A popular project among
microcontroller aficionados is to build a radio-controlled clock. Tiny
receiver boards are available, with a pre-adjusted ferrite antenna, that
receive and demodulate the DCF77 time signal broadcast from Mainf
lingen in Germany. DCF77 has a range of about 1,000 miles. All the
microcontroller need do is decode the signal and output the results on a
display. The reception quality achieved by these ready-made boards
tends to be proportional to their price.
In areas of marginal reception a higher quality receiver is needed,
and a small selective preamplifier stage will usually improve the
situation further. The original ferrite antenna is desoldered from the
receiver module and connected to the input of the preamplifier. This
input consists of a source follower (T1) which has very little damping
effect on the resonant circuit. A bipolar transistor (T2) provides a
gain of around 5 dB. The output signal is coupled to the antenna input
of the DCF77 module via a transformer.
The secondary of the transformer, in conjunction with capacitors C4
and C5, forms a resonant circuit which must be adjusted so that it is
centered on the carrier frequency. An oscilloscope is needed for this
adjustment, and a signal generator, set to generate a 77.5 kHz sine
wave, is also very useful. This signal is fed, at an amplitude of a few
milli-volts, into the antenna input. With the oscilloscope connected
across C4 and C5 to monitor the signal on the output resonant circuit,
trimmer C5 is adjusted until maximum amplitude is observed.
It is essential that the transformer used is suitable for
constructing a resonant circuit at the carrier frequency. Our proto-type
used a FT50-77 core from Amidon on which we made two 57-turn windings.
It is also possible to trim the resonant frequency of the circuit by
using a transformer whose core can be adjusted in and out. In this case,
of course, the trimmer capacitor can be dispensed with.
Elektor Electronics 2008