I'm working on the desing of an active beamforming antenna. I'm a beginner
and I'm not able to explain why if I have a 21Mhz bandwidth I need an ADC
of 105MSPS to digitalize the received signal.
Which is the minimum sample rate and resolution of the ADC and DAC on A/D
and D/A conversion of a signal of 21Mhz bandwidth and why? It should be
determinated by the Nyquist freq. but...then why (for wimax) I'm only
finding high speed ADCs??
I think I don't understand anything...please help!!!!
Reply by mnentwig●October 24, 20072007-10-24
some key issues:
- there will be adjacent channels that are much stronger than your own
signal. The neighboring channels can't be suppressed effectively with
analog filters prior to the ADC, so the ADC must have high enough rate to
include them to prevent aliasing.
- ADCs (sigma-delta) can be designed to produce most of the inevitable
noise in higher frequencies, where there is no wanted signal
- Higher oversampling will require less bits for the same SNR, because the
quantization noise spreads over a wider bandwidth. It might be more
efficient to implement a high-rate low-bit ADC than vice versa.
Hint: Try to find an application note for an actual WiMAX receiver, doing
the whole system design from scratch may be a bit of work. And watch out
for LO phase noise/drift between different receivers...