1: Overview of Phase Noise
Phase noise refers to the phase variation or instability in a signal or oscillator over frequency. It is a measure of the phase deviation or fluctuation relative to an ideally stable oscillator. The presence of phase noise means that there are small changes or perturbations in the phase of the signal in time, which can have a negative impact on some applications.
Phase noise is usually expressed in the form of Phase Noise Density or Phase Noise Power Spectral Density. They describe how phase changes with frequency. In the frequency domain, phase noise density shows the deviation of a signal's phase from frequency. It is plotted with frequency deviation in units of hertz (Hz) as the horizontal axis and relative phase deviation in decibels/hertz (dB/Hz) as the vertical axis. A phase noise density plot is usually a curve or collection of curves that demonstrates the phase stability of a signal at different frequency offsets. Phase noise can have a variety of effects on system performance. In communication systems, high phase noise may cause phase distortion in signal transmission, affecting signal demodulation and decoding. In the field of precision measurement, phase noise can introduce measurement errors and reduce the precision and accuracy of the system. Therefore, the control and reduction of phase noise is critical for many applications. Good signal sources or oscillators usually have low phase noise levels, which means they produce signals that are relatively stable with small phase changes. This can be achieved through optimized circuit design, noise suppression technology, and constant temperature control.
Phase noise refers to the fluctuation or change in the phase of a signal over time. It is a measure of the frequency stability of a signal and reflects the uncertainty or jitter in the frequency components of the signal. Phase noise is usually expressed as the change in phase relative to an ideal continuous wave (CW) signal and is measured in units of decibels per Hertz (dB/Hz) or decibels per Hertz (dBc/Hz). The smaller the value of phase noise, the better the phase stability of the signal. The characteristics of phase noise include the following aspects:
1. Phase Noise Density: The power density of phase noise in the unit frequency range. It describes the level of phase fluctuation of a signal at various frequency offsets. Usually expressed in dB/Hz.
2. Noise Power Spectral Density: The square root of the phase noise density, used to represent the phase noise power within the unit frequency range. Also often expressed in dBc/Hz, it represents the phase noise level relative to the signal amplitude.
3. Phase noise curve: Plot the relationship between phase noise density or power spectral density and frequency offset. The phase noise curve is usually a curve that increases with increasing frequency offset, and the shape may be linear, parabolic, or other forms.
4. Integrated Noise: The cumulative effect of phase noise within a certain frequency range. By integrating the phase noise curve, the integrated noise within a specific frequency offset range can be calculated. Integrated noise represents phase stability over the entire frequency range
2: Phase noise test
To test phase noise, you need to go through a lot of calculations or have a spectrum analyzer that supports testing phase noise. Generally, spectrum analyzers support phase noise mode testing. The spectrum analyzer used here is KEYSIGHT N9030B, which supports 2Hz ~ 3.6GHz. frequency range.
The interface after the spectrum analyzer is turned on is as follows. Only one test window is displayed here, which is the spectrum scanning window based on the test. Multiple test windows can be added through the "+" on the screen. Especially when testing different indicators at the same frequency, for example, if you want to test a clock, you can test the accuracy of the clock in window 1 and the phase noise of the clock in window 2.
To test phase noise, you need to first adjust to the phase noise test mode, which can be selected through the touch screen or through the buttons. By pressing the button and clicking the MODE/MEAS button in the menu, you will enter the test project selection interface.
The operation of adding a test window here is performed on the touch screen. Click "+" on the screen to add a window to configure the test mode, which will add +1 to the number of the current window (Spectrum Analyzer 2). Test select " Phase Noise—log Plot—Decade Table".
After clicking "OK" to complete the configuration, the following test window will appear. The window name is the test item + the window of the test item (Phase Noise 1). Phase noise is generally within the bandwidth of the test center frequency 100hz to 1Mhz.
The crystal oscillator output of the test equipment is connected to the RF Input interface of the spectrum analyzer through a coaxial cable. The equipment is powered on normally. Click AUTO TUNE in the phase noise test window and wait for the following interface to appear. This is the phase noise of the 50M clock. Generally, testing requires a relatively clean environment, otherwise the test error will be relatively large. It is recommended to "AUTO TUNE" multiple times to check the stability.