Radio Frequency Integrated Circuit Design - Webs

246 Radio Frequency Integrated Circuit Design
Figure 8.1 Example of periodic waveforms.
in one discrete oscillator frequency and possibly at multiples of the oscillator
frequency. A signal with power at only one discrete frequency would correspond
to an impulse function if plotted in the frequency domain. However, all real
oscillators have less than perfect spectral purity and thus they develop ‘‘skirts’’
as shown in Figure 8.2. These skirts are undesirable, and we would like to
minimize them as much as possible. Power in the skirts is evidence of phase
noise, which has resulted in oscillator power bands around the intended discrete
spectral lines. Phase noise is any noise that changes the frequency or phase of
the oscillator waveform. Phase noise is given by
PN = Po
No
(8.1)
where Po is the power in the tone at the frequency of oscillation and N o is the
noise power spectral density at some specified offset from the carrier. Phase
noise is usually specified in dBc/Hz, meaning noise in a 1-Hz bandwidth
measured in decibels with respect to the carrier.
Since oscillators are designed to run at particular frequencies of interest,
long-term stability is of concern, especially in products that are expected to
function for many years. Thus, we would like to have minimum drift of
oscillation frequency due to such things as aging or power supply variations.
In addition, oscillators must produce sufficient output voltage amplitude for
the intended application. For instance, if the oscillator is used to drive the LO
switching transistors in a double-balanced mixer cell, then the voltage swing
must be large enough to switch the mixer.
Figure 8.2 Spectrum of a typical oscillator.