The test design George shows in #187 is not optimal for measuring a phase shift of an open cable. It is designed to measure the settling time caused by a load which has a characteristic impedance different from that of the wires.
When the line and load matches, the current will settle in zero time at the source. When the load impedance is below the line impedance, the line will take a finite time to achieve the current demanded by the load in response to the source voltage. This finite time is a consequence of the transit time within the cable conspiring with the reflection coefficient of the load.
The entire scope of the test is to show how a varying load impedance will cause a varying delay in current settling. I designed it so that there was no loop in the ground to the scope. Also, to maintain the reflection coefficient of the source, the CVR MUST be an extremely low inductance and very low resistance. Hence the CVR must be designed carefully.
My test design can also be driven with variable sine, and the phase shift of the current caused by the load mismatched to the line can be measured as well. However, trying to elicit phase shifts in the audio band where the temporal shift is in the 10 uSec range is not a trivial exercise. That is why my first choice is a voltage step signal, and watch the current rise to the load defined current.
john
When the line and load matches, the current will settle in zero time at the source. When the load impedance is below the line impedance, the line will take a finite time to achieve the current demanded by the load in response to the source voltage. This finite time is a consequence of the transit time within the cable conspiring with the reflection coefficient of the load.
The entire scope of the test is to show how a varying load impedance will cause a varying delay in current settling. I designed it so that there was no loop in the ground to the scope. Also, to maintain the reflection coefficient of the source, the CVR MUST be an extremely low inductance and very low resistance. Hence the CVR must be designed carefully.
My test design can also be driven with variable sine, and the phase shift of the current caused by the load mismatched to the line can be measured as well. However, trying to elicit phase shifts in the audio band where the temporal shift is in the 10 uSec range is not a trivial exercise. That is why my first choice is a voltage step signal, and watch the current rise to the load defined current.
john
Last edited: