[USRP-users] x310 Master clock rate

Ian Buckley ianb at ionconcepts.com
Wed Apr 22 14:33:40 EDT 2015


X310 - 3x 47tap halfbands in both TX and RX directions (and then CIC)
-Ian
On Apr 22, 2015, at 2:44 AM, Marcus Müller via USRP-users <usrp-users at lists.ettus.com> wrote:

> 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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> USRP-users at lists.ettus.com
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