Hello!

Please refer to the attached screendump from LTSpice. I am viewing the (0.8V peak to peak, 30 MHz sinusoidal AC) voltage before and after it is passed through a series combination of 340R resistor & 47pF capacitor, and then I measure the phase difference between the two voltages (labelled IN & OUT in the circuit), as follows:

Denote the times of the two sets of two consecutive peak voltages (of input & output waveforms) thus: T_in1, T_in2 and T_out1, T_out2 Then the phase difference Phi is given in degrees by:

180 x (1 - [(T_out2 - T_in1) / (T_in2 - T_in1) ])

Eyeballing the curves (the double entendre just struck me!), I estimated these times as follows:

T_in1 = 83.57504 uS T_in2 = 83.60839 uS and T_out2 = 83.60688 uS

from which I calculate the phase difference to be 8.15 degrees. Noting that the voltage and current AC waveforms are in phase across the resistors, I am trying to reconcile this measured phase difference with that given by the phasor diagram formula:

Phase difference = Arc Tan (impedance of capacitor / total circuit resistance) which in this case translates to arctan | (-112.8758 / 362) | = arctan (0.31181) = 17.32 degrees.

Please tell me what I am doing wrong. Many thanks!

 

I think your eyeballing or calculation is a bit out. Mine gives a time shift of about 1.6nS between the in and out waveforms, corresponding to a phase shift of 17.4 degrees.

 

I think your eyeballing or calculation is a bit out. Mine gives a time shift of about 1.6nS between the in and out waveforms, corresponding to a phase shift of 17.4 degrees.

Thank you Alec! Could you please give details of how you measured the time shift?
Edit: Am I right in the following calculation:

Phase shift = 180 degrees * (1.6 nS / 30 MHz) = 180 * 0.048 = 8.64 degrees

Edit2: I realised my error! I needed to multiply by 360 degrees not 180 degrees, so twice 8.64 = 17.4 degrees. Many thanks!

 

Could you please give details of how you measured the time shift?

I plotted both waveforms, left-clicked the title of one trace to get a cursor line, moved that down to the 0V position, then click-and-dragged the mouse to get the time difference between the zero-crossings of the two traces.