1000Hz Signal Injector

This circuit is useful for injecting a signal source in audio systems. It was specifically designed for use in broadcast radio applications. The circuit provides an accurate, low-distortion (less than 0.1% THD) 1000Hz sine wave with an amplitude adjustable from 0 to 4V peak-peak (+5.22dBu). Its excellent frequency accuracy and low phase noise is derived from an inexpensive, readily-available 4.096MHz quartz crystal.

Integrated circuit U1 contains an oscillator section which is controlled by crystal X1 to establish a 4.096MHz square wave. U1 also contains a ripple counter that divides the crystal frequency by 4096, providing a square wave of 1000Hz at output Q11. The 1000Hz square wave is fed to an 8-pole low-pass Butterworth filter comprised of four Sallen-Key sections, U2A through U2D. Potentiometer RV1 is inserted between the third and fourth filter stage to provide amplitude control. Transformer T1 provides isolation and ensures 0 DC offset voltage at the output.

A 5V supply is provided by low-dropout voltage regulator U3. The circuit uses 9mA, and may be powered from a 6V to 9V battery.

Capacitors C1 and C6 are ±10% X7R ceramic types; C2, C3 and C12 are ±5% NP0 ceramic types; C4, C5, C7-C11 and C13 are ±5% metallized polypropylene types; C14 and C15 are 16V, ±20% tantalum or aluminum electrolytic types. Resistors R1 and R2 are ±5% carbon film types; R3-R10 are ±1% metal film types.

For best performance, isolate the U1 circuit from the filter so that square-wave counter signals are not coupled into the audio output.

November 27, 2013

Text and image ©2013 by Arthur Harrison

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