5.3.2 AM Receiver with Synchro Detector
In previous project, the NE612 was in fact used as the AM signal detector. The LF signal exiting the mixer is product of the simultaneous (synchronous) action of the station signal and voltage from the local oscillator upon it. That is how the term “Synchro Detector” emerged. There’s also a possibility to use a station carrier instead of local oscillator’s voltage, so that the station signal gets beaten by itself, however strange this may sound. Electronic diagram of one such device is given on pic.5.9.
The station signal, which the input circuit (C, L) is tuned at, is led to the regulating Gate of the BF960 MOSFET. Under the effect of this voltage, the AC current that creates voltage drops on resistors R2 and R3 runs through the transistor. These two voltages, taken between the S and ground and D and ground, are mutually shifted in phase for 180°, and are being led over the coupling capacitors C2 and C3 to pins 1 and 2 of the NE612, i.e. on one input of the mixer. On the other mixer input the Drain signal is brought, over C4, and beating occurs in the mixer, the result of which is the LF signal on pin 4. This signal is, over C8, being led onto the volume regulation potentiometer and the audio amplifier.
* The unwanted (and parasite) products of mixing, that are manifested as whistling, squeaking etc. are being suppressed by the C7 capacitor. If the obstructions still exist, the capacitance of C7 is to be increased and/or the R* resistor added.
* The voltage stabilization of the DC voltage on pin 8 is performed by the ZPD6.2V Zener diode and resistor R5. A diode with smaller voltage is also possible to be used, say, 6.2 V and similar. If the supply voltage is less than 12 V, the resistance of R5 should be decreased.
