[USRP-users] Which bandwidth does uhd::usrp::multi_usrp::set_rx_bandwidth set?

Martin Braun martin.braun at ettus.com
Fri Feb 22 19:30:25 EST 2019


Janos,

the purpose of the the API call is to configure analog filters, and not the
sampling rate. If your analog bandwidth is lower than the sampling rate,
there's nothing wrong with that. In fact, for the AD9361-based devices,
this is not unusual, since the analog bandwidth is 56 MHz, but the max
sampling rate is 61.44 Msps. The reason you would want to do that is
because your actual signal might be smaller than 56 MHz, but you want to
sample at 61.44 Msps because that's easier on your DSP.

For dboards that don't have configurable filters, the experience is,
unfortunately, not entirely consistent. UBX currently (and I don't really
like this) simply accepts any value you write, and does nothing with it.
However, if you do get_rx_bandwidth(), you will read back what you wrote.
For the dboards with variable analog filters, though, you get the actual
value.

-- M

Some daughterboard just do nothing and accept any value you write (e.g.,
the UBX).

On Wed, Feb 13, 2019 at 5:22 AM Janos Buttgereit via USRP-users <
usrp-users at lists.ettus.com> wrote:

> Hi everyone,
>
> after having read
> https://kb.ettus.com/About_USRP_Bandwidths_and_Sampling_Rates I wonder
> what uhd::usrp::multi_usrp::set_rx_bandwidth does for setups with
> Daughterboards that have a fixed analog bandwidth? According to the API
> documentation it sets "the RX bandwidth on the frontend“ which to me seems
> to refer to the analog bandwidth. So does calling this function only result
> in any changes if the setup contains frontends with a configurable analog
> bandwidth?
>
> Now what happens if I set a sample rate that is lower than the supported
> analog bandwidth? This should obviously decrease the data rate  of the
> stream from/to the host, however what happens before? Is the signal still
> sampled at a sample rate that is high enough to handle the analog bandwidth
> and then digitally filtered and decimated/interpolated by the FPGA to match
> the required host sample rate? E.g. in fact  the sample rate setting is
> more relevant for the actual bandwidth of a setup than the bandwidth
> setting which is only relevant to a few frontends?
>
> I’d be happy to get a short feedback if I got it all right or if I
> misunderstood something?
>
> Best Regards
> Janos
> _______________________________________________
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> USRP-users at lists.ettus.com
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>
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