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Need for Channel Select Filter (Read 3727 times)

aaron_do

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Need for Channel Select Filter
Nov 28^th, 2006, 11:50pm

Hi All,

I have a question regarding the channel select filter (CSF). Basically I determined my CSF attenuation requirements based on adjacent and alternate channel requirements. These set the level of the adjacent and alternate channel that the receiver must be able to withstand and maintain an acceptable BER. The alternate and adjacent channels will mix into the desired channel by odd ordered nonlinearity and be interpretted as noise which degrades SNR hence BER. So based on BER we can define a required IIP3.

The channel select filter appears in the baseband circuitry usually before any signal amplification. So my question is, if we can amplify the received signal up to the required level for ADC while keeping within IIP3 requirements, do we need a channel select filter? i.e. will the unwanted signals affect the demodulation/ADC if they do not appear in the desired band?

Also, am I right in saying that out-of-channel noise must be supressed since when viewed in the time domain it still affects the SNR?

thanks,
Aaron

there is no energy in matter other than that received from the environment - Nikola Tesla

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ACWWong

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Re: Need for Channel Select Filter
Reply #1 - Nov 29^th, 2006, 4:33am

Hi Aaron, please see my 1p's worth...

aaron_do wrote on Nov 28^th, 2006, 11:50pm:

I have a question regarding the channel select filter (CSF). Basically I determined my CSF attenuation requirements based on adjacent and alternate channel requirements. These set the level of the adjacent and alternate channel that the receiver must be able to withstand and maintain an acceptable BER.

Agreed.

aaron_do wrote on Nov 28^th, 2006, 11:50pm:

The alternate and adjacent channels will mix into the
desired channel by odd ordered nonlinearity and be interpretted as noise which degrades SNR hence BER. So based on BER we can define a required IIP3.

This is also true. This predominately happens in the "bottle neck" of your system linearity/IIP3 lineup, usually 1st mixer (or 2nd mixers if you have them)... but this depend on your own system design.

aaron_do wrote on Nov 28^th, 2006, 11:50pm:

The channel select filter appears in the baseband circuitry usually before any signal amplification. So my question is, if we can amplify the received signal up to the required level for ADC while keeping within IIP3 requirements, do we need a channel select filter? i.e. will the unwanted signals affect the demodulation/ADC if they do not appear in the desired band?

I don't agree. In all of the transceivers I have worked on, there is a fair amount of gain ahead of the channel filtering (LNA's, mixers, first low noise pre-amp). But it is also true the bulk of the receiver gain (normally just more than half of it, again depends on your system) is post -channel filter, and the reason is as follows....
You must remember the effect of the adjacent and alternate channel signals themselves and not just the distortion they cause inband due to IIP3. Often the adjacent and alternate channel signals are larger than the wanted and are mixed down to frequency which your circuits will react too unless filtered. The issue comes down to a "headroom" problem. Without a channel filter, it is likely that your analogue amplifiers will saturate (heavily compressed) long before enough gain has acted to amplify the signal you want, purely due to the straight down-conversion of the adjacent and alternate channel signals. So I would think it very unlikely any receiver sysytem of any robustness could do without a channel filter even if they had the best ADC in world on the back end...

aaron_do wrote on Nov 28^th, 2006, 11:50pm:

Also, am I right in saying that out-of-channel noise must be supressed since when viewed in the time domain it still affects the SNR?

I agree with this.

I hope my comments help,
cheers
aw

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aaron_do Senior Fellow Offline Posts: 1398	Re: Need for Channel Select Filter Reply #2 - Nov 29^th, 2006, 6:26am Hi aw, thanks for the reply. I understand your point on the gain compression, however what I was really trying to ask is will the out of channel signals affect the ADC/demodulaion, assuming the desired signal is sufficiently amplified and has a good enough SNR. cheers, Aaron
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loose-electron

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Re: Need for Channel Select Filter
Reply #3 - Nov 30^th, 2006, 8:00am

AW's comments are pretty solid, and I do agree. - Just a minor comment -

Eventually you do end up seperating the out of band (OOB) from the signal of interest (SOI) - Where and how that happens depends on your system.

Most strategies I have been involved with include enough analog filtering/gain to properly span the ADC, while not aliasing the ADC with OOB signals that are over Nyquist. After you get there, the ADC output gets dumped to a DSP filter (FIR) which is pretty "brick wall" except for the SOI.

Some DSP demodulators also reject OOB as a function of the de-mod process.

Your mileage may vary, but that is pretty typical of a lot of modern receivers in cell phones and 802.11 type devices.
There are many different options in receiver architectures out there.

The world would like to attach an ADC directly to an antenna, and then do the rest in DSP, but an ADC with 6GHz sampling at 24 bits is not on the near horizon. �:o

The "software defined radio" is only partially available. Maybe in a few years. (Never say never in EE) � Grin

Jerry

Jerry Twomey
www.effectiveelectrons.com
Read My Electronic Design Column Here
Contract IC-PCB-System Design - Analog, Mixed Signal, RF & Medical

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