Figure 3 Fig. 3: Phase noise plot showing determining factors [6] From this plot it is fairly easy to come up with guide lines for minimizing phase noise in oscillatordesigns. Use devices with low flicker noise. Since the 9dB/octave section is dominated by this quantity, reduction of circuit flicker noise is of great concern. BJTs have a much lower flicker noise than Fets mak- ing them more suitable for low phase noise applications. The 6dB/octave section implies that Q of the circuit being higher is of great interest as is noise factor and power. As will be described below, OCXOs utilize crystals with higher Qs than TCXOs. Higher drive power is also desirable because that is the driving factor for the phase noise floor (frequencies above ƒ
o /(2Q
l )). This comes with a trade-offbecause higher drive levels usually result in some phase noise degradation close in to the carrier. â–  Phase Noise Measurement Measuring phase noise is not an easy feat. Most manufacturers of spectrum analyzers do not have the resolution to meas-ure the phase noise of a crystal oscillator manufacturers directly. Figure 4 shows the normal configuration for phase noise testing. An ‘ideal’ source that is the same frequency as the oscillator under test is mixed against the oscillator. This produces the products of these two signals and also the difference. Using a low pass filter, the products are stripped away leaving only the difference, which will be the oscillator noise, if the ‘ideal’ source is exactly the same frequency as the oscillator.
Figure 4

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Figure 4. Phase Noise Setup [2]