Mixer Isolation Question

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James Kutia

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Hi there. I want to use a frequency mixer with a LO power of 6 dBm and an expected RF input of -60 dBm. If the specified LO-RF isolation is 35 dB, what would be the effect of this insufficient isolation?

Would there still be an IF output of |Frf-Flo| at a lower power or would this not be seen at all?

Thanks
 

At the IF output of the mixer, always will be a signal |Frf-Flo|, whatever LO-RF isolation is.
If is a heterodyne mixer (RF different frequency than LO), there is not too much impact of the LO-RF isolation, on the IF signal.
If is a Direct Conversion mixer (RF almost equal to LO), any LO leakage at RF input will produce a DC offset at IF output.
 

You will just see -29dBm LO signal at the input that is quite low.Since the RF signal frequency is different than LO, there is no trouble with it.( I presume that your LNA hasn't poor linearity)
 


That is great news, thanks for your help.
Can I assume the same will apply with a circulator, i.e: if my received signal (approx -85 dBm) is at a slightly different frequency to the transmitted signal, will the receiver still receive the different freqeuncy even though approximately 10dBm will leak between the TX and receiver ports?

Thanks
 

In case if this is a full-duplex system (RX in the same time with TX) you have to do a system analysis and verify if the +10dBm from TX doesn't produce intermodulation or saturation of your receiver.
 

the effect: you would have a big leakage signal at the LO. You would also have a boatload of intermodulation products too. If you had designed your frequency plan, and provided filters, it would be an acceptable system still. If you ignored this step, you might be sorry.
 


Thanks for the help, I'll "plug" it into ADS and see how it looks
 

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