The modernization of a crystal radio
Radio, 2001, №1
A crystal radio... For many decades, it is one of the first designs built by novice amateurs. The crystal radio is an interesting introduction to the world of radio receivers. It allows the young enthusiasts of Radio Engineering to carry out a variety of exciting experiments with the radio receiving the local radio stations. However, what can be improved in this long-known device? But, as the author of this articles says, the potential for growth has not yet been exhausted.
In the simplest receivers (Fig. 1 a) the resonant tank is overloaded by the detector impedance. Although the volume and the sensitivity are quite acceptable, the selectivity is insufficient. Because of the low quality factor Q of the tank circuit, it is often to listen simultaneously to two or three radio stations.
Assume that the receiver is tuned to the middle of MW frequency range (1 MHz). The inductance of the coil L1 is 200 uH, the capacitance of the capacitor C1 is 120 pF (typical values). Its reactive resistance is about 1.2 Kilohms and the impedance of the resonant circuit in Q times more. With the quality factor of the coil (with no load) Q = 200 we get 240 Kilohms. For the frequency range of LW the resonant impedance of the circuit is close to 1 Megohm!
At the same time, the input impedance of the detector is considered to be equal to half the load resistance, the load is a high-impedance headphones with an impedance at audio frequencies is only 10...15 Kilohms (the full impedance of the headphones is more than the value shown on their case because of the inductance of the headphones capsules).
It is easy to see that the tank circuit L1C1 is shunted too much, and its real Q is less than 10 (the ratio of the load resistance to the reactance of the tank circuit). Making the coupling with the detector circuit weaker, you can improve the quality factor Q, and hence the selectivity increases. The volume will almost not change because a voltage of a signal across a resonant tank circuit with a higher quality factor Q is higher, this will compensate the decrease of the signal across the detector. The coupling is usually adjusted by connecting the detector to a selected tap of the coil (Fig. 1b).
If we adjust the coupling, it is useful to optimize the resonant tank. In [1-3] it was shown that the maximum efficiency of the antenna circuit is achieved when the antenna circuit is directly connected to the upper end of the resonant coil L1 without a coupling capacitor. Tuning are provided by changing the inductance of the coil, as the capacity of the resonant tank is used the capacity of the antenna. If the antenna is large and its capacity is significant, then it is necessary to include a tuning capacitor in series with the antenna (Fig. 1b).
This receiver works better than the previous one and has a higher selectivity, but it isn't convenient to regulate the coupling between the detector circuit and the resonant tank, because it would require a multi-tap coil. Therefore, the process of adjustment is not smooth.
There is a method of impedance matching using a capacitive coupling, where the capacitive resistance of the capacitor is equal to the geometric mean of both impedances. In our example (the impedances of 240 Kilohms and 6 Kilohms is matching), it will be about 40 Kilohms ( R=(R1*R2)0.5 ), and the corresponding capacity is only 4 pF! (C=1/(2*Π*F*Rc)). It turns out that the coupling can be adjusted by an ordinary trimmer of KPK or KPM type.
VD1, VD2 - D18 (an old USSR Germanium diode); C1 - 5..180 pF; C2 - 8..30 pF; C3 - 680 pF
But the coupling capacitor breaks the DC current path of the detector circuit. To avoid this problem it is possible to add the second diode to the circuit (Fig. 2). It seems we get a detector with a voltage doubler. In fact, because of the small capacitance of the capacitor C2 there is no voltage doubling effect. During the negative half-cycle of the signal across the tank circuit L1C1, the capacitor C2 is charged through the diode VD1, and during the positive half-cycle the capacitor C2 discharges through the diode VD2 and the load. The headphones BF1, shunted by the bypass capacitor C3 to smooth out ripple, is the load of the detector.
The smaller the capacity of the capacitor C2, the less the charge and the energy, respectively, taken from the tank circuit. The coupling network is adding to the tank circuit a small reactive (capacitive) resistance, which is automatically compensated while tuning of the tank circuit in resonance with the oscillations of the input signals.
