Abstract

Given the importance of oscillators in communication systems, in this work we

studied the most important parameter of this block which is the phase noise and

introduced two useful techniques for improving this parameter. These techniques

are body biasing and tail current shaping. Finally, to achieve a minimum phase

noise, these techniques were combined together and applied to a CMOS cross-

coupled oscillator. To prove the usefulness of these techniques, a circuit was

designed and simulated in CMOS 0.18m technology, for each one.

Body biasing is a commonly used technique in digital circuits for correcting the

threshold voltage. In this work we utilized this technique for improving the

performance of an oscillator by changing its operation mode to class-C which has a

lower phase noise.

We can also reduce the phase noise by lowering the effective value of Impulse

Sensitivity Function. We do this by changing the noise modulation function of the

devices used in the oscillator, especially the tail current source, through tail current

shaping. Thus, a new sinusoidal tail current shaping method is introduced in this

work which has several advantages over previous ones, such as lower amounts of

injected noise and power.

After performing the simulations on the final design of the presented oscillator, the

measured power consumption was 4.6mW and the phase noise at 1MHz offset

frequency was -129.51dBc. Therefore, the achieved Figure of Merit was

190.73dBc. Compared to a conventional oscillator with the same quality factor of

LC tank and output swing, the presented design has 10.44dBc less phase noise.

Key words: oscillator, phase noise, MOSFET, body biasing, bias current shaping