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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 an´LCD.

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 overload.

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 here).

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 here.

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 here. 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.

LC oscillator coil with 14 mm ferrox cube core

LC oscillator coil with 14 mm ferrox cube core

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 112.0 kHz.

4.2.2 TCA440 with external crystal oscillator

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 described here.

4.2.3 TCA440 with external LC-VCO oscillator

This alternative works with a frequency controlled LC-VCO and an ATtiny25. It is described here.

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.

4.3 The xtal filter for 32.768 kHz

Measuring the passband curve of 32.768kHz crystals

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.

Filter pass band curve of one 32.768kHz crystal

Filter pass band curve of one 32.768kHz crystal

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.

4.4 Automatic control of the DCF77 signals

The gain control as well as the frequency adjustment can, for test purposes, be adjusted with resistor trimmers. A usual trim potentiometer with 270° is sufficient.

More comfortable is when a micro-controller does that work. Measuring, adjusting and control of the AGC and AFC can be done with an ATtiny45, as shown here in detail.

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