Instead of the capacitor CR, that was used for fine tuning in the previous project, a capacitive (varicap) diode can be used. It’s a special HF diode which is polarized by exposing it to DC voltage in order to be non-permeable (+ to the anode, - to cathode). By changing the voltage diode’s capacitance also changes, which allows for it to be utilized as variable capacitor. If, acc. to pic.5.13-a, the DC voltage between the cathode and anode (UAK) varies from U1 to U2, diode’s capacitance goes from Cmax till Cmin.
The electronic diagram for the electronic fine tuning circuitry is given on pic.5.13-b. Diode capacitance is changed by moving the slider of the P1 potentiometer. By means of trimmer TP the necessary Cmax is set, and when this is done TP can be replaced by an ordinary resistor. All the components are mounted on the PCB, together with other parts of the receiver, except the P1. It is mounted on the front panel, and connected to the PCB with 3 ordinary wires.
* The variable capacitors that were used for tuning in all the receivers described so far are solid, lasting, reliable
* In case of SW band, the P2 potentiometer is added for fine tuning.
The optical indication of the tuning, with and knob with arrow is the simplest solution possible. More prettier one is using a small movable-coil instrument (V), such as those used as battery indicators in industrial devices, or for tuning indication and similar. The connecting is done acc. to the diagram on the left part of the pic.5.13-c. In series with the instrument, the TP potentiometer is attached. Its resistance depends on the maximum instrument current, and can be found experimentally. For start, you may use a 1 MOhm linear trimmer, with its slider at lowest position (so that its resistance is maximum). Put the P1 slider also at the lowest position. Turn on the receiver. Start moving the P1slider upwards, and observe the instrument needle. if it soon goes to the end, you’ll have to take a trimmer with greater resistance or to add another resistor in series with it, so that when the P1 slider gets to its rightmost position, the needle goes somewhere around the middle of the full scale. If the needle, with P1 in topmost position, moves too little, you’ll need a smaller resistance trimmer. When you succeed in having the needle in the middle of the scale with P1 in topmost position, start moving the TP slider until the needle reaches the end of scale. The circuit is well adjusted if the needle goes from zero to full scale while P1 slider is moved from bottommost to topmost position. The instrument can have any shape, but the most appropriate (and cheapest) is square, like the one on the picture.
From all the signals in the reception antenna, the one that is created by the local transmitter is by far the strongest one, due to the fact that it is hundreds, sometimes even thousands times closer than other radio transmitters. That signal can be so strong that it can jam normal reception of other stations. In case of simpler receivers its programme is heard,more or less, in all the positions of the variable capacitor. The solution for this problem is the so-called seal circuit, which serves to weaken the signal of the local transmitter, so that it doesn’t interfere (but is still strong enough for normal reception, when the receiver is tuned at it).
Using a variable capacitor in the seal circuit (pic.5.14-a) isn’t an economical solution. It is much better, considering both economy and space, the solution given on pic.5.14-b. A block capacitor C1 and a variable inductance coil are used in the seal circuit. As shown on the framed part of picture, the coil is wound on the plastic body, with ferrite core. The number of quirks is found experimentally about couple of hundreds of quirks made with as thin copper wire as possible). The capacitance for C1 is also found experimentally (couple of hundreds of pF). The earlier mentioned IF transformer can also be used as a coil. With labelling acc. to pic.4.3-a, legs No.2 and 3 are used, the others are “hanging” (they are not soldered). C1 capacitance is also found experimentally. It is also possible to wind the coil on a piece of ferrite rod, as shown on pic.5.14-b, and setup to be done with trimmer Ct
5.5.4. Dual Tuning
The author of this book, as great radio techniqe lover (amateur, in French), owns great collection of over 150 pieces of various old-timer radio receivers. There is one among them that is over 60 years old, at which the tuning is being done by two knobs. With first one the receiver is set roughly to the desired station, which is usually barely heard at that moment. The second knob is then turned until the optimum reception is achieved, which is significantly better than before, and in case of weak stations - extremely better.
The selectivity of simple receivers that were described in previous chapters can be significantly increased by using the aforementioned dual tuning. The electronic diagram is shown on pic.5.15-a. Another oscillatory circuit, made of L1 and C1 connected in series, is inserted between the antenna connector and input circuit of the receiver (it can be any of the earlier described AM receivers). As with the earlier mentioned parallel oscillatory circuit, the resonance frequency of the serial circuit is given by the Thompson pattern:
The serial oscillatory circuit has very small impedance (compared to the parallel circuit whose impedance is very big on the resonance frequency). The dependance of the impedance (”resistance”) of the serial oscillatory circuit from the frequency is shown on the diagram on pic.5.15. As you can see, the serial circuit acts as a resistor of very small impedance only for the station that it’s tuned at. For all other stations, it behaves as a huge resistor (impedance). All in all, from all the signals in the antenna, the biggest current, and therefore the biggest voltage on the input circuit is created by the transmitter that both serial and parallel oscillatory circuits are set to. The tuning is done as it has already been described, first with C (so-so), then with C1 (much better).
* Greater experimenting opportunities with dual tuning provides the diagram on pic.5.15-b. Once again, it’s the serial resonance (in circuit L1, C1), and parallel resonance (in circuit L, C), that are being used. The coils are placed side-by-side, in order to generate magnetic coupling between them. The tuning is done as previously explained, but now we also have a possibility of changing the amount of magnetic coupling between the coils by moving them closer or farther, which affects the antenna’s influence on the L, C oscillatory circuit, therefore changing its selectivity and sensitivity.
5.5.5. Separation of Stages - Preventing the Oscillation
On of the significant problems that occur at devices that comprise more cascade-linked amplifying stages is the occurrence of the feedback over the conductors that connect those stages with the positive pole of the battery, or the power supply. By the way, the feedback is a phenomenon when part of the signal exiting an amplifier gets on its input. Under certain conditions, this feedback causes the oscillation of the stage, which in devices that have the loudspeaker on output, manifests itself as strong whistling, squeaking and similar.
On of the ways to prevent this feedback is given on pic.5.16, where a block-diagram of a radio receiver that has four amplifying stages with active components (transistors or IC’s) that require the battery supply is shown. Separation of stages for the AC current (preventing the feedback) is accomplished by the LF filters with resistors and capacitors. Resistors are from couple of hundreds of Ohms to 1 kOhm. Capacitances of C1 and C2 are from couple of tenths till couple of hundreds of nF, and of C3 from couple of hundreds of nF to about 100 mF. The stage PCBs should be designed in such way to make the contact where right end of the capacitor is soldered as close to the contact where the positive end of the power supply voltage is brought (e.g. on pic.5.9, the right contact for C6 should be as close as possible to the contact where pin 8 of NE612 is soldered).
In the devices supplied from the battery, the C5 capacitor, which has capacitance of couple of hundreds of micro Farads, serves to take the role of the battery when it gets emptied a little bit, and strong tones have to be reproduced at the loudspeaker (in simple terms, C5 acts as a small accumulator that helps the worn-out battery to give enough power to the power amplifier, when necessary. When its help isn’t needed, the capacitor is refilled). This capacitor is not needed when the receiver is supplied from the adaptor that already has an electrolytic capacitor on its output, and when the wires that connect the adaptor to the receiver are not longer than about 15 cm.
