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urgent, LPF to suppress the hamonics - RF Cafe Forums

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 Post subject: urgent, LPF to suppress the hamonics
Posted: Thu Apr 16, 2009 8:02 am 
 
Lieutenant

Joined: Thu Apr 16, 2009 7:45 am
Posts: 2
Dear all,

I want to filter the all harmonics above 125MHz using Low pass filter (BW : 300MHz, specification) but I could not suppress them even I used a good response LPF . Note that, this harmonics are from mixing an RF signal and an LO signal.

Is an idea about it ??

Thnak you in advance

N


 
   
 
 Post subject: Re: urgent, LPF to suppress the hamonics
Posted: Mon Apr 27, 2009 7:35 am 
 
General
User avatar

Joined: Fri Feb 17, 2006 12:07 pm
Posts: 343
Location: London UK
Hi nassim
You will not be able to filter the second harmonic of 125MHz if the -3dB bandwidth of the LPF is 300MHz.
If your max frequency is 125MHz, then the -3dB cutoff frequency should be no higher than say 140MHz, in order to ensure that at the second harmonic (250MHz) the level is well attenuated. There may be other products generated by the mixer, depending on your system topology. Thus you should ensure that the expected products in all conditions are above the low frequency -3dB cut-off frequency. What are all the frequencies present at the mixer ports?

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