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DD4WH edited this page Aug 23, 2018 · 15 revisions

This is Teensy DCF77, a software aiming at adding ultraprecise time data to your Teensy by receiving and decoding long wave time signals from DCF77, located at Frankfurt/Main, Germany. This should be possible with your Teensy and very few external hardware parts. [joint effort by Frank Boesing and Frank DD4WH]

Hardware Setup

You need a Teensy 3.x and the Teensy audio board (use the MIC input of the audio board for connecting the antenna, see below). Very little external hardware is needed:

The inductor and capacitor form a resonator that resonates at the frequency of your desired time signal longwave station. Add a ground connection and some metres of simple wire as an antenna, and you´re done!

Use the formula given in the figure or one of the many calculation tools on the WWW for resonators to calculate which capacitor you need for the given inductance or your inductor.

http://wetec.vrok.de/rechner/cskreis.htm

A few hints on how to maximize the success with your setup:

  • connect GND of the MIC input of the Teensy audio board to a ground connection (the heating for example): NEVER EVER USE THE GND CONNECTION OF YOUR MAINS CONNECTOR! (Do not even think about that!)
  • always use the coupling capacitor (100nF) to prevent MIC bias short circuit!
  • avoid/eliminate local noise: plasma TVs, switching power supplies, laptop displays, the Teensy itself! The resonating circuit should be placed a bit away from the Teensy and its oscillators, use wires of at least 20cm length to connect the resonating circuit to the Teensy MIC Input
  • if everything fails: buy a DCF77 receive module (available from 8 to 15 EURO) and connect the output to the Teensy, this should be bullet proof

SOFTWARE:

  • the Teensy audio board takes the MIC input signal and digitizes it with 192ksps (so you are able to receive up to 96kHz)
  • that is a Direct Sampling Receiver similar to the really expensive and professional SDRs
  • the signal is bandpass-filtered around 77.5kHz
  • the signal is fed to a 1024-point FFT for visual inspection AND for peak analysis in order to extract the time information bits
  • the signal is multiplied with a local oscillator working at 76900Hz
  • the signal is lowpass-filtered (however, the biquads are not very good at low frequencies, be careful here!)
  • you can hear the audio signal from DCF77 with a 600Hz tone (77500Hz-76900Hz) that´s the principle of a DC (direct conversion) receiver

For more information, have a look at this thread in the Teensy Forum:

https://forum.pjrc.com/threads/40102-Time-signal-LF-Receiver-Teensy-DCF77-with-minimal-hardware-effort

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