5.3.1 NE612 Synchrodyne AM Receiver
If the author remembers well an article that he read in a professional magazine many years ago, the synchrodyne receiver is the ancestor of the superheterodyne receiver. Sometime at the beginning of the 20th century this device was called the Heterodyne receiver, and it was first constructed by Levvy. Armstrong improved it and gave new name to the new device, by adding the prefix SUPER to the old name.
The electronic diagram of this device is given on Pic.5.7. This receiver, as well as that on pic.4.2 has got the local oscillator with oscillatory circuit connected between pins 6 and 7. However, frequency of this oscillator is not greater for the value of fm, but is in fact equal to the frequency of the station we wish to listen to: fm=fS. Because of that, the important design difference compared to the diagram from pic.4.2 is that on pic.5.7 capacitors Co and Cto are not used, but the capacitor C which is obtained by connecting the legs O and A, acc. to pic.3.7. Its capacitance can be changed from 12 pF till 218 pF, so that the oscillator frequency, in case of MW reception, goes between 500 kHz to 1500 kHz. The oscillator voltage is emanated in the mixer by the signals from all stations coming from the antenna. The result of emanation with signal of the station whose frequency is equal to the oscillator frequency is the LF signal (speech, music, Morse Code etc.) that serves for performing the modulation in the transmitter. This signal is obtained on pin No.4, from which it is then led, over the 1 ìF capacitor, to the volume potentiometer and audio amplifier. Products of mixing the oscillator voltage with other stations’ signals are also obtained on that pin.
They are being suppressed by the LF filter that comprises the R* resistor and C* capacitor. The device we were testing did not, however, contain R*. It is to be installed if some disturbances occur (whistling or similar), and its optimum value is to be found experimentally. If necessary, greater capacitance of C* is also to be tried out.
* As mentioned earlier, it is very important for the supply voltage of the NE612 to be stable. This values even more for the synchrodyne then the superheterodyne receiver. The voltage control is done by the stabilizer, made with 78L06 IC. It is being placed in the low-power transistor package, either metal (as for BC107) or plastic (as for BC547), and its maximum current is about 100 mA (pic.5.7-b). A simpler stabilizer, made with the Zener diode, can be used instead, as on pic.5.9.
* Instead of factory-made coil LO, the self-made one can also be used. The simplest solution is to use the one from pic.3.6, in which case the mid leg is not used. Over this coil, the feedback coil should be winded, acc. to pic.5.7-c (its ends are marked with 4 and 1). When connecting with capacitor C and pins 1 and 7 of NE612, care should be taken to join properly: coil ends 1 and 3 with ground, 2 with capacitors C and 560 pF, and 4 with 1 nF capacitor. It is, of course, possible to use smaller coil, wound on a smaller body, with more quirks of thinner wire. Its inductance should be about 350 mH, and the number of quirks required is to be found by testing. The feedback coil (4-1) has app. 3x fewer quirks than the oscillatory circuit coil (2-3).
* On the pin 5 of the NE612 the LF signal is also obtained. It is the same as the one on pin 4, but has a 180° phase shift compared to it (in simple words, while one signal increases, the other one decreases, and vice versa). That gives us the opportunity to use the dual audio amplifier in the LF part, that has two amplifiers, with inverting and non-inverting inputs. As shown on pic.5.8, the counter-phase LF signals from NE612 are led onto the same inputs. The output signal has 2x greater amplitude, therefore making the output power 4x greater than when only one input is used (as on pic.5.79).
