If I resign from TCS one month before the appraisal, will I still receive the revised appraisal letter during my notice period?

But I can resign after April in order to get the revised CTC?

You can resign after getting the letter. If you resign in April and it is marked in system before letter release, you'll not get the letter.

It might be possible that you resign (send email) but hr did not start process in system and you get the letter. Not sure what will happen in that case.