[USRP-users] x310 Master clock rate

Marcus Müller marcus.mueller at ettus.com
Wed Apr 22 05:44:54 EDT 2015


Hi!

Thanks for following up on this.
> With the master clock set at 200Mhz and a sampling rate of 40Msps the
> following warning appears:
>
> UHD Warning:
>
> The requested interpolation is odd; the user should expect CIC rolloff.
>
> Select an even interpolation to ensure that a halfband filter is enabled.
>
> interpolation = dsp_rate/samp_rate -> 5 = (200.000000 MHz)/(40.000000 MHz)
>
>

To explain the warning:
The FPGA decimates the RX samples down, coming in at the master clock
rate from the ADC, to the sampling rate you select.
To do that without introducing aliasing, it has to low-pass filter them.
The FPGA filter chain has multiple stages; if I remember correctly, the
filters are in this order:
1. 1x CIC filter for odd factors in decimation
2. 3x 31-tap half band filters that each implement a good decimation for
factors of 2 each.

Now, if you use an odd decimation (200MHz/40MHz==5), there's no factor
of 2 in your decimation, and hence you only use the "not-as-good" CIC
filter, and are likely to see effects of filter roll-of at the edges of
your band.

In TX direction, it's quite the same problem:
1. 1x 17-tap half band interpolators
2. 1x 5-tap half band interpolators
3. 1x CIC

Therefore: if your measurements show that your signal is OK for your
application, just ignore the warning (it's just a warning, not an
error); if you think this is bad for the signal you want to work with,
I'd recommend using a 50MS/s sampling rate -- that would have a
decimation of 4 and should work beautifully. Of course, this means that
your host computer would have to implement the interpolation.

How wide is the signal you're planning to transmit spectrally? Is it
significantly less than 40MHz wide?

> When we increase the sampling rate to anything above 40Msps, the
> following discrepancy is seen in the data.
> If you look at the above image there seems to be some missing data.
I can only guess here, but is it possible UHD printed an "O" when you
tried to transmit this?
That would imply that your computer was too slow to supply the samples
in real time, and since the USRP did not have anything to transmit,
there is a "pause".

Best regards,
Marcus Müller

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