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17 Apr 2012

SYM-RFT-77 DCF77 module with standard antenna (60x7mm)

Using a better antenna to improve DCF77 reception on long distances

We are in the prototyping phase of building a Nixie clock using 1N-14 Nixie tubes. The clock is designed around a PIC16F886 MCU, 74141N BCD decoder/driver and CNY74 optocouplers using common circuit topology. High DC voltage (+ 180VDC ) is generated using MAX1771 step-up switching regulator, which is quite efficient (if you use appropriate components).

Our clock will have some nice features:

  • Compact design
  • Manual time configuration
  • DCF77 time synchronization
  • Sync success indicator
  • HV shutdown during sync (to reduce noise received by DCF module)
  • Super-capacitor time backup
  • Thermal protection
  • ICSP connector etc.

When clock is complete we will release it as open source-hardware here at Electronics-Lab.com

We decided to use DCF77 signal as time reference for two main reasons, it’s quite easy to receive it and it’s very accurate for the reason that carrier signal is generated from atomic clocks.

But, what about receiving and decoding DCF77 signal? 

The easiest way is to get a DCF77 Time Receiver Module, like SYM-RFT-77 or similar (check on ebay). This module comes with a receiving antenna tuned at 77.5kHz and probably using U4224B receiving IC which demodulates the received signal and provides digital pulses every sec (output goes high every second).  That’s cool ehh?

receiving DCF77 in Thessaloniki - Greece - image: www.compuphase.com

What about receiving the signal more than 1500Km away from transmitter (Frankfurt) in a noisy environment (red dot in map)? We tested the SYM-RFT-77 module and we saw that digital output is quite noisy, even late at night, when shortwave propagation is better (assuming optimal antenna orientation to Frankfurt). If we were lucky enough (or weather conditions were good) we managed to receive a clear DCF signal for some time.

So, we decided to look for a better DCF 77.5kHz antenna for our module and found C-MAX Time Solutions GmbH. They were kind enough to send as a CMA-77-100 clock antenna. This antenna measures around 100x10mm and it’s bigger than standard antenna (60x7mm) came with the module.

In the photo below you can see three 77.5kHz antennas. The first is the antenna came with a module based on CME6005 IC (found on ebay), the second one is the antenna came with SYM-RTF-77 module and third is the CMax antenna.

DCF77 antenna comparison

Testing  SYM-RTF-77 with CMA-77-100 antenna

To test out the new antenna we used two  SYM-RTF-77 modules side by side, one with the standard module antenna and the other with CMA-77-100 antenna.

We looked the digital output of the modules on oscilloscope and the results were impressive!! The signal received by module using CMA-77-100 antenna (OSC CH I) was almost perfect compared to the signal received by standard antenna  (OSC CH II) which was really noisy and hard to recognize the bit sequence.

DCF77 digital out - 100mm antenna (CH I) compared to 60mm antenna (CH II)

Check the data capture above. We see at CH I two random DCF bits. The first one is 200ms long which coresponds to logic 1 and the second one is around 50ms long which isn’t a standard duration (we expect it to be 100ms long) and we assume that it’s logic 0. Yet, the signal is much better the one we see on CH II.

CME6005 IC based module using Cmax antenna

Conclusion

It seems that antenna selection affects received signal quality,  bigger length and better tunning plays significant role in reception in long distances form DCF transmitter. So if you plan to use a DCF receiver on long distances, look for a better antenna.

Authors: Michail P. – Kwstas K.

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