[time-nuts] A simple sampling DMTD

[Jan-Derk Bakker]

Dear Scott,

You are entirely correct, sampling at 100ksps is mathematically the same as
sampling at 10Msps and then decimating by a factor of 100. The reason I'm
doing it in this way is driven by two practical considerations:

- my ADC, the LTC2140 (selected for bandwidth, dynamic range, aperture
jitter and availability) has a pipelined design and cannot be clocked as
slow as 100ksps without degrading performance
- my CPU, the Atmel/Microchip XMega (selected for its peripherals, toolset
support and familiarity for both me and my students) is an 8-bit AVR
running at 30MHz and cannot directly process 10Msps

A simple D-flipflop based decimator was the easiest way to bridge this gap.
The DMTD design has connectors for a (forthcoming) FPGA daughterboard to
properly sinc-filter (and I/Q demodulate and...) the incoming samples.

Sincerely,

JD 'walk first, then run' B.

On Mon, Nov 4, 2019 at 6:38 PM W7SLS <w7sls.scott at gmail.com> wrote:

> Hello,
>
> (It has been a more few years since I designed / developed DSP for
> spectrum analyzers for a major company, thanks for your patience.)
>
> I recall that:
>
>         sample @ fs -> decimate (toss samples) by a factor of N
>
> is equivalent to
>
>         sample @ fs / N
>
> If we wanted a lower sample rate, we would:
>
>         sample @ fs  -> lowpass filter at (say) 80% of (fs/2) / N ->
> decimate
>
> Otherwise all of the energy > 50% of (fs / N) gets aliased into your data.
>
> Why not just sample at 100 kHz?
>
> What am I missing ?
>
> Thanks for considering,
> Scott W7SLS
>
> > On Nov 3, 2019, at 12:59 PM, Jan-Derk Bakker <jdbakker at gmail.com> wrote:
> >
> > Is this not caused by the fact that I'm currently subsampling the ADC
> > (conversion rate 10Msps, rate into the microcontroller 100ksps by
> dropping
> > 99 put of 100 samples)?
>
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com
> To unsubscribe, go to
> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com
> and follow the instructions there.
>