Does the La Crosse UltrAtomic clock actually use a crystal filter or do they digitally filter the signal? Has anyone ever looked inside of one of the clocks? Just curious.
Ray AB7HE -------- Original Message -------- Subject: Re: [time-nuts] WWVB SDR discussion From: Mark Haun <[email protected]> Date: Mon, August 10, 2020 2:18 pm To: [email protected] No more so than for a GPSDO using a microcontroller. In this case, the MCU would steer an external oscillator using an appropriately long time constant, based on a phase tracking loop. In my tentative block diagram, the oscillator is a cheap 26 MHz OCXO which supplies the MCU clock and (as a byproduct) the internal ADC sample rate. The basic plan for an STM32L4 was something like 26 MHz clock input from OCXO 78 MHz MCU clock (x3 via internal PLL) 3.12 MHz ADC sample rate (78 MHz / 25) 13x downsample to 240 kHz using DFSDM block (hardware CIC) sign flip and deinterleave into I and Q channels (I, Q, -I, -Q, ...) downsample again, to ~ 10 Hz or so; do SDR stuff at this rate carrier tracking loop runs back outside the MCU to the OCXO control input, via PWM DAC With care, the internal ADC gives you about 11 good bits at the high sampling rate. Assuming white noise (questionable I suppose), this yields something like 18 noise-free bits at the low rate where the phase tracking and decoding happens. Prerequisite is an analog input centered around 60 kHz with less than 3.6Vp-p amplitude. There are no particular bandwidth requirements (Nyquist is 1.5 MHz!) as long as it is narrow enough to avoid clipping from strong interfering signals. I am having a hard time with this part, unfortunately. My tuned loop seems still too broadband, even after a couple more poles of op-amp filter. I have a bunch of 60-kHz tuning-fork crystals and wanted to try a crystal filter like the "pros" do in cheap consumer clocks, but I can't figure out a circuit to do this (weak on analog). So if anyone wants to tackle that and send me a working design, I'm happy to do the easy stuff, i.e. the DSP/SDR parts ;) Mark 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.
