UWB for Around-the-Body Data Streaming - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 57

Fig. 12 a 3-stage DCO schematic. b Schematic of each stage

fF/unit. For such small step values, monotonous behavior needs to be ensured by ther-
mometer encoding. Different from the medium tuning back, digitized MOS varactors
are chosen over switched-MIM capacitors, to avoid the effect from the on-resistance
of the switches. The 1 MHz tuning step is achieved by connecting the minimum-
sized varactor in series with two customized capacitors. The customized capacitor is
realized by the fringing capacitor between top metal with a size of 1.4um

×

14um
to achieve < 0.5fF/unit capacitance. Each tuning range is required to overlap with
the adjacent ones to ensure continuity in the frequency sweep, avoiding gaps created
by process variations and inaccuracy of parasitic capacitances estimation.
A phase-aligned technique is implemented by tying one side of the differential
output to VDD while the other side to ground using switches as in Fig. 14 . Such
technique has two advantages. First, the DCO output is ensured to start from a fixed
phase each time when the DCO is enabled by the “EN” signal. Second, it avoids