This seems to indicate to me that the mixer input is not 50 ohm but a
lot lower. I tried terminating all other ports with 50 ohms with no
noticeable difference. What am I missing?

Why can't I see the correct level in to the device. I have plenty of
drive before the pad but it reduces when connected to the SBL-1.

I am missing something. Can anyone prompt me?




--
vk3akc

You are on the right track: The SBL-1 LO port input impedance is not 50 ohms
as you have correctly measured. On top of that, it is rather unlinear, i.e.
drive level dependent.

Reason: LO drive has to turn mixer quad diodes in and out of conduction. The
RF current needed for that depends on the instantaneous voltage/current
point on the diode characteristic.

Conclusion: Important thing is that the three ports of the SBL-1 are
terminated in 50 ohms, i.e. 'see' a low swr/return loss from the mixer's
viewpoint. Your 3 dB pad adds nicely to that. You have done just the right
thing.

Ease back and enjoy the result!

vy 73
OZ7S Sven