[time-nuts] Commercial software defined radio for clock metrology

[Bob Camp]

Hi

The Max V FPGA on the Bemicro card is pretty impressive. The gotcha is with the Quartus
side of things. Altera is not willing to let you have the DSP stuff (NCO’s, CIC’s, FIR’s) for free
the way Xilinx is.

Bob


> On May 28, 2016, at 4:47 AM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
> 
> A low pass filter will reduce the source broadband noise aliased into the ADC output signal.
> 
> Using the LVDS ADC outputs rather than the CMOS outputs may help in reducing noise generated on the board. NB the ADC performance is specified when the LVDS outputs are used.
> 
> This SDR setup appears to have a higher PN (at least 2 ADC's per signal are required  to achieve lower PN ) than a Timepod, however it appears to be better at measuring ADEV than a Timepod.
> Bruce 
> 
>    On Saturday, 28 May 2016 6:02 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> 
> On Fri, 27 May 2016 18:58:35 -0700
> Hal Murray <hmurray at megapathdsl.net> wrote:
> 
>> bruce.griffiths at xtra.co.nz said:
>>> All the filtering and down mixing is done in the digital domain.
>>> Anitialiasing filters in front of the ADCs are also be required. 
>> 
>> What sort of bandwidth is expected?
>> 
>> The usual trick with audio ADCs is to have a low cost analog filter that 
>> does't have a sharp corner but lets everything you want through, sample at a 
>> high rate - say 16x, run that through a digital filter with a sharp cutoff, 
>> then decimate down to the desired sample rate.
> 
> The daughterboards for the USRP N210 that allow direct access to the
> ADC inputs do not contain any filters. They kind of expect band limited
> signals at the input.
> 
> Given that they used 10MHz signals from H-masers and used 100Msps,
> I would say that the Niquist condition does hold. One might probably
> increase the noise performance a little bit by using a low pass filter.
> 
>             Attila Kinali
> 
> -- 
> Reading can seriously damage your ignorance.
>         -- unknown
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