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DCF77 receiver amplifier with transistors
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2 Transistorized DCF77 receiver amplifier

An RF amplifier for DCF77, transmitting on a frequency of 77.5 kHz, has to

2.1 Amplifier and driver for DCF77 RF

Amplifier with transistors The amplifier has three stages, each equipped with a usual NPN small signal transistor (you can use any available type):
  1. 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 capacitive divider.
  2. 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.
  3. The third stage is an emitter follower without any amplification and drives the AM rectifier stage, it just decouples the high resonance resistance from the low impedance of the diodes.
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 roughly 1,000.

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 reduces their resistance, which is R = UDiode / IDiode. At the highest current here, IDiode is (5 - 2 * 0.6) / 1 k = 3,8 mA, so the diode resistance is RD = 0.6 / 3.8 = 158 Ω. As both diodes are parallel to ground (the upper one direct, the lower one via the capacitor) the resistance of the two diodes is 79 Ω.

With a capacitor of 1 nF, its capacitive reactance 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 the 0.0385 fold, or with a factor of 26. Both attenuators reduce the gain of the amplifier by the 676 fold.

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. Transistor amplifier on the breadboard 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.

AGC to diode attenuator PNP driver The signal of DCF77 is highly amplified, the gain can be reduced by applying current through the diode attenuators. I use a trim resistor which drives the base of a PNP transistor with 1 kΩ on its emitter (with the collector on ground) to get enough diode current. This stage is also required when driving the AGC with a pulse-width modulated signal from an AVR. This allows a fine tuning of the gain.

2.2 Rectification

Rectifier for the AM signal This here is the 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.

Rectifier voltage for different input voltages 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.

Unload curve with C=470nF and R=33k 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 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 level.

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.

Long term averaging of the amplitude drop 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: 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.

2.3 Automatic regulation

AGC interface for the TN45 controller 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 here.

2.4 The pass-band curve of LC filters

A test generator for 70 to 80 kHz 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.

Pass band of the 3,3mH LC filter 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 (3 dB).

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.

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