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Alignment-free FM Detector Schematic Circuit Diagram

Simple and Alignment-Free 455-kHz Quadrature Detector

This 455-kHz quadrature detector designed for narrow-band FM signals offers two significant benefits: it is delightfully straightforward and does not necessitate any alignment procedures. At its core lies the renowned NE612 IC, an 8-pin DIL package containing a double-balanced mixer and oscillator.

Alignment-free FM Detector Schematic Circuit Diagram 1

Buffering and Signal Processing

The incoming signal undergoes initial buffering through T1 and is then directed to the input of the NE612. Simultaneously, a small portion of the signal is channeled to the mixer using a low-value capacitor (C4). The circuit functions in such a way that when the input frequency aligns with the resonant frequency of the parallel LC network, the signal on pin 7 leads by 90 degrees concerning the signal on pin 2. This phase angle expands as the input frequency rises and contracts as the frequency decreases. As the signals on pins 2 and 7 are multiplied, the output level peaks when the signals are in phase and reaches zero when they are in anti-phase.

This configuration represents the detector’s operational state. Consequently, a fluctuating input frequency results in an output signal with a variable amplitude. The detector’s operational range inversely correlates with the Q factor of the parallel resonant network.

Alignment-free FM Detector Schematic Circuit Diagram 2

Alignment-free FM Detector Schematic Circuit Diagram 3

Optimal Performance and Robustness

This circuit functions optimally with an input signal level ranging from 0.5 to 2 Vpp. Its linear operation across a broad spectrum (420–500 kHz) eliminates the need for alignment. Moreover, typical variations in the inductance and capacitance values within the resonant circuit have minimal impact. Within the operational range, the output level fluctuates by approximately 1 V, resulting in a detection sensitivity of about 13 mV/kHz. This sensitivity proves sufficient for most narrow-band FM applications utilizing an intermediate frequency of 455 kHz.

Flexible Power Supply and Easy Assembly

The circuit accommodates a supply voltage within the range of 4.5 to 8 V, ensuring adaptability to various power sources. Its current consumption is limited to around 2.5 mA, enhancing energy efficiency. Utilizing the provided compact printed circuit board, assembling this FM detector is remarkably straightforward and can be accomplished in less than half an hour, even for those with limited experience.

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