Merely selecting every 100th sample throws away the opportunity to reduce the effective ADC noise by digital filtering before decimation.
Bruce > On 05 November 2019 at 10:08 Jan-Derk Bakker <[email protected]> wrote: > > > 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 <[email protected]> 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 <[email protected]> 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 -- [email protected] > > To unsubscribe, go to > > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
