An automatic swing control (ASC) feedback is presented for relaxation voltage-controlled oscillators (VCO's). The ASC feedback maintains the oscillation swing at a constant level by dynamically adjusting the threshold voltage of the oscillator. The feedback mechanism provides a very wide linear frequency range and pushes saturation in the frequency characteristic of the oscillator to higher frequencies. As a result, operation close to the maximum achievable frequency, set by the inherent delay of the oscillation loop, becomes practically possible. Also, the sensitivity of the VCO frequency characteristic to process and temperature variations is reduced. These features make the ASC oscillator particularly suitable for autorate operation in serial-digital video applications. A detailed description of the design and experimental results of a trimmable VCO are given in this paper. It is shown that the frequency range is increased from 20-500 to 20-800 MHz when the ASC feedback is applied. In addition, statistical data from more than 1000 samples from several batches before and after trimming are provided. The results confirm the manufacturability of the ASC feedback approach. The implementations were done in a semicustom 14-GHz bipolar process.
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