Re: [Discuss-gnuradio] bladeRF - gr-osmosdr - gqrx - bandwidth?

[Brian Padalino]

On Fri, Sep 13, 2013 at 3:58 PM, Alexandru Csete <address@hidden> wrote:
> On Fri, Sep 13, 2013 at 3:53 PM, Brian Padalino <address@hidden> wrote:
>> On Fri, Sep 13, 2013 at 2:19 AM, Ralph A. Schmid, dk5ras
>> <address@hidden> wrote:
>>> Hi,
>>>
>>> I am using bladeRF with gr-osmosdr, gnuradio 3.7 and gqrx, so far a fine
>>> radio. One thing, when using gqrx...I am limited to USB2.0 at the moment, so
>>> I use a sampling rate of 8 Ms/s, and in this mode I have big problems with
>>> images from the neighbour frequencies that are still within the 26 MHz
>>> bandwidth, or whatever the maximum BW is.
>>>
>>> Is there some command to set this bandwidth to a smaller value? I assume
>>> that this would improve my RX experience a lot :) A quick look at osmocom
>>> showed me nothing, but maybe I missed smth.
>>
>> The gr-osmosdr interface allows for a bandwidth setting to be changed here:
>>
>>   
>> http://cgit.osmocom.org/gr-osmosdr/tree/lib/bladerf/bladerf_source_c.cc#n581
>>
>> The driver will figure out which bandwidth is closest to what you want
>> with a minimum of 1.5MHz and a maximum of 28MHz for the low-pass
>> filters.
>>
>> It will definitely make a world of difference by actually applying the
>> LPF and removing the aliasing.
>>
>> As for official support for gqrx, it's next on our list.  We need to
>> get on the gqrx mailing list and figure out what code we need to
>> write.
>>
>
> Actually, it was a deliberate choice not to have explicit support for
> that API call since it seemed unnecessary. Wouldn't one always want to
> use the narrowest analog bandwidth corresponding to a given sample
> rate? If yes, the setting may as well happen as part of the sample
> rate configuration and best handled at a layer that knows about the
> specific device. Am I wrong?

One may want to listen to an LTE signal that is 5MHz wide, but sample
at the standard 30.72MHz sample rate.  In this case, I would think the
LPF might want to be set smaller than 28MHz (maybe 8MHz?) but still
sample at 30.72MHz?

Maybe another case is wanting to see 5MHz worth of bandwidth, seeing a
weak signal, move the weak signal into the LPF region, and tighten the
filter such that you could increase the gain without clipping the ADC
in case there is a near-by blocker?

I agree you can shoot yourself in the foot very easily by separating
the two of them and have lots of noise alias in, but keeping them
separate, in my mind, is still a very reasonable thing to do.

Maybe, for the case of gqrx, an option for gr-osmosdr at device
opening (and passed to the constructor) is to have a bandwidth setting
(auto or manual) that when put into auto mode will do as you suggest,
and set the appropriate low pass filter bandwidth as well as warn if
the sample rate is too low, and there will be aliasing due to the LPF
not being tight enough?

What are your thoughts?

Brian