Re: [Discuss-gnuradio] RFX2400 intermediate frequency

[John Orlando]

<snip>
> thanks for the fast response Eric =) but in the documentation by firas abbas
> in page 9 (http://rapidshare.com/files/121692530/USRP_Documentation.pdf), it
> said that the DDC is downconverting the signal from IF to baseband. so when
> the signal digitize by the ADC, it is still in IF right? its mean the
> daughterboard is responsible to downconvert the on air received signal to
> the IF signal. the ADC will sampled the IF signal and pass it to the DDC
> then the DDC will downconvert the IF to the baseband plus slowing it down so
> that it can be transferred trough the USB 2.0 bus.. am i right? plus what
> about the diagram that i give just now
> (http://gnuradio.org/redmine/wiki/gnuradio/UsrpRfxDiagrams) in the diagram
> it show that ADC is sampling a signal which is filtered by a 20MHz low pass.
> so the IF signal is still at 20MHz right? please correct me if i am wrong..
> thank you very much for the help =)

The DDCs are only used if the front-end RF can't tune to the exact
frequency requested by the user.  The DDCs are capable of doing a
final digital mixing (from a low IF, typically) to center the
bandwidth of interest at DC, but these aren't used with all
daughterboards.

So, in the case of RF daughterboards that employ quadrature
downconversion, and are able to tune to the exact frequency of
interest (i.e., downconvert the RF bandwidth to center it at DC),
there is no IF.  The A/D converters are simply digitizing the baseband
signal, with one A/D digitizing the I channel, and one digitizing the
Q channel.  Google for "zero IF receiver" and you'll find more
details.

The 20 MHz low-pass filters are there to reduce wideband noise, and to
help with anti-aliasing.  They don't imply that there is a mandated 20
MHz IF.

-- 
John Orlando
CEO/System Architect
Epiq Solutions
www.epiq-solutions.com