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DCF77 direct amplifier with TCA440
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3 A DCF77 receiver direct amplifier with a TCA440

Lots of parts are needed for a gain-adjustable receiver amplifier for 77.5 kHz (see here). So using an integrated circuit can reduce the number of necessary parts. As currently AM radio reception are slowly dying out, the production of such ICs is very limited. There are only ICs in production, that The way out from that dilemma is to use ICs that are not produced any more but are still available in specialized shops. Old fashioned AM superhet ICs fit our needs for DCF77 reception perfectly. For a direct receiver concept, only the internal pre-amplifier, oscillator and mixer is unnecessary and has to be disabled. The integrated IF amplifier, equipped with a gain control input pin, can be used to amplify the 77.5 kHz directly with enough gain.

Here the ancient TCA440 is used. It needs only a few external components. It is still available in specialized electronic shops, search for it on the internet.

3.1 The TCA440

The TCA440 integrates, in its 16 pin DIP case
  1. it provides a differential amplifier on its pins 1 and 2, that can be used as a RF input stage, its gain can be reduced applying positive voltages on pin 3,
  2. it receives a symmetric oscillator frequency on its pins 4 and 5 and couples it back on pin 6,
  3. it has an additive mixer on board, with its outputs on pins 16 and 15,
  4. integrates a gain regulated IF amplifier with symmetric inputs on its pins 12 and 13 and its output on pin 7,
  5. has a gain amplifier stage on pin 9, that regulates the IF gain and has an output for attaching an instrument to display field strength on pin 10.
The pins 14 (plus) and 8 (GND) provide the operating voltage. The IC integrates Copying of this using discrete parts would be an extreme effort, filling a Euro sized board.

Additional data, the internal structure and applications as medium wave receiver can be seen in the data-sheet by Siemens.

3.2 Schematic for a DCF77 direct amplifier with TCA440

DCF77 direct amplifier schematic with TCA440 The received signal from the cross antenna, with frequency regulation and buffered with an N-FET from here is symmetrically applied to the IF amplifier of the TCA440. Pre-amp, oscillator and mixer are switched off and are not used here.

The output of the IF amplifier goes to a low-resistance (approx. 2 kΩ) resonance circuit. The AM-RF is then rectified with a double diode rectifier already described here.

Frequency (of the cross antenna receiver) and gain regulation can be made using trim resistors or with an ATtiny45 controller as described here.
Mounting the TCA440 direct receiver on a breadboard That is how the amplifier and rectifier is mounted on a simple breadboard.


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