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

[jimlux]

On 5/27/16 6:15 PM, Bob Camp wrote:
> Hi
>
>
>> On May 27, 2016, at 8:17 PM, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:
>>
>> On Thursday, May 26, 2016 06:40:26 PM Bob Camp wrote:
>>> Hi
>>>
>>> Very interesting paper, thanks for sharing !!
>>>
>>> One question:
>>>
>>> In many DMTD (and single mixer) systems, a lowpass and high pass filter are
>>> applied to the signal coming out of the mixer. This is done to improve the
>>> zero crossing detection. It also effectively reduces the “pre detection”
>>> bandwidth. My understanding of the setup in your paper does not do this
>>> sort of filtering. It simply operated directly on the downconverter signal.
>>> Is this correct? I may have missed something really obvious in a quick
>>> read of the paper…..
>>>
>>> Thanks!
>>>
>>> Bob
>>
>> All the filtering and down mixing is done in the digital domain.
>> Anitialiasing filters in front of the ADCs are also be required.
>>
>> A 2  (or more) receive channel SDR board would be a nice tool to use for this
>> provided the FPGA is large enough.
>
> So … is there an explicit high pass in the FPGA other than the dc offset elimination high pass?
> There is obviously a lowpass function in the decimating FIR’s and the CIC. That appears to be
> optimized simply for sample rate rather than for noise. Thus the same question applies to
> low pass as well.
>


If they are using the stock USRP load, it's a digital down converter: 
NCO mixes with input samples, CIC decimator and bandpass filter 
(actually low pass I/Q), followed by a couple FIR filters.

Then, you can implement whatever further filtering and processing you 
want in software, most commonly done in gnuradio (by far most common) or 
simulink/Matlab.