Current compensation method for adjustment free stereo multiplex voltage control oscillator in FM receivers

Abstract

An alternative solution of an adjustment free stereo multiplex (MPX) voltage control oscillator (VCO) circuit is proposed. The design approach taken was the utilization of a compensation circuit to change the VCO chargingldischarging current, and hence, the natural frequency (f,,). Manual adjustment of the external resonator was passed over which translated to a reduction in labor and timing cost. I. m O D U C T I O NThe history of stereo broadcasting began in 1961. Two channels of audio information were modulated with a single carrier for compatibility with the existing monophonic FM receiver. Frequency-division multiplex (FDM) was employed to utilize the existing 200kHz station bandwidth.The L+R stereo channel occupies the range of frequency from OkHz to 1%". The L-R signal is amplitude modulated with a 38kHz sub-carrier to produce a double sideband suppressed camer signal that occupies a frequency band of 23kHz to 53kHz. A 19kHz pilot is transmitted to be used in the stereo demodulator process.To recover the composite signal, a decoding circuit was introduced. Baseband FM signal was injected to the PLL to obtain synchronization with the VCO's output. It was known that a non-coherent demodulation would cause distortion at the output. To ensure low noise and high fidelity decoding, small PLL lock-range or high accuracy of VCO's natural frequency, f,, is essential. Current technology uses three methods to realize a precise VCO's fn, (a) External RC resonator, (b) External crystal and internal reactance circuit and (c) Zener Zapping method [ 11.The first method is the most popular one. Manual adjustment of capacitance value is needed to compensate for process variation in this method. The second method requires an external crystal resonator. Although it avoids expenditure in the tuning process, the external oscillation circuit exists as a potential noise source to the system. The third method omits all external components but it involves a fabrication tuning process that induced a higher cost. FM Stereo Signal at Baseband 4 I Lock Detection Circuit Current circuit Compensation Inquadrature Half Counter (1 9 H z ) I I Fig. 2. Modified PLL BlocksThe tactic engaged in our paper to ensure a high accuracy f, is a compensatory current to VCO in addition to its normal biasing current. Modified PLL blocks are shown in Fig. 2. The supplementary current is introduced in discrete linear steps. It was realized by inserting a lock detection circuit and a current compensation circuit to the original PLL. II. VOLTAGE CONTROL OSCILLATORThe proposed high precision VCO is shown in Fig. 3 It is a modification from the conventional emitter-coupled multivibrator [2]. Two alternatively conducting current paths bring about the oscillating action. The selection of path is determined by Schmitt-trigger circuit, which monitors the voltage across the timing capacitor, C1. When the capacitor charges through 'path l', its voltage rises and eventually reaches the upper triggering level. The Schmitt-trigger will then activate the current path i...