In Option 3L5/3L4/3L3, Port 1 and 2 can be selectively AC or DC coupled. DC coupling (Default setting) allows both DC and AC signals through, while AC coupling accepts only AC signal. In DC coupling, the port 1 and 2 has 50 Ω input impedance. In AC coupling, a blocking capacitor is inserted at the ports 1 and 2. It may cause the signal level difference between AC and DC coupling at lower frequency.
Press System > S-Param Port Couple, then select AC or DC.
When the port coupling is set at AC, there are limitations as follows:
DC bias can not be turned ON for the port 1.
The minimum frequency is 100 kHz.
When the port coupling is set at AC and the frequency is set below 100 kHz, the trigger becomes hold state.