The amplifier has two stages, each equipped with a usual NPN small
signal transistor (you can use any available type):
The first stage is a voltage amplifier with a resonant LC circuit
for 77.5 kHz in its collector. To reduce load influences from the
next stage on the resonant circuit, the capacitor of the LC is divided
into a large and a small capacitor and so divided by 4.5 with that
The second stage is a similar voltage amplifier but with a slightly
smaller inductivity and larger capacitors. As the next stage is not
interfering the LC resonant circuit, no voltage division is made here.
The gain of the first two stages is extremely high, due to the very high
resistance of the LC circuit in the collector at resonance (approx.
161 kΩ in stage 1 and 70 kΩ in stage 2). And this
high gain is frequency-specific. Each stage amplifies the signal by
I also tried an additional stage of a similar design. With that stage
I had to reduce the gain because of self-oscillation. I tried the diode
attenuator as well as reducing the emitter capacitors and increasing
the emitter resistors: it is all the same, the applicable gain of
stage 3 is of no use, so it does not make any sense to add it.
The diode attenuator on the first and second stage input works as
follows. Increasing the current through the diodes (red curve) reduces
their resistance, which is R = VDiode / IDiode.
At the highest current here, IDiode is (5 - 2 * 0.65) / 1 k =
3,7 mA, so the diode resistance is RD = 0.65 / 3.7 =
176 Ω. As both diodes are parallel to ground (the upper one
direct, the lower one via the 100nF-capacitor) the diodes are parallel
and the resistance of the two parallel diodes is 88 Ω.
With a capacitor of 1 nF, its capacitive reactance is ZC
is: ZC = 1 / 2 / Π / 77500 / 1E-9 = 2,053 Ω.
The capacitor and the two diodes make up a resistor divider that
attenuates the signal to its 88 / (88+2,053) = 0.041-fold, or with a
factor of 24.4. Both attenuators in the two stages reduce the gain of
the amplifier by the 595-fold.
The diagram shows the attenuation of a single stage (red line) and of
the second stage (blue line) of the two-stage amplifier versus the AGC
voltage applied. The AGC voltage reduces the gain of the two stages
smoothly (note that the PNP drive stage reaches saturation at 2.8 V,
so the curves are theoretical above that!).
In stage 1 the diode attenuator following adds 1 nF to the C12
capacitor. That reduces the resonance frequency of the LC combination
slightly. This effect is calculated in the Libre-Office spreadsheet
here. The resonance
frequency shifts from 77.8 down to 77.15 kHz, still is within
the bandwidth of DCF77 and has no negative consequences.
The same calculation sheet calculates the influence of the emitter-base
capacitor of the transistor, in this case the stage 3 capacity. This
has positive consequences as it reduces the resonance frequency down
from 77.95 to 77.82 kHz. Because the tolerances of the parts used
have a much higher influence, this is rather of an academic nature.
The DCF77 signal is amplified by a very high gain, so that the gain
has to be reduced with the diode attenuator. I have used a trim resistor
with 10 kΩ and a simple PNP driver stage with 1kΩ in the
emitter. That allows to adjust the gain to ensure that the rectifier
stage is not overdriven.
If you mount the amplifier and leave the AGC input (base of the PNP)
open, the AGC is at full +5V and the gain is minimized. If you
shortcut the AGC input with minus, then the amplifier swings. I have
measured 72.5 kHz, which is near the resonance of the first
collector resonance circuit. The difference between 72.5 and 77.5 is
not so dramatic because the selectivity of the collector stage is not
very high. If you apply approx. 100 mV to the AGC input, the
In a second test configuration I attached the base of the PNP driver
to a potentiometer resistor of 100 kΩ, with both ends
attached to the operating voltage. With the potentiometer the gain
can be adjusted very sensitive. If you use manual adjusting, it can
be recommended to reduce the variability of the potentiometer with
additional fixed resistors to reduce the sensitivity.
