[time-nuts] WWVB SDR discussion

[d.schuecker at avm.de]

Hi,

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:

#define CMUL(_a,_b,_c)  \
    { _c.re = _a.re*_b.re-_a.im*_b.im; \
      _c.im = _a.re*_b.im+_a.im*_b.re; }
#define CROT(_a,_w) 
    { fcomplex_t hh; \
      CMUL(_a,_w,hh); \
      _a.re=hh.re*hh.re+hh.im*hh.im; \
      if(_a.re<1.0f){ \
         hh.re *= (1.0f+1e-6); \
         hh.im *= (1.0f+1e-6); \
      } else { \
         hh.re *= (1.0f-1e-6);  \
         hh.im *= (1.0f-1e-6);  \
      } \
      _a.re=hh.re; \
      _a.im=hh.im; \
    }


The sample rate is not an integer multiple of the signal frequency, that 
is not necessary. 


> 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.
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.

I want to phase track at least 2 time broadcasters eg Anthorn and DCF77 
and try to do a hyperbolic navigation with the phase difference. The LC is 
tuned by MOSFET switched capacitors. A STM32 should suffice for the thing. 
See, how far ( or near, in terms of navigation ) I can get :).

Cheers
Detlef
DD4WV
 





 

Detlef Schücker


"time-nuts" <time-nuts-bounces at lists.febo.com> schrieb am 10.08.2020 
23:18:29:

> Von: "Mark Haun" <mark at hau.nz>
> An: time-nuts at lists.febo.com
> Datum: 11.08.2020 00:29
> Betreff: Re: [time-nuts] WWVB SDR discussion
> Gesendet von: "time-nuts" <time-nuts-bounces at lists.febo.com>
> 
> 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
> 
> On 10-Aug-20 11:45 AM, paul swed wrote:
> > Hello to the group.
> > Have been looking forward to seeing how the STM32 SDR project might be
> > going.
> > SDR is a weak spot for me. So been reading. And believe the answer is 
that
> > a SDR solution may work for AM code and even BPSK code to an extent. 
But
> > doesn't the sampling destroy the quality of the incoming signal for
> > establishing a locked reference?
> > Regards
> > Paul
> > WB8TSL
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