UWB for Around-the-Body Data Streaming - Ultra-Wideband and 60 GHz Communications for Biomedical Applications - page 58

Fig. 13 Fine tuning bank in the 3-stage ring oscillator
Fig. 14 a Implementation of
the phase align technique.
b Start-up behavior at the
output of each stage of the
DCO
the phase noise accumulation between data bursts. That feature is critical since
in the proposed system, the LO carrier frequency is only calibrated, and left free
running during the TRX operation. Such choice enables the effective duty-cycling,
and thus minimizes the power consumption. On the other hand, the frequency drift
that happens in the oscillator are usually low-frequency phenomena. By turning off
and on the oscillator at the duty-cycled ratio, the LO signal is effectively high-pass
filtered.
The 3-stage DCO provides 3-pair of different output at phases of 0, 60, and
120 ◦ , while quadrature signals are needed for the receiver. Therefore, an IQ buffer
implemented to generate the IQ signal from the DCO output. The I-branch is a two-
stage resistive-loaded buffer to amplify the zero-phase DCO output; in the Q-branch,
the 60 and 120 ◦ phase DCO outputs are combined to generate the quadrature output
Fig. 15 . A symmetric loading between the I and Q buffer is necessary to ensure the
quadrature output.
The DCO is evaluated with its full tuning range: in total 32,678 tuning steps
covered by coarse, medium, and fine steps. A tuning range of 4.7-9.1 GHz with
an average tuning step of 1 MHz is measured. Figure 16 [ 13 ] illustrates the overall
tuning curve, showing that the DCO can be tuned from roughly 5-10 GHz with