The design of the PNP driver stage with 1 kΩ to plus
and two 1 kΩ before the two diodes of the attenuators,
reaches saturation when the AGC exceeds 2.5 V: the diode
current is not rising any more because the two diode pair currents
exceed the driver current through the 1kΩ to plus. The diode
current (in the diagram in green, right side scale) is then constant,
limiting the possible attenuation.
Because I live in 28 km distance to the DCF77 antenna and so
have strong reception signals, the attenuation with the 1kΩ
resistors was large enough to reduce the field strength. If you
live even closer to Mainflingen, you might need more attenuation,
reduced resistors of 470Ω and 220Ω are shown in the
Or, you can decide to use a Mercedes-Benz-type driver with two
opamps, which drives the diode currents in a super-linear manner
(from 0.0 V AGC voltage on), with max. 12 mA diode current
and to achieve an attenuation of 1,514-fold with that. But this
is the version for the electronics lover only.
The rectification can be made with a diode rectifier or an ATtiny25
controller (see here for more or
here for a
2.2.1 Diode rectifier
This here is the diode rectifier for the amplitude-modulated RF signal. Two
Germanium or Schottky diodes rectify and double the DC made from the
RF, with two capacitors of 470 nF. The resistor of 33 kΩ
unloads the capacitors during the amplitude drop of the DCF77 signal,
with a half-life time of approximately 10 ms. The following RC
with R=10kΩ and C=470nF reduces humming of the 77.5kHz signal,
and a clean signal, to be fed into an ADC stage of an AVR.
The first stage decouples the diode rectifier from the second stage
of the amplifier, so that the resonance circuit of the second stage
is not overloaded by the low diode resistances. This stage has no
amplification, it just reduces the source impedance.
This is the produced DC voltage for different AC voltages. The
rectifier does not work below 0.4 Vpp input voltage due
to the diodes. But it provides enough DC voltage for normal
RF or IF signals.
This is the unload curve of the RC combination with C=470nF and
R=33kΩ. With t = 0.69*R*C = 0.01 s it is steep enough
to detect the 100 resp. 200 ms long amplitude drops when
DCF77 transmits a zero or a one.
In red the delayed drop on the R=10kΩ/C=470nF filter can be
seen. It is slightly delayed, but drops down with a similar speed
like the voltage on the input.
Calculation of those curves was performed with the OpenOffice
spreadsheet here. The sheet
SimRectifier simulates for a frequency of 77.5 Hz
and for the diverse parts of the rectifier and for a selectable
resolution. Fields that require an input are with a green
background color. It simulates
the voltages on the two rectifier capacitors (in columns C
and D and their sum in column E,
the drop in amplitude with a selectable level, starting
at a selectable time, and
the voltage on the RC filter output.
From that simulation the ripple of the voltage on the rectifier
capacitors was also calculated. It is below 0.25 mV and
remains below one digit of a 10 bit ADC. Only if the capacitor
values down to one tenth or the reduction of the resistor down
by a factor of 20 yields one ADC digit. This would increase
the amplitude drop speed, but would also decrease the voltage
Even though the rectifier RC does not produce humming I added
the 10k/470nF RC filter. In practice humming was larger than
simulated here, so this filter was necessary for a clean signal.
The rectifier hardware and the calculation tool can also be used
for smaller frequencies, e. g. for the 32.768 kHz IF of a
superhet. It doesn't work with an IF of 455 kHz or
10.7 MHz, though.
The amplitude drop when receiving zeroes and ones and during the
2-second long missing drop when the minute is over leads to a
reduction of the long-term average of the signal, when averaged
over a time period of longer than one minute (as done in an AVR
or in a long-term RC filter). The following parameters were used
in this simulation:
Signal DC of
2 V without amplitude drop,
0,4 V with amplitude drop,
averaging by an RC filter of
with a time constant of t = 8,5 s.
Such a RC combination would be chosen if the DCF77 signal would
be used to adjust the gain of the RF or IF amplifier, e. g. in
a TCA440 or for a diode attenuator. In those cases the reception
of a zero or a one shall not lead to a relevant gain adjustment.
