Re: [Discuss-gnuradio] aUaUaUaUaUaU and requested rx frequency not supported problem

[mleech]

On Fri, 11 May 2012 19:08:38 +0200 (CEST), address@hidden wrote:

> Hi Everybody,
> I am using USRP with daugherboard of range 1MHz to 250MHz. I am running
> the program with gnu-radio 3.6.0 on Ubuntu 11.10. I am trying to run the
> basic fm receiver program as shown in the video.
> http://www.youtube.com/watch?v=KWeY2yqwVA0
>
> Whenever I try to execute the code, I get following things on grc. I have
> double checked that the daughterboard works for the range. When I see the
> fft plot of the received signal I see that I have just noise all around.
> The hardware does not support the requested RX frequency:
>    Target frequency: 100.000000 MHz
>    Actual frequency: -28.000000 MHz
> aUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaUaU
>
> I have also tried to see if the antenna choices are correct. I checked it
> with uhd_usrp_probe
> The important part of the result was
>   RX Subdev: AB
> |   |   |   |   Name: Basic RX (0x0001) - AB
> |   |   |   |   Antennas:
> |   |   |   |   Sensors:
> |   |   |   |   Freq range: -250.000 to 250.000 Mhz
> |   |   |   |   Gain Elements: None
> |   |   |   |   Connection Type: IQ
> |   |   |   |   Uses LO offset: No
> |   |   |     _____________________________________________________
> |   |   |    /
> |   |   |   |       RX Subdev: BA
> |   |   |   |   Name: Basic RX (0x0001) - BA
> |   |   |   |   Antennas:
> |   |   |   |   Sensors:
> |   |   |   |   Freq range: -250.000 to 250.000 Mhz
> |   |   |   |   Gain Elements: None
> |   |   |   |   Connection Type: QI
> |   |   |   |   Uses LO offset: No
>
> So, when I try to work with uhd_fft. I got same problem with following
> results
> uhd_fft --spec B:AB
>
> linux; GNU C++ version 4.6.1; Boost_104601; UHD_003.004.001-39-unstable
>
> -- Opening a USRP1 device...
> -- Using FPGA clock rate of 64.000000MHz...
> Using Volk machine: ssse3_64
>
> UHD Warning:
>    The hardware does not support the requested RX frequency:
>    Target frequency: 100.000000 MHz
>    Actual frequency: -28.000000 MHz
>
> Could you help me out here?
> Thanks in advance.
>
> -- Pranav Sakulkar
>
>
>
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> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio

The aU is because your audio subsystem is experiencing underruns, probably because you have a sample-rate issue in your flow-graph.

 

The UHD warning is because the BASIC_RX has no downconversion hardware at all, which means that any frequency above Nyquist (32Mhz for USRP1 and 50Mhz for N2XX) requires that the FPGA/ADCs sample the appropriate alias frequency, and UHD is warning you that's what it's doing.  But without analog filtering in the front-end, you'll end up with *unwanted* aliases as well.  The BASIC_RX is very "BASIC", and it's really intended as an interface card to "proper" RF front-ends, with filtering, and probably downconversion to an IF.