Phase techniques of absolute phase noise measurements

There are three dominant techniques used to measure the phase noise of oscillators: the direct spectrum analyzer approach, the phase-lock-loop (PLL) techniques, and the discriminator techniques.[46] The most direct and probably the oldest method used to measure the phase noise of oscillators is the direct spectrum analyzer method. Here the signal from the device-under-test (DUT) is input to a spectrum analyzer tuned to the DUT frequency. The sideband noise power can be directly measured and compared to the carrier signal power to obtain the phase noise spectrum. This method actually measures the total sideband noise, including AM noise and phase noise. If AM noise is much less than the phase noise, the measurement can be considered as to be the phase noise. The sensitivity of this method is limited by the internal local oscillator (LO) noise of the spectrum analyzer, and the inability to track any signal drift limits the close-to-carrier noise measurement capability of the analyzer.

When the AM noise is relatively high to the phase noise, a phase detector is required to separate the phase noise from the amplitude noise. The phase detector converts the phase difference of the two incident signal in to a voltage at the output of the detector. When the phase difference between the two input ports of the detector is set to 90 degree, the voltage output will be zero volts. Any phase fluctuation from quadrature will result in a voltage fluctuation at the output. When the quadrature is not maintained, an error can be introduced into results based on the amount of the phase delta from quadrature. The error is 20 log [cos (phase deviation from quadrature)] (dB). Phase detectors are usually constructed by the double balanced mixers, and typically required large power signals at the input port to operate properly.

One of the signals must be of high power to switch the diodes in the detector, allowing the other signal to be of lower power.

There are two different measurement techniques which use a phase detector, along with associated filters, low noise amplifier, and baseband analyzer: one is the PLL with reference source measurement technique, and the other is FM discriminator measurement technique. Within the PLL with reference source measurement technique, another source is used to provide the reference phase signal for the phase detector. This is the standard measure of phase fluctuations described in NIST Technical Note 1337. Fig. 3-9 shows a block diagram of the method suggested by NIST.

Fig. 3-9 General block diagram described in NIST technical note 1337.

Signals from two oscillators at the same nominal frequency are applied to the mixer inputs. The PLL is used to controlled either of the two sources and establish phase quadrature at the input ports of the phase detector. This means that one of the two sources used in this method must have DC voltage control capability for phase

locking. A very narrow band PLL is used to maintain a 90 degree phase difference between these two sources. The phase detector operation is such that when the input signals are 90 degrees out of phase (in quadrature), the output of the mixer is a small fluctuating voltage proportional to the phase difference between the two oscillators.

By examining the spectrum of this error signal on the spectrum analyzer, the phase noise performance of this pair of oscillators may be measured. If the noise of one oscillator dominates, its phase noise is measured directly. A useful and practical approximation when the two test oscillators are electrically similar is that each oscillator contributes one-half the measured noise power. When three or more oscillators are available for test, the phase noise of each oscillator may be accurately calculated by solving simultaneous equations expressing data measured from the permutations of oscillator pairs. The frequency difference between the two sourced at the phase detector must be less than 10% of the peak-tuning-range (PTR) for PLL to close. High PTR will cause the increase in the system noise floor. This feature makes this technique is not suitable for measuring the high-drift-rate low phase noise sources which requires high PTR. Lower power from the DUT or the reference source can cause the phase detector noise floor to rise or the phase detector to not operate. Low noise amplifier prior to the phase detector can help to solve this problem, but the residual noise of the amplifiers will add to the phase detector phase noise floor. The increase in the system noise floor will degrade the sensitivity of the phase detector.

The residual noise of the amplifier becomes a limiting factor in the overall system measurement noise floor.

The oscillators with SAW or STW resonators have both low phase noise and high drift rate characteristics. So, the direct spectrum techniques and the PLL with reference source measurement techniques are not fulfilling the requirement while measuring the low phase noise oscillator with piezoelectric resonator. FM

discriminator technique is the hopeful candidate for measuring these low phase noise SAW oscillator. The theory and design consideration are describe as follows.