Understanding RF Mixer Isolation

An RF Mixer is a 3 port device with an RF Port, LO Port, and IF Port. Isolation is a measure of the amount of power that leaks from one port of the mixer to another. The Higher the isolation between the ports, the lower is the leakage between them.

There are three main types of isolations found in Mixers:

LO-RF Isolation: This is a measure of how much the LO (Local Oscillator) signal is attenuated when traveling from the LO port to the RF port of the mixer (at the LO frequency). The LO-RF isolation can be seen as the leakage of the LO signal into the RF port.

This is of most importance when a mixer is being used to downconvert a signal as the LO power can leak into the RF circuits and cause interference in the amplifier and other RF circuits. This is also of importance in upconversion mixers when the LO and RF Frequencies are close to each other - as the LO Signals that get though might not get filtered out and can cause interference.

LO-IF Isolation: This is a measure of how much the LO (Local Oscillator) signal is attenuated when traveling from the LO port to the IF port of the mixer (at the LO frequency). This type of isolation is most important when the LO Frequency and IF Frequency are close to each other and thus leakage of the LO signal to the IF circuitry can cause the IF amplifier to saturate or cause interference in other IF circuits.

RF-IF Isolation: This is a measure of how much the RF signal is attenuated when traveling from the RF port to the IF port of the mixer. Since the RF signal power levels are usually much smaller than the IF signal power levels, this Isolation does not matter as much as the other two types of isolations. RF-IF isolation of a mixer is usually over 20 dB.