One can see that the average voltage is at 1.85 V (approx.
93% of the 2 V on the input) and differs only by +/-10 mV
during a zero or a one. During a minute change, the level change
is slightly higher and around +/-25 mV.
From that one can see that long-term averaging (via software or
with an RC filter) is an appropriate method.
The rectification with an ATtiny25 controller is in detail described
assembler software for the ATtiny25 can also be found there.
This can directly be attached to the second stage of the amplifier
and does not need an emitter follower like the diode rectifier, due to
high resistance of the ADC3 input stage in the ATtiny25.
The signal of the second amplification stage is fed to the AD converter
of the ATtiny25. This measures the amplitude of the signal with a very
high sampling frequency, rectifies it (by subtracting 0x0200 from the
10-bit ADC result and inverting the result if negative = rectification),
detects the maximum from a large series of measurements (256), averages
those over 16 maxima, divides it by 2 and runs the 8-bit TC1 with that
in PWM mode.
TC1 produces a pulse-width modulated signal that is averaged by a
three-stage RC filter, that produces a stable DC voltage with a very
low ripple of between 0 and 5 mV, depending from the amplitude on
the ADC3 input. This DC is transferred to the decoder controller's
"AM in" that derives a) the DCF77 bits and b) the AGC and
AFC signals from that.
Do not try to integrate the functions that the tn25 rectifier performs
into the decoder's controller: the very fast ADC sampling rate eats up
the complete clocking of the controller with 8 MHz, so that there
is no time left for the complex functions of the decoder.
The Duo-LED that can be attached to the rectifier controller (red/yellow
or red/green 2-pin-Duo-LED, red anode to OC0B, resistor of 270Ω)
can serve as a signal strength indicator (with decreasing red and
increasing green brightness, if signal strength is too large to regulate
the amplifier it turns fully green). If you do not need that you can
switch this off by changing the software's configuration.
2.3 Automatic regulation
For the automatic regulation of the frequency (AFC) and of
the gain (AGC) as well as for the complete decoding of the
DCF77 signal, including a serial interface for transmitting
the data to another controller that displays date and time
received, a controller of the type ATtiny45 has been developed.
This reads the rectified voltage, analyzes the voltages,
derives controls and adjusts AFC and AGC via two PWM channels
and, by detection and checking of voltage drops, decodes the
zeroes and ones received from DCF77.
The description of that TN45 controller can be found
In order to determine the filter properties of the two LC
resonance circuits in the collector of the amplifier stages
a generator was designed and built that allows to measure
those filters around 77.5 kHz. It produces sine waves
with adjustable frequencies between 70 and 80 kHz.
The amplitude of the oscillator is 4 Vpp at an
operating voltage of 5 V.
Those are the filter curves with different coupling
capacitors. The maximum of resonance with a 330pF
capacitor is not at 79 kHz (as calculated) but
by 5 kHz lower at 74 kHz. This is, on one
hand, due to the coupling capacitor (when fully in
parallel 70.2 kHz), but is also due to straying
effective values of L (5%) and C (10%).
The curve is rather broad and not very steep. It covers
+/-2.5 kHz for the amplitude drop down to half
When decreasing the coupling capacitor to 68 pF
(with a ZC of approximately 30 kΩ)
the resonance frequency increases. Reactance of the LC
is larger than 11 kΩ at resonance.
Due to the high bandwidth of the LC circuit it does not
make much sense to adjust the two LC circuits for 77.5 kHz.
Compared to a simple resistor in the collector, an un-adjusted
LC circuit is an immense advantage. Especially RF far away
of the 77.5 kHz (short wave, 90 kHz power supplies,
etc.) are not amplified.
Those who need it more narrow, because their power supply,
energy saving lamp or old valve TV transmits at 80 kHz,
can use a superhet with a more narrow filter, as also described
on this web page.
This is the transistor amplifier on the breadboard. Make sure that
the cross antenna is at least in a distance of 15 to 20 cm of
the inductivities to avoid feedback and self-oscillation.