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Design >> RF Design >> Problem with mixer LO leakage
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Message started by kbcmu on Sep 28^th, 2008, 9:44am

Title: Problem with mixer LO leakage
Post by kbcmu on Sep 28^th, 2008, 9:44am

I found a design flaw in an RF front end I built. There is too much LO leakage into the RF signal on upconversion. I am using a standard mixer with high LO-RF isolation (~40dB) but the RF signals can be very low power (-90dBm) and the LO leakage can cause significant interference. The LO is at 2.396GHz and the RF signals range from 2.401GHz to 2.486GHz (The IF signal ranges from 5-90MHz). I am not using a quadrature mixer.

I'm looking for an inexpensive solution to reduce the LO leakage by about 40dB without redesigning the RF front end.

One solution I thought of is to add an IF mixer before the RF front to upconvert the original IF signal to a higher frequency before the RF mixer. The RF LO will now be at a lower frequency and will cause less interference, but leakage from the IF LO will still cause a spike at 2.396GHz.

Another solution may be a sharp notch filter on the RF output. I did some searching but I didn't find an RF filter that will remove the leakage and preserve the signal.

Any Advice?
Thanks

*Just a note about my application. This is an RF front end for a wireless channel simulator similar to the Propsim C8. The upconverted RF power level is so low because we are simulating signals going through the wireless channel. The output of the RF front end connects directly to the antenna port of a radio.

Title: Re: Problem with mixer LO leakage
Post by didac on Sep 29^th, 2008, 6:54am

Hi,
If I understand correctly you are upconverting low power levels to mimic the channel losses, is it right?, Why don't you upconvert a higher power level to avoid the problems of LO leakage and then you put a resistive attenuator fixed or variable after the mixer to mimic the losses?, it's the only solution that I can think about that doesn't imply a change of the mixer.
Hope it helps,

Title: Re: Problem with mixer LO leakage
Post by kbcmu on Sep 29^th, 2008, 5:37pm

Yes, we are trying to mimic the power levels over the air, but we also have a number of signals from different radios we are combining together before upconversion, each with a different attenuation. Basically we have a real-time channel simulator for a wireless network. Adding a variable attenuator would work to reduce the LO component, but it would be too complex to add to the rest of the channel modeling system.

Thanks