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Re: [Discuss-gnuradio] How to configure SBX with two channels, one for c

From: Alex Zhang
Subject: Re: [Discuss-gnuradio] How to configure SBX with two channels, one for communications, one for sensing?
Date: Mon, 21 May 2012 20:57:26 -0500

A little confused. 

I am using the gnuradio-companion to configure the uhd.usrp_resource. 
What is the difference between the rf_freq and the center_freq? How the tune_request (is it a UHD method?) can be reflected in the gnuradio-companion setting?

On Mon, May 21, 2012 at 6:29 PM, Josh Blum <address@hidden> wrote:

On 05/21/2012 04:12 PM, Alex Zhang wrote:
> Thanks!
> I am trying to set up two channels on RX2 as Nick said. And the subdev is
> set as "A:0 A:0". Please see the attachement for the GRC flow graph.
> But my problem is that it seems that both the channels can not receive the
> expected signal(I am using an OFDM transmitter for test).
> The single channel with frequency of 908M or 928M works well as I can see
> the OFDM spectrum from the observing FFT plot window.

What did you enter for ch0 and ch1 center frequency? I recommend this:

uhd.tune_request(908e6, rf_freq=918e6,

uhd.tune_request(928e6, rf_freq=918e6,

Its important the RF frequency be the same for each channel, otherwise,
the second tune for channel1 will override the tuning for channel0.


> On Mon, May 21, 2012 at 5:37 PM, Josh Blum <address@hidden> wrote:
>> On 05/21/2012 03:09 PM, Alex Zhang wrote:
>>> Hi,
>>> What I want to do is try to use the two antennas of SBX simultaneously.
>>> The TX/RX is used for duplex communications and the RX2 is for sensing.
>> The
>>> two antennas are working different frequencies.
>>> But this solution seems to be unfeasible, as someone said the receiver
>>> chain of SBX is the same. The TX/RX and RX2 can not work simultaneously.
>>> As suggested in the community, I am trying to using 3 frequecies on one
>>> antenna (TX/RX): f1, f2, f3.
>>> f1(tx) and f2(rx) are used for FDD duplex communications,and f3 is for
>>> sensing path. And f2 and f3 are within the 40MHz difference.
>>> My problem is how to set it in USRP source block for this kind of
>>> configuration? Does anybody has such example? And I met the error like
>>> below:
>>> Runtime error:vector::_M_range_check
>>> which seems to be caused by incorrect setting of subdev. However, I don't
>>> know what the valid value fo subdev in such kind configuration.
>> Correct, you can only receive from one antenna at a time. If your RX
>> frequencies of interest are reasonably near in frequency, lets say 40
>> MHz, you can use multiple DSPs in the FPGA as a kind of 2 channel
>> filterbank.
>> You can set the subdev/frontend spec to "A:0 A:0", this will map the
>> frontend A:0 to both DSP0 and DSP1. If you setup the USRP source for 2
>> channels, DSP0 will be output 0 and DSP1 will be output 1.
>> Be careful with the tune_request so that you are setting the rf
>> frequency to the same setting, but with different target frequencies to
>> get the DSPs to shift appropriately. There should be some mention of
>> tune request in the docs inside the USRP source block properties dialog
>> in GRC.
>> -josh
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>>> Discuss-gnuradio mailing list
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