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Simple sine synthesizer generates 19-kHz pilot tone for FM baseband signal

-April 14, 2005

A multiplex signal comprises baseband information transmitted on a stereo analog FM-broadcast system, plus one or more SCA (Subsidiary Communications Authorization) channels (Figure 1). This Design Idea presents a low-cost method of generating the basic 19-kHz pilot tone. The 19-kHz pilot tone comprises a baseband signal, and the L+R and L–R signals consist of DSBSC (double-sideband-suppressed-carrier) modulation centered at 38 kHz. For a receiver to correctly demodulate the signal, the transmitted pilot tone and L–R signal must synchronize at their respective zero crossings. In addition, any distortion in the pilot tone produces harmonics that can interfere with adjacent sections of the signal.

The low-distortion, 19-kHz pilot-tone generator comprises a resistive voltage divider, R1 through R11, connected between the VCC and –VCC supply rails (Figure 2). The resistors' values are weighted to provide N=8 approximate sampled values of a sine wave and are relatively low to present "stiff," low-impedance sources to eight-channel analog multiplexer IC1. An up/down counter, IC2, drives IC1 and takes advantage of a sine wave's inherent symmetry to enhance the resolution and reduce the distortion of the 19-kHz pilot sine wave.

In effect, analog multiplexer IC1 acts as a zero-order hold circuit, producing an N times Nyquist oversampled sine wave of frequency fSIN, plus several attenuated alias frequencies centered at: fALIAS=m×(2×N×fSINE), where m=1, 2, 3. For most applications, a simple passive RC filter at the multiplexer's output adequately removes the alias frequencies. Binary counter IC3 generates a 608-kHz clock signal plus a 19-kHz up/down control signal for counter IC2, and sections of hex inverter IC1 serve as a crystal oscillator and buffer.

You can expand the basic circuit by duplicating the resistor network, multiplexer, and up/down counter. An external audio source drives the resistor network's upper and lower ends with L and R audio signals that have undergone lowpass-filtering to eliminate components with frequencies greater than 15 kHz. A 1.216-MHz signal clocks the second up/down counter and a 38-kHz up/down control signal derived from higher frequency taps on counter IC2. The added circuitry generates the baseband L+R channel, and the L–R modulation in synchronism with the 19-kHz pilot tone because all clock pulses originate from a common counter. To produce the composite multiplex signal, the outputs of both analog multiplexers sum in an external network.

Using the specified components, the circuit generates a 19-kHz pilot tone with harmonics 60 dB below the fundamental and synchronous with the maximums of the suppressed 38-kHz carrier. The same circuit structure produces L+R- and L–R-channel generation without changing components' values. Potentiometer P1 allows a 90±10° fine phase adjustment to correct distortion and to resynchronize at zero crossing.

 

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