NE612 heterodyne detector

Posted on Feb 7, 2014

This is a schematic for a straightforward direct-conversion heterodyne detector, having a tuning range of about 10 kHz to 120 kHz. It has been optimised for the use of an electret microphone. The current consumption is very low at only 8 mA, which allows it to operate for about 100 hours with a set of rechargeable batteries. When the frequency is

NE612 heterodyne detector
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set to about 35 kHz, it is even possible to hear the 32, 768 Hz vibrating crystal inside an electronic wrist watch held close to the microphone! The signal from the MCE-2500 electret microphone is amplified by the pre-amp, then it is mixed with a square wave from the variable oscillator and finally low-pass filtered and amplified for headphones. The microphone is a special small type of electret. It is able to pick up sounds up to at least 60 kHz and possibly even further although the signal-to-noise ratio gets worse with higher frequency. My microphone draws about 0. 45 mA, relatively independent of supply voltage. By feeding it with a 4k7 resistor, its DC voltage sits around 2. 1 volts lower than the battery voltage. Because of the current-source behaviour of the microphone, the 4k7 resistor will dominate the source impedance. The amplifying element is the leftmost transistor, which is a low-noise type BC550C. The `C` indicates a high current gain, which is desirable because it means that the base current will be low. In the datasheet a low noise setpoint has been given of 200 uA collector current for a source impedance of 2k. Since in this case we have a higher source impedance of 4k7 ohms, a lower collector current of 100 uA has been chosen. The capacitor from emitter to ground has been chosen to give a rising gain vs. frequency characteristic. Maximum gain (about 50) is reached at about 100 kHz. The rightmost...

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