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We proposed a method to realize the fine frequency-tuning step using tiny capacitors instead of MIM capacitors for a digitally controlled oscillator (DCO). The tiny capacitors are realized by the coplanar transmission lines in a 6 metal layers (M6) foundry of 0.18 um CMOS technology. These transmission line based capacitors are designed by using electro-magnetic field simulator, and co-designed by using SPICE simulator. Finally, we fabricated the proposed DCO in 0.18 um CMOS technology, and tested. The measured phase noise of DCO was -118.3 dBc/Hz (@1MHz offset frequency), and the oscillating frequency tuned from 4.86 GHz to 5.36 GHz in the minimum frequency-tuning step of 18 kHz.