Hi Aaron, please see my 1p's worth...
aaron_do wrote on Nov 28th, 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 28th, 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 28th, 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 28th, 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