Re: [Discuss-gnuradio] USRP MUX questions

[Javs]

I am a new user to this group and I am beginning to 
understanding the bridge between the software and
usrp. Let me attempt to answer atleast on of your
questions here. The gurus can correct me if I am wrong
:-)

> Are the connections from the daughterboards to the
> ADC's
> hardwired in the USRP the way it is shown in the
> figure,
> ie, RX-A to ADC-0, RX-B to ADC-1 and the IF output
> of the
> TV-RX to both ADC-2 and ADC-3 ?

I think this is the default wiring:
RxA (Jumper 19 )-ADC 0 
RxB (Jumper 18 )-ADC 1
TvRx-Real - ADC 2
TvRx-Imaginary-ADC 3

However,you can change these setting using software
muxes documented in usrp_standard.h and usrp_basic.h

>Why is there a need for 4 DDC implemented in the
FPGA?

I was thinking about it for some time too. But in a
different way. Why not use four DDC's (real A, cmplx
A, real B,cmplx B)with just one input and one output?
Haven't delved deeply into it but i think the answer
lies in the architecture of the AD9862, or the way the
FPGA code was designed. Quoting Matt here: "The way
the USRP FPGA code is designed, samples sent over the
USB bus are always complex.  This is not inconsistent
with using real samples on the IF, since you
downconvert to 0 IF in the FPGA, and so you need both
I and Q." 

> According to usrp_wfm_gui.py the correct MUX seting
> for listening to
> FM is    0x f0 f0 f0 f2.
>  To start with, why are we zeroing all Q 
> channels? If this is done then we should be getting
> always a complex  number with zero imaginary part...

Thats the intention. To receive only the real samples
and provide a zero for all the imaginary samples.

HTH,

Javs

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