AVR single chip controllers AT90S, ATtiny, ATmega and ATxmega DCF77 superhet receiver with xtal filter
4 DCF77 superhet receiver with xtal filter
Those who want to have the Mercedes of a DCF77 receiver, home-brew themselves a
superhet with a crystal filter! The DCF77 receiver RF signal (e. g. from a
cross antenna) of
77.5 kHz is
amplified in a pre-amp, then
mixed with an oscillator signal to form a different frequency (here:
32.768 kHz), which is then
filtered with an LC circuit and a crystal, after that
amplified in an Intermediate Frequency (IF) amplifier, its output then
is again filtered with an LC circuit and rectified in a two-diode stage
as shown here
with the generated DC filtered in an RC stage, and then
the DC is measured, checked and decoded in an ATtiny45 controller, with
time and date information serially transmitted to
be received, decoded and displayed on anŽLCD.
With that, you can be absolutely shure that no one besides you (and me, of
course) has such a homebrewed Mercedes in its garage: it is unique and perfect.
4.1 Advantages of a superhet over any other concepts
Superhets are better than direct receivers because the Intermediate Frequency
(IF) can be filtered with a small bandwidth (here: of a few Hz). So any
interferences from other sources (random noise, strong RF from nearby short
wave transmitters, from switching power supplies or switched power saving
lamps as well as all other electromagnetic fields can be completely sorted
out and eliminated. So it is possible to receive the DCF77 signal in a very
far distance and in a noisy environment, where other receivers do not work.
As the IF amplifier works on a different frequency, the IF signal can be
amplified without getting self-oscillation. This also makes it more sensitive
than direct receiver concepts.
4.2 The superhet schematic
This is the schematic of the Mercedes.
The symmetric output signal from the cross antenna's FET buffer stage
is fed into the pre-amplifier stage of a TCA440 on its pins 1 and 2.
The gain reduction of the pre-amp stage on pin 3 is turned off. IF
you are in the absolute near-field of DCF77 (say: less than 10 km)
you can apply 1 or 2 V here to not drive the mixer stage into an
On the oscillator pins 4 and 5 the oscillator signal of 77.5 + 32.768 =
110.268 kHz is supplied. This signal ois either generated in an
LC circuit (by using the oscillator output signal on pin 6, see
here) or with an xtal oscillator (see
The mixer products are filtered with a LC circuit made of a fixed coil
of 15 mH and a capacitor of 1.5 nF. To filter the only product
of interest, the 32.768 kHz, one or up to three 32kHz xtals follow.
The properties of such a crystal filter are in detail shown
The output of the crystal filter is fed into one of the two symmetric
input pins (pin 12) of the IF amplifier, with the other input on pin
13 being blocked to ground potential via a 1µF capacitor.
The emitter output of the IF amplifier on pin 7 is connected with a
second LC combination with L=100µH and two parallel capacitors
of 220 nF and 15 nF. The signal is then fed into a 2-diode
rectifier and RC filter stage to yield the amplitude as DC. This is
further measured and analyzed in a controller as described
The superhet comes in two variations: with the oscillator signal
produced by a LC combination, or
with a crystal oscillator and rectangle-to-sine filter.
4.2.1 TCA440 with an LC oscillator circuit
If you want to use the built-in oscillator in the TCA440
the following is necessary. Prepare an 18mm ferrite core
with an AL value of 2,850 nH per winding2.
The core can be trimmed with a screw or with the trim capacitor
to 110.268 kHz. Use a frequency counter or the rectified
DC to adjust.
An alternative to that would be to chose a 14-mm ferrox cube
core with an AL of 250 nH/w2. The components
change slightly, with a trimming range from 108.4 to
As the oscillator has a very narrow frequency band where the
superhet is optimally tuned and due to the fact that LC
combinations on the TCA440's tend to change their frequency,
e. g. with changing temperatures, I thought about
alternatives to the LC controlled oscillator stage, using
crystals. Two alternatives were designed and tested.
The first alternative is to clock an ATtiny25 with a crystal
and dividing this by a constant divider. This solution is
This alternative works with a frequency controlled LC-VCO
and an ATtiny25. It is described
4.2.4 Mounting the superhet
That is how the capacitor and xtal grave looks alike on a breadboard,
here with a LC oscillator.
To the left the buffer stage with the FET can be seen (the
antenna can not be seen). The frequency of the input stage can be
adjusted with the left trim resistor. Then the TCA440 with the
oscillator coils follow. Above to the right the three tiny
crystals and the 1µF grave can be seen. On the lower
part the three 470 µF capacitors of the rectifier
can be seen. The trim resistor to the right regulates the
gain of the IF amplifier.
To measure the filter properties of 32.768kHz crystals, one can use
this oscillator. It generates a 32kHz sine wave signal with an
adjustable frequency. The adjustment is made with Medium Wave
varactor diodes, for which a BB212 or a variable capacitor for
medium wave can also be used.
The crystal is fed with the low-resistance signal output of the
sine wave generator and has an output resistor of 1kΩ.
This is the resulting pass-band curve. It is less than 10 Hz
wide, especially the falling edge is rather steep.
When measuring slightly above the resonance frequency a moderate
feedback on the oscillator took over control, so one single data
point showed an unexpected value.
Remarkable is that the selectivity far from the resonance is rather
limited. This is caused by the stray capacity of the crystal.
Therefore the crystal filter shall always be combined with an
LC filter, to reduce frequencies far from the xtal resonance.