On Fri, Aug 31, 2018 at 10:14:27AM -0400, Marcus D. Leech via USRP-users wrote: > On 08/31/2018 09:26 AM, Flo A. via USRP-users wrote: > >Hei! > > > >I have a very general question regarding the function of the digital mixer > >as part of the USRP motherboard-DDC: > > > >Since the RF signal is already mixed to baseband by the synthesizer > >component of e.g. the LFRX daugtherboard, I do not understand why we need > >another mixer after the ADC as part of the DDC. > > > >Probably this is more a question related to DDCs in general but it would > >be great if you could provide me with an explanation for that. > > > >Thanks in advance cheers! > > > Neither the LFRX, nor the BASIC_RX have on-board synthesizers and mixers, so > in that *particular* case, the DDC is crucial to bringing the > RF signal down to baseband. > > For synthesized boards, the DDC is also used to "fine tune" the baseband > conversion, since real-world synthesizers have a finite frequency > step-size. > > The other part of the DDC is doing rate conversion (filtering and > decimation), since the ADC operates at a fixed sample rate of 100Msps > on the N2xx.
Also, you can tune very accurately (in time) with the DDC, because it doesn't have any kind of ringing effect as LOs do when re-tuned. Say you're doing frequency hopping: It's much easier to do this with digital tuning than with LO tuning (of course, you're limited to the 80 MHz of your passband). -- M
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