How to check the second hamornic emission and elminate it

[Eric.Lee]

The CDMA cellular phone designed working at 450MHz , however, it present large emisson at 900MHz under EMC test, what will be the problems most probaly? 3x very much.



I'm a newbie in RF and wish your direction.




Best Regards,
Eric

 

[dingjingfeng]

the linearity of your design is low
you can also adding a harmonics filter to filte the second harmonics out.

 

[Eric.Lee]

Re: How to check the second hamornic emission and elminate i

the linearity of your design is low
you can also adding a harmonics filter to filte the second harmonics out.

you mean add a harmonic filter after the PA? and what kind filter should i use?
ps:There is a saw lowpass filter after the PA already.

does the emission diectly corelation with the secondary harmonic in the circuit? or there is any other reason?


Best Regards,
Eric

 

[dingjingfeng]

usually the harmonics(after Saw)=harmonics(before Saw)-attenuation of SAW

may be the stop band of this SAW is not enough for secondary harmonic

 

[biff44]

Re: How to check the second hamornic emission and elminate i

You may have to add a lowpass filter in series with the SAW to achieve the desired performance.

Also it is possible that the SAW filter is capable of attenuating the 2nd harmonic sufficiently, but because of a bad board layout you are still radiating the 2nd harmonic through some other path.

How far out-of-specification is the 2nd harmonic now?

 

[Eric.Lee]

Re: How to check the second hamornic emission and elminate i

Had changed the PA chip, the PA used befroe presented a large 2nd harmonic...
now testing...

 

[springf2000]

the pa should have enough linearity