Hi Detlef, On 11-Aug-20 3:46 AM, Detlef Schuecker via time-nuts wrote: > you do not necessarily need a variable ( physical ) oscillator. You mix > the signal down in the digital domain. A 'digital local oscilator' is a > mere complex value, which is rotated and the power is adjusted: My proposed block diagram does actually have a digital LO, only mine is 1, 0, -1, 0... (in-phase) and 0, -1, 0, 1... (quadrature). You could of course use an variable-frequency NCO, but I need a physical oscillator in any case to clock the MCU. I am also thinking in terms of a WWVB-DO where I want a stable local reference to steer. (Although in fairness, for WWVB I think you probably want stability over the diurnal propagation variation, and my crappy OCXO has no chance at that.) >> 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" > Good point. > Firstly I tried a tuned resonant LC circuit with a BF245 preamplifier to > keep a high Q. Besides having minimal analog design experience, I think what is confusing me is these crystals have a really high impedance far away from their parallel resonance. Even at series resonance the motional resistance is in the 10s of kohms, IIRC. So I'm not exactly sure how to deploy one in place of an LC tank circuit, e.g. in a collector or drain circuit. > Secondly an accurate ADC is an option. With 24Bit you get more than 100dB > dynamic range, so you dont care about a 60dB stronger nearby interferer.
Fancier ADC shouldn't make any difference. Even though the STM32L4 ADC has an SNR of (IIRC) ~ 69 dB, you can sample at several MHz. After downsampling to something like 100 Hz (post LO), the SNR is well over 100 dB, which should be plenty. (Of course, this presupposes that there are not strong interferers or SMPS noise spurs, etc. within a 100-Hz BW centered on 60 kHz. But if there are, a fancy ADC wouldn't help you anyway. The main thing is to make sure the interference isn't causing any clipping or nonlinearity before you sample.) I should note that my big tuned air loop and preamp, which is modeled after Joe Magliacane's, may be adequate even though I can't see WWVB on the oscilloscope. I was hoping to build confidence there before hooking it up to the MCU and doing SDR stuff. I also want a small loopstick version of this so I can embed it in my own nixie wall clock, hence the interest in crystal filters, etc. 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.
