should not have to worry about RF effects, so you should not need to worry about 1/4 or 1/2 wavelength matching for an experiment with this simplicity.

What are you using to measure the voltage, a multimeter, an oscilloscope, etc.?

Most measuring equipment has an input impedance of something like 1 to 10 megaohms, so your large resistor value could be loading down your measuring equipment.

 

[Catalyst]

I'm using a digital oscilloscpe with a 1MegaOhm probe.

Would I be better using a smaller resistor such as 10 K , 100 K ??


Thanks

 

[kevinj]

Use a 10K resistor. 1Mohm is too large because your probe is 1Mohm.