I can show you one way. The cellview sandbox.tb_meas_rout:schematic contained in the attachment and shown below illustrates the SST voltage-mode transmitter model and the XMODEL instrumentations that can measure its output resistance while the transmitter is running. Particularly, the parts inside the dotted box measure the output resistance seen at the 'tx_out' node. Since this node is connected both to the transmitter output and to a 50-ohm transmission-line channel, this resistance is expected to have a value of 25-ohms when the transmitter output resistance is equal to 50-ohms.

The output resistance can be measured by injecting a small-signal AC current change (Δi_test) into the node and measuring the resulting AC voltage change (Δv_test). Then, the output resistance R_out is equal to Δv_test/Δi_test. We do this by injecting a small-amplitude, low-frequency sinusoidal current into the node and measuring the resulting amplitude of the same-frequency sinusoidal change in its voltage. To measure the sinusoidal amplitude of a given frequency, we can use a coherent detection technique, which multiplies the voltage signal with the injected current signal and measures its average over an integer multiple of the sinusoidal period. For example, with Δi_test = I₀·sin(ωt), we expect Δv_test to be I₀R_out·sin(ωt), and the average value of Δi_test·Δv_test to be I₀²R_out/2. Hence, we can derive R_out by dividing the measured average value by I₀²/2. In the testbench shown below, the 'sin_gen' and 'isource' primitives inject the sinusoidal current into the node and the 'multiply', 'scale', 'trig_rise', and 'meas_avg' primitives measure the output resistance (R_out) from the resulting voltage change of the node.

The simulated waveforms using this testbench are shown below. The testbench measures the output resistance at 'tx_out' by injecting a 0.1mA-amplitude, 1MHz-frequency sinusoidal current and measuring the resulting change in its voltage. After 10 sinusoidal periods, the 'meas_avg' primitive produces a value of the measured output resistance. In this example, the measured output resistance value is 25-ohms, which implies that the transmitter output resistance is 50-ohms.

We can repeat this simulation while varying the number of driver units enabled. The corresponding simulated waveforms are shown below. When the number of driver units enabled is 3, 4, and 5, the measured output resistance is 28.57, 25, and 22.22-ohms, implying that the transmitter itself has the output resistance of 66.67, 50, and 40-ohms, respectively.

Attachment: meas_rout_20231130.tar.gz

I can show you one way. The cellview sandbox.tb_meas_rout:schematic contained in the attachment and shown below illustrates the SST voltage-mode transmitter model and the XMODEL instrumentations that can measure its output resistance while the transmitter is running. Particularly, the parts inside the dotted box measure the output resistance seen at the 'tx_out' node. Since this node is connected both to the transmitter output and to a 50-ohm transmission-line channel, this resistance is expected to have a value of 25-ohms when the transmitter output resistance is equal to 50-ohms.

The output resistance can be measured by injecting a small-signal AC current change (Δi_test) into the node and measuring the resulting AC voltage change (Δv_test). Then, the output resistance R_out is equal to Δv_test/Δi_test. We do this by injecting a small-amplitude, low-frequency sinusoidal current into the node and measuring the resulting amplitude of the same-frequency sinusoidal change in its voltage. To measure the sinusoidal amplitude of a given frequency, we can use a coherent detection technique, which multiplies the voltage signal with the injected current signal and measures its average over an integer multiple of the sinusoidal period. For example, with Δi_test = I₀·sin(ωt), we expect Δv_test to be I₀R_out·sin(ωt), and the average value of Δi_test·Δv_test to be I₀²R_out/2. Hence, we can derive R_out by dividing the measured average value by I₀²/2. In the testbench shown below, the 'sin_gen' and 'isource' primitives inject the sinusoidal current into the node and the 'multiply', 'scale', 'trig_rise', and 'meas_avg' primitives measure the output resistance (R_out) from the resulting voltage change of the node.

The simulated waveforms using this testbench are shown below. The testbench measures the output resistance at 'tx_out' by injecting a 0.1mA-amplitude, 1MHz-frequency sinusoidal current and measuring the resulting change in its voltage. After 10 sinusoidal periods, the 'meas_avg' primitive produces a value of the measured output resistance. In this example, the measured output resistance value is 25-ohms, which implies that the transmitter output resistance is 50-ohms.

We can repeat this simulation while varying the number of driver units enabled. The corresponding simulated waveforms are shown below. When the number of driver units enabled is 3, 4, and 5, the measured output resistance is 28.57, 25, and 22.22-ohms, implying that the transmitter itself has the output resistance of 66.67, 50, and 40-ohms, respectively.

Attachment: meas_rout_20231130.tar.gz