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Hey everyone,

I'm trying to measure the stray capacitance in a circuit comprised of an rf signal generator, an oscilloscope, a coaxial cable (short-circuited) and some capacitors. I measured the resonant frequencies of the coaxil cable for varying values of capacity (0pF to 850pF) between 4MHz and 100MHz. I then used the following equation to plot X vs capacity and find the corresponding characteristic impedance and stray capacitance:

$$X = ZC_{x} + ZC_{s}$$

Where X is:

$$X = 1/(2πf tan(2πfl/v_p))$$

(I was only given the function for X, so not entirely sure what it describes, I'll be grateful to anyone who is able to offer some insight or background theory)

l = length of test cable, 2m

f = frequency

v = phase velocity

Results from graph:

Z = 55.167

Cs = 29.5pF

Does this value seem sensible for the given frequency/capacitance range?

I then calculated the phase velocity of the coaxil cable to be around 1.94*10^(-8), and that the stray capacitance increases the phase velocity by around a mere 7m/s. Again is this expected for the given range?

Thanks for all the help, in advance.

I'm trying to measure the stray capacitance in a circuit comprised of an rf signal generator, an oscilloscope, a coaxial cable (short-circuited) and some capacitors. I measured the resonant frequencies of the coaxil cable for varying values of capacity (0pF to 850pF) between 4MHz and 100MHz. I then used the following equation to plot X vs capacity and find the corresponding characteristic impedance and stray capacitance:

$$X = ZC_{x} + ZC_{s}$$

Where X is:

$$X = 1/(2πf tan(2πfl/v_p))$$

(I was only given the function for X, so not entirely sure what it describes, I'll be grateful to anyone who is able to offer some insight or background theory)

l = length of test cable, 2m

f = frequency

v = phase velocity

Results from graph:

Z = 55.167

Cs = 29.5pF

Does this value seem sensible for the given frequency/capacitance range?

I then calculated the phase velocity of the coaxil cable to be around 1.94*10^(-8), and that the stray capacitance increases the phase velocity by around a mere 7m/s. Again is this expected for the given range?

Thanks for all the help, in advance.

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