Fig: Phase Noise measurement using a Reference Oscillator and a Phase Detector:

In Phase Noise measurement using a Reference Oscillator and a Phase Detector method:-

• The reference oscillator is synchronized to the measured oscillator by means of a PLL of a very small bandwidth.
• The PLL sets the phases of the two oscillators to a difference of 90°.
• The Phase Noise of the DUT is eliminated within the loop bandwidth.
• The sum noise power of the reference and the test oscillator obtained outside the loop bandwidth is present at the output of the phase detector.
• This output signal is amplified by means of an LNA (Low Noise Amplifier) and displayed on a Spectrum Analyzer starting at a frequency of 0Hz.

This method offers the advantage of a very wide dynamic range, provided that the reference oscillator is of a very high spectral purity. Two identical oscillators are used for measurements on crystal oscillators, and the assumption made that the two oscillators have the same Phase Noise. In this case, 3 dB is subtracted from the result because the noise powers add up. Also, this method yields the widest measurement coverage (e.g. the frequency offset range is 0.01 Hz to 100 MHz). This method is insensitive to AM noise and capable of tacking drifting sources.

The disadvantages of this method are:

• The method requires two oscillators at the same frequency that have to be synchronized to each other.
• An extra PLL and a Low Noise Amplifier are needed.
• Calibration is complex because the gain of all components is included in the result. Calibration is made by mistuning the two oscillators relative to each other and measuring the AC voltage obtained at the output of the LNA.
• Requiring a clean, electronically tunable reference source, and that measuring high drift rate sources requires reference with a wide tuning range.