JD, Thanks for your cogent reply. I appreciate it.
Best Scott Sent from my iPhone > On Nov 4, 2019, at 5:03 PM, 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.
