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LNA and Filters - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

 Post subject: LNA and filters
Posted: Mon Oct 12, 2009 12:41 am 

Joined: Wed Oct 07, 2009 6:27 am
Posts: 6
In the receiver chain which should come first? LNA or BPF. In most of the schematics, I see BPF coming first. I have a wide band LNA. Can i use it at the beginning of the filter chain(because I need high sensitivity)? What will be the problems in this?

 Post subject: Re: LNA and filters
Posted: Mon Oct 12, 2009 4:29 am 

Joined: Sun Oct 04, 2009 5:17 pm
Posts: 6
Hi veerasamy,
The BPF is usually placed before the LNA stage to protect it from out-of-band interference. In this case the total noise figure is increased by a quantity equal to the insertion loss of the filter itself.
Strong out-of-band interferer can generate high intermodulation products depending from the IP3 of your LNA. Very strong interferer can also cause distortion, if they are close or higher than P1dB.


 Post subject: Re: LNA and filters
Posted: Thu Jan 14, 2010 3:01 am 

Joined: Thu Jan 14, 2010 2:57 am
Posts: 4
Hi veera,

Mostly you will not be using a bpf if your antenna is narrowband and you use
an impedance matching circuit. But the rejection of unwanted signals is always
important consideration before an LNA for good performance otherwise more
power will appear @ input of lna which will affect its linearity!!!

 Post subject: Re: LNA and filters
Posted: Tue Jul 13, 2010 9:36 am 

Joined: Tue Jul 13, 2010 8:57 am
Posts: 1
we want to design a receiver in 2.4Ghz with advanced design system software,& we don't know how to find the moduls of LNA & mixer & band pass filter & LPF & amplifier & oscillator &...
help us please..

 Post subject: Re: LNA and filters
Posted: Tue Jul 13, 2010 3:13 pm 

Joined: Thu Jan 14, 2010 2:57 am
Posts: 4
I have used models of various components while designing the wideband frontend for SDR.

I remember that the Avago technologies provides the model for their LNA.
Johansons dielectric also provides models for their passives.

Posted  11/12/2012

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