Variable gain amplifier gain step
we working on design of front end LTE receiver, and i want to know the relation between the VGA gain step and the input of following block(ADC), or how i choose the step of VGA ?
The step size is determined by the usable dynamic range your ADC affords you. Every bit of extra precision that you have (but don't strictly need for subsequent numerical processing) buys you 6 dB of dynamic range.
For example, if you can successfully demodulate your data with only 4 bits of precision (i.e. 16 discrete levels) and your 8 bit ADC has 7 usable (i.e. unaffected by circuit noise) bits, then your signal can shrink from full scale (8 bits) to 5 bits (4 essential + bottom noise bit(s)) before demodulation breaks. This corresponds to 3 (bits) x 6 dB = 18 dB of amplitude change before the VGA needs to take a corrective step. Therefore the step size needs to be no greater than 18 dB. (Smaller steps would be desirable though :)