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Re: [Discuss-gnuradio] multi_file usrp tuning options

From: Eng. Firas
Subject: Re: [Discuss-gnuradio] multi_file usrp tuning options
Date: Mon, 4 Jun 2007 20:37:01 -0700 (PDT)

Dear Ematlis,

I want to draw your attention to this remake:
 When using std_4rx_0tx.rbf FPGA file, the 4 FPGA DDCs does not have low
pass filter (the HBF) and you have to do the low pass filtering by software.


ematlis wrote:
> Ok, great.  This simplifies things; I was afraid I was going to have to
> create 
> a modulating circuit to modulate those low-frequency timing signals so
> they 
> could be captured along with the 2 MHz am-modulated signals using a single 
> tuning frequency.
> I have some questions about the deinterleaver.  Firstly, how does it know
> how 
> many channels are interleaved?  Does it determine this information from
> the Mux 
> settings, or from the number and type of subdevices detected, or something 
> else?
> Secondly, in terms of computational efficiency, is there any advantage to 
> performing the deinterleave operation earlier in the flow-graph sequence,
> or 
> later?  For example, I'm doing AM demodulation.  So, currently my flow
> graph 
> consists of the usrp block connected to a low-pass filter, connected to a
> mag 
> block, connected to a high-pass filter (to remove dc), connected to a
> block 
> that corrects for gain, connected to a block that factors in a
> "calibration 
> constant" for the data path, and finally the file-write.  I also branch
> off to 
> some fft sinks.  Is it necessary for me to introduce a deinterleave block 
> immediately after the usrp block or can I put it anywhere?
> Thirdly, if all I was doing was capturing to file, is the deinterleaver 
> strictly necessary?  I could write a program to deinterleave the data in
> the 
> file in post-processing, correct?
> thanks!
> eric
> On Fri, 1 Jun 2007, Eric Blossom wrote:
>> On Fri, Jun 01, 2007 at 09:27:55AM -0400, address@hidden wrote:
>>> Excellent.  Just so I understand how this is done- when one tunes
>>> different subdevices to different frequencies, is there one LO on the
>>> USRP
>>> which is being switched between these frequencies, or is there more than
>>> one LO?
>> In general, the tuning is split between an LO on the daughterboard and
>> the DDCs in the the FPGA.  In the case of the Basic Rx and LF RX,
>> there is no LO on the daughterboard, so all the tuning is handled by
>> the DDC.  When using std_4rx_0tx.rbf, there are 4 DDCs available in
>> the FPGA.
>> u.tune(...) handles adjusting the LO (if any) and the DDCs
>> transparently for the common case.
>>> Also- Can I tune all four subdevices independently, or am I restricted
>>> to
>>> using the same frequency on a given daughterboard?
>> With the Basic and LF Rx everything is independent, since there's no
>> LO on the daughterboard.  In the case of daughterboards with LO's,
>> life is a bit more complicated and you'll have to explicitly control
>> the LO on the daughterboard, and then explicitly control the 2 DDCs
>> that are being fed from the given daughterboard.  You of course need
>> to ensure that that two frequencies that you want within the IF
>> passband of the daughterboard.
>> To see how this is currently handled, take a look at the
>> implementation of "tune" in gr-usrp/src/usrp.py
>>> Finally- I would guess that at a minimum the decimation factor set in
>>> the
>>> fpga must be the same for all subdevices.  Is this correct?
>> Yes, the decimation rate applies to all subdevices.
>>> thanks again,
>>> eric
>> You're welcome!
>> Eric
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