Hi The problem with the crystal is that it has a temperature coefficient. As a narrow band filter, it will have a *lot* of delay. Crystal resonance moves (with temperature) and the delay changes.
How much delay depends a lot on a bunch of fiddly details. A 10 to 100 Hz wide bandpass could easily have delay in the > 100 ms range. How much change? The crystal could be in the 10’s of ppm / C range (might be lower, could be higher). At 60K Hz 10 ppm is 0.6 Hz. With a modest sort of basement temperature swing you *could* get 10% of the delay changing around. Yes, that’s a bunch of guesses. Net result would be delay variation in the 1 to 10 ms range. Bob > On Aug 11, 2020, at 3:37 PM, paul swed <[email protected]> wrote: > > Mark > His antenna hit a preamp as I recall about 20 db of gain. To see something > on a scope add 40 more db approx. Unfortunately a purely broadband solution > will show 40 db of pure garbage these days. Using the 60 KHz watch crystals > $2.00 for 20 out of China you can most likely find a reasonable match. > Thats what I did. It is hi Z so it feeds one side of an opamp. Look at the > spectracom schematics to get a sense of what to do. I made a small socket > to plug them in and found the one that worked. As an alternative you can > build a bandpass filter with opamps lots of variations. Anything to get the > received bandwidth reduced. Look at Johns front end also. > Regards > Paul > > On Tue, Aug 11, 2020 at 3:20 PM Mark Haun <[email protected]> wrote: > >> 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. >> > _______________________________________________ > 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.
