AVR single chip controller AT90S, ATtiny, ATmega and ATxmega DCF77 direct amplifier with TCA440
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
include AM and FM receivers in one chip (e. g. CD2003 and many more).
For DCF77 the FM part is unnecessary and consumes current for nothing,
offer very primitive amplifiers without gain regulation, unusable
for changing voltage levels (e. g. TA7642 ZN414), or that
are unavailable in normal electronic shops (such as the MAS6181 or
the TDA1572), or that
require a large number of external capacitors to work (e. g. SA602/612).
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
3.1 The TCA440
The TCA440 integrates, in its 16 pin DIP case
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,
it receives a symmetric oscillator frequency on its pins 4 and 5
and couples it back on pin 6,
it has an additive mixer on board, with its outputs on pins 16
integrates a gain regulated IF amplifier with symmetric inputs on
its pins 12 and 13 and its output on pin 7,
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
21 diodes, and
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
The received signal from the cross antenna, with frequency
regulation and buffered with an N-FET from
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
Frequency (of the cross antenna receiver) and gain regulation can
be made using trim resistors or with an ATtiny45 controller as
Note that the TCA440 has a gain of 1.0 if you leave the AGC input
open. To wake-up the TCA440, apply a lower voltage to the AGC pin,
and he wakes up and gains.
That is how the amplifier and rectifier is mounted on a