In this experimental design the coil L1 is wound on a 12 mm in diameter plastic pipe with one layer of 0.2 mm (AWG 32) copper enameled wire, the coil has 240 turns. The ferrite rod of 10 mm in diameter made of ferrite 400NN (μbeg=400, μmax=800) is used for adjustment. The tuning range is from 200 kHz (when the capacitance of C1 is maximum and the ferrite rod is fully retracted) to 1400 kHz (with the removal of the rod and decreasing the capacitance of the C1).
At the apartment with a small antenna (about 7 m) and a ground (a central heating system) the receiver showed excellent results, received all Moscow LW and MW radio stations. By adjusting the coupling with the trimmer C2, it was able to get sufficient selectivity at the normal volume level.
There is another advantage of the receiver - because the detector is powered by a current going through a high impedance of the coupling capacitor C2, the "step" on the current-voltage characteristics of diodes is smoothed out. By the way, the usefulness of the detector powered by the current has been reported in . In our receiver a silicon diodes (with a threshold of 0.5 V) works almost as well as germanium diodes (with a threshold of 0.15 V). Moreover, it was possible to connect to the receiver a low-resistance (50-70 ohms) headphones, it is absolutely unacceptable in the traditional version. But in this case the bigger capacitance of the coupling capacitor is required - up to 40...50 pF. The sound volume will be less because of the significant losses in the direct resistance of the diodes.
The high sensitivity of the detector described above to weak signals came to the idea to try a simple resonant tank-free version of the receiver (Fig. 3). It was easy to build - all components have been soldered to the terminals of the headphones, and a 1.5 meter of insulated hookup wire with the clamp "Crocodile" at the end worked as antenna. With the "Crocodile" the antenna can be attached to the trees or other high objects. The headphones cord has some stray capacity Cstray to the operator and further to the ground was used as the counterweight (instead of ground). Even with such a primitive version it was able to listen to some of the most powerful radio stations.
This receiver almost does not perceive low frequency interferences, for example, from the mains power line because the small capacitance of the coupling capacitor C1 prevents it. The audio frequency current is completely shorted in the isolated network of the headphones BF1 and the diodes VD1, VD2.
I cannot say that the circuit diagram of this receiver is something new. Half-bridge rectifier that's used in it, was known long ago - it was used in the indicator of electrical field . By the way, nothing prevents to use a full-bridge circuit based on four diodes, connect it to the tank circuit or antenna by a capacitor of small capacity.
A similar circuit has been described in , but, unfortunately, the author incorrectly interpreted the principle of operation of the receiver. The correct receiver circuit is shown in Figure 4. It differs from the author's circuit only in the presence of a stray capacitance Cstray between the headphones and the the earth, the stray capacitance acts as a coupling capacitor and matches the tank circuit with the detector circuit. By a happy coincidence, the capacitance Cstray was close to optimal. But the author didn't took it into account! As experimental results, it is proved to be excellent, as it follows from the publication .
At the end, let's go back to the circuit, shown in Figure 2 and bring it to the attention of the radio amateurs. This crystal radio set has shown excellent results. Experiments with it not less interesting and attractive than with the more complex electronic devices.
1. V. Polyakov. About powering a radio with a "free energy". - Radio, 1997, № 1, с. 22, 23.
2. V. Polyakov. "Eternally speaking" radio. - Radio, 1997, № 5, с. 23. 24.
3. V. Polyakov. Antennas for receiving. - Radio, 1998, № 2, с. 44-46.
4. Psurtsev V. "Inventing" the amplitude diode detector. - Radio, 1986, № 1, с. 33-36.
5. Shepelev G. A simple indicator of the electrical field. - Radioamateur, 1993, № 6, с. 24.
6. Besedin V. One more... . - Radioamateur, 1994, № 6, с. 34.
V. Polyakov, Moscow