[time-nuts] wtd: WWVB info

Bob Camp kb8tq at n1k.org
Thu Aug 6 21:51:43 EDT 2015


> On Aug 6, 2015, at 1:43 PM, Jim Lux <jimlux at earthlink.net> wrote:
> On 8/5/15 8:27 PM, Donald wrote:
>> On 8/5/2015 7:55 PM, Bob Camp wrote:
>>> Hi
>>> Analog Devices has some very nice ADC’s that are directly targeted at
>>> doing this general sort of thing. They do not have any “odd” filtering
>>> approach
>>> that creates issues. Some of the early 192 KHz audio parts did not do
>>> very well
>>> past 1/4 the clock rate.
>> What type of FFT horse power would be needed at 192 Khz ?
> not very much..
> It's a fairly narrow band signal, It might be more computationally efficient to do a Bandpass FIR and decimate.

If you have an FPGA to do the “lots of bits” add(s), a higher order decimator followed by a filter probably will do just fine. 
With some care you can get a few hundred db of filtering that way (math wise). Even without much thought, you can get 
past 150 db. You likely will hit IMD issues on your ADC *long* before that. The decimator (which is computationally simple)
does all the high speed heavy lifting. The filter(s) run(s) at a much slower rate so it’s not terribly MIPS intensive. 


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