R
Member
LO leakage showing up at the IF port is one of those mixer problems that looks like a bad part until you measure it properly. I chased one recently where the IF spectrum had a strong tone exactly at the LO frequency, about 25 dB above where it should have been. Here is how I tracked it down.
First, confirm what you are actually seeing. Put a spectrum analyzer on the IF port with the RF port terminated in 50 ohms and no RF drive. If the LO-frequency tone is still there, it is leakage, not an intermod product. Measure the level against the mixer datasheet's LO-to-IF isolation spec — if you are 10 dB worse than spec, the problem is in your board, not the part.
Second, check the LO drive level. Overdriving the LO pushes the mixer diodes harder and worsens feedthrough; underdriving it hurts conversion loss and can make the isolation look worse than it is. Sweep the LO power a few dB around the datasheet value and watch the leakage level. In my case the leakage dropped 8 dB just by backing the LO off by 2 dB.
Third, look at the IF filtering. A low-pass or band-pass filter right at the IF port is the cheapest fix, but check its stopband at the LO frequency — a filter that is fine at the IF band can be nearly transparent an octave up. Measure the filter response out to the LO frequency before trusting it.
Fourth, check balance and layout. In a double-balanced mixer, poor balance in the balun or asymmetric routing degrades isolation directly. On the bench, swapping in a known-good evaluation board is the fastest way to separate a layout problem from a part problem. Also check grounding: a shared ground return between the LO and IF paths is a classic feedthrough route.
Finally, record the numbers that matter: LO power, leakage level at the IF port with RF terminated, and the same measurement with the RF port driven. If the leakage does not move when RF is applied, it is pure feedthrough and filtering or layout is the fix, not a different mixer.
What is your go-to first check when LO leakage shows up at the IF port? I am curious how others separate part, drive level, and layout causes.
First, confirm what you are actually seeing. Put a spectrum analyzer on the IF port with the RF port terminated in 50 ohms and no RF drive. If the LO-frequency tone is still there, it is leakage, not an intermod product. Measure the level against the mixer datasheet's LO-to-IF isolation spec — if you are 10 dB worse than spec, the problem is in your board, not the part.
Second, check the LO drive level. Overdriving the LO pushes the mixer diodes harder and worsens feedthrough; underdriving it hurts conversion loss and can make the isolation look worse than it is. Sweep the LO power a few dB around the datasheet value and watch the leakage level. In my case the leakage dropped 8 dB just by backing the LO off by 2 dB.
Third, look at the IF filtering. A low-pass or band-pass filter right at the IF port is the cheapest fix, but check its stopband at the LO frequency — a filter that is fine at the IF band can be nearly transparent an octave up. Measure the filter response out to the LO frequency before trusting it.
Fourth, check balance and layout. In a double-balanced mixer, poor balance in the balun or asymmetric routing degrades isolation directly. On the bench, swapping in a known-good evaluation board is the fastest way to separate a layout problem from a part problem. Also check grounding: a shared ground return between the LO and IF paths is a classic feedthrough route.
Finally, record the numbers that matter: LO power, leakage level at the IF port with RF terminated, and the same measurement with the RF port driven. If the leakage does not move when RF is applied, it is pure feedthrough and filtering or layout is the fix, not a different mixer.
What is your go-to first check when LO leakage shows up at the IF port? I am curious how others separate part, drive level, and layout causes.
