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mixer LO to IF port leakage problem.

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eejli

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My chip has a measured LO leakage at the mixer IF output. It is in worst case only 25dB lower than the desireable IF signal. what could be the reasons and what is the bad side of this leakage?

Thanks.
 

eejli said:
My chip has a measured LO leakage at the mixer IF output. It is in worst case only 25dB lower than the desireable IF signal. what could be the reasons and what is the bad side of this leakage?

Thanks.
Click to expand...

Usually, the LO leakage at IF output can be remove by low pass filter.
Because it is very far away from IF. So do you use this low pass filter?
 

yes, the chip has a low pass filter after the mixer. My concern is what kind of issue the big LO leakage could have though it can be filtered out after the mixer.
 

eejli said:
yes, the chip has a low pass filter after the mixer. My concern is what kind of issue the big LO leakage could have though it can be filtered out after the mixer.
Click to expand...

Got it.
If the LO is far away from IF, it can be filtered easily, but if they are very close, the LPF will be difficult to realized.
So, I think if LO is close to IF, we should use the double balance MIX.
 

When there is poor isolation between LO and IF circuits noise from LO will go to IF and compromise the signal to noise ratio. Also LO signals may interact with IF signals and create unwanted products. But this is the task of frequency planning to avoid such problem.

Best regards,
RF-OM
 

RF-OM said:
When there is poor isolation between LO and IF circuits noise from LO will go to IF and compromise the signal to noise ratio. Also LO signals may interact with IF signals and create unwanted products. But this is the task of frequency planning to avoid such problem.

Best regards,
RF-OM
Click to expand...

Yes, LO will interact with IF Signals, but if the frequency of LO is thousands times of frequency of IF,
does it also interact with IF, and is the effect serious?
 

To jecyhale,

The effect depends on frequency plan and design. If frequency planning and physical implementation were done right system performance should met the spec. This is designer's work to do so.

Just one simple situation from real life: due to layout imperfections and wrong LO filter type the S32 for mixer were low and odd harmonics of LO had almost no attenuation. As results system sensitivity was compromised and design started from scratch.

BR,
RF-OM
 

To RF-OM,
and how to design frequency plan that avoid the problem in 2.4GHz ISM band?Any criteria we should obey?
BR,
John
 

There is a lot of tools for frequency planning. usually they are based ob so called spur charts. I usually use my own and redesign them if it necessary to accommodate to particular need. You may check www.rfcafe.com. There was one good tool.

BR,
RF-OM
 

also u can use , a tool from agilent , called what IF ?
also the spur chart is a great tool , u can check the books for it

Khouly
 

You can use a band pass filter centred around your IF with required bandwidth of onward processing.
 
IF filter usage is mandatory for almost any application.
 
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