What causes intermittent JTAG chain errors? How do I fix it?

I rolled back my Quartus and JTAG driver versions to 18.1.2.277. This did not solve the JTAG issue discussed above, but it did solve another problem. With this version (only) I was able to get the pam4_xcvr_com.qpf project that comes with the BTS to compile, load and run. The transceiver toolkit in Quartus also works now. (It seems necessary to remove the Max V from the JTAG chain.) I have two questions:

(1) the SI Quickstart Guide says to hook the board SMA outputs to an oscilloscope, but it doesn't say what kind of scope, or how to trigger it. We haven't been able to display the PAM-4 eye on our DCA using any of the output clocks. How do we trigger it? Is an external clock source needed? (We've tried that too, without success so far.)

(2) when all PAM4 channels are turned off, we still see a signal on the SMA outputs. It looks like a ~7GHz distorted sine wave. We can actually trigger this one with the PLL output clock signal (at ~156 MHz). What signal is this?