"Alfred Lorona" wrote in message
...
Most mixers I've seen are not matched to 50+j0. They are simply terminated
in 50+j0. Is this the best or optimum termination or does it matter?

A commutating (or reversing switch) type mixer, such as a diode
ring mixer (DBM) will, ideally, present the same impedance at its input
as it is terminated in. This is because, in theory, the output is
always connected to the input, but the phase of the connection
reverses with time. Thus in a downconverting DBM, if the IF is
terminated in 50 ohms, the RF input will have a 50 ohm input
impedance. If the IF is terminated in 75 ohms, then the RF input
will have an impedance of 75 ohms.

In practical DBMs, transformers are used. These are optimized for
a particular impedance, so you will get best performance at the
specified impedance level.

The LO port input impedance has no particular value, being dependent
on drive level, diode turn-on voltage, etc. Typically, it is considerably
less than 50 ohms in a 50 ohm mixer. The manufacturer still advises
driving with a 50 ohm source so that the transformer will be optimally
terminated on at least the primary. I have driven DBM's with voltage
sources and current sources and they will often work OK that way.

Rick N6RK