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.
