The phase noise is described from the frequency domain, which is an important indicator for describing signal quality. There is an important influence on the Doppler radar system, radio communication, spatial signal transmission. Measurement of signaling signals is often encountered in work. This article first starts from the definition of signal phase noise, focusing on the use of signal analyzers for phase noise measurement methods and precautions.
1, what is the phase noise?
In the frequency domain, an ideal sine wave signal is a single profile; the actual signal includes some discrete lines in addition to the main signal, which is the random amplitude and phase jitter, on both sides of the normal signal. The form of modulation appears. All instability of the signal in the frequency domain is always the noise ribbon on both sides of the carrier, and the sideband noise is an index of indirect measurement with the frequency of the radio frequency signal power spectrum-related noise power. The sideband noise can be expressed as the frequency band sideband noise and the modem ribbon noise. Most of the splitter band power of the signal measured by the phase noise test system is small, so that the sideband noise measured by most signals is the frequency band sideband noise (ie, the phase noise is also known. Sideband phase noise). It is defined as the ratio of the power and signal total power of the 1 Hz bandwidth, in which the unit is DBC / Hz. On the signal / spectrum analyzer, the sideband noise is the sum of phase noise and amplitude noise, and is usually the sideband noise read from the spectrometer when the noise of the descriptor is much smaller than the phase noise (less than 10 dB). Noise.
At 290K ambient temperatures, the noise power substrate is -174dBm / Hz. Due to the contribution of phase noise and modem noise on thermal noise, the contribution of phase noise on thermal noise is -177 dBm / Hz, which is 3 dB than thermal noise. If the carrier power is small, for example, -20dBm, phase noise is limited to -157 dBC / Hz (-177dBm / Hz - (- 20dBm)). The following table is a corresponding table of carrier power and phase noise limit.
2, phase noise measurement
In the frequency domain, the commonly used phase noise measurement method mainly has direct spectrum analytical instrumentation, phase detecting instrument method, diaphragm method, and dual channel mutual level. It should be noted that the measurement method is also different in different situations. Typical phase noise measurements can be done by a professional phase noise test system, but the price of these professional equipment is quite expensive, and the spectrum analyzer or a new generation of signal analyzers is relatively common instruments, and the requirements for some phase noise indicators are not very stringent. In addition, the measurement of phase noise indicators can be performed with signal / spectrum analyzers.
The method of phase noise measurement by spectral analysis is called a direct spectrum analysis instrument method. The method can not only display the measured value of phase noise on the analyzer, but also accurately display whether other discrete signals are available, it is simple, flexible and easy to use. After the measured signal can be directly applied to the RF input port of the analyzer, the analyzer is directly analyzed by the analyzer; it is also possible to obtain a suitable intermediate frequency signal after the signal is mixed with the phase noise indicator. This intermediate frequency signal is analyzed by the analyzer.
With the development of instrumentation technology and function, most of the high-grade signal / spectrum analyzers have phase noise measurement options. The option measurement interface and the operation method are similar to the professional phase noise measuring instrument. After the signal is connected to the analyzer, set the appropriate carrier frequency or perform the carrier automatic search, then set the frequency offset range of the measurement, wait for the measurement to end, one complete The sideband noise curve is presented on the interface. If the value of some points is interested, the tag function is opened, and the corresponding frequency offset is set, and the measurement result of the point is displayed.
Signal / Spectrum Analyzer Phase Noise Option Measurement Interface
If the analyzer does not have a phase noise measurement option, some corresponding settings can be tested, the test results, and phase noise measurement options are the same. The following test step is a general purpose analyzer with a analyzer having a spectral analysis function to measure phase noise test steps.
(1) After connecting the measured signal to the analyzer, set the center frequency and appropriate analysis bandwidth (ie, bandwidth) as the measured signal frequency, and the analyzer can be labeled to the center frequency using the analyzer. "The tag function reduces the bandwidth, and then uses" marked to the center frequency "until the carrier signal is in the center of the screen (the bandwidth is equal to or slightly greater than twice the measured frequency).
(2) Set a suitable reference level on the analyzer, the reference level setting should be greater than or equal to the actual output level value of the measured carrier signal. This step can be used with the analyzer "marked to the reference level". In this process, if the measured carrier signal is smaller, the small attenuator gear position should be used to ensure that the signal is not compressed.
(3) Set appropriate resolution bandwidth and video bandwidth: If the two parameters of the analyzer are automatically associated settings, it can be free from the operation. If not, you need to manually set, these two parameters should not set too much, too big. Impact the accuracy of the indicator measurement, but too little will affect the measurement speed. You can choose a larger intermediate frequency bandwidth, then gradually decrease until the measured value of the phase noise is no longer reduced.
(4) Open the "Track Average" function; open "Tag", search "Peak", then open "Differential Tag", set the difference mark value to the measured frequency bias, then open the noise mark from "Tag function" Function. The current marker displayed is the phase noise value of the carrier signal at the frequency offset, and the unit is DBC / Hz.
3. Precautions for measurement
(1) Select high performance signal analyzer or spectrum analyzer
First, this should be used to select a low-end noise analyzer because the lower limit of the measured phase noise depends on the bottom noise of the analyzer. The analyzer acts as an ultra-pending analysis device. The final measurement result is the result of superimposed on the external input signal with the internal vibration signal of the unit. If the phase noise indicator of the external input signal is higher than the indicator of the analyzer itself, the measurement results It is actually the phase noise of the analyzer. The results of the measurement are correct when the phase noise indicators of the external signal is poor than the analyzer indicator (more than 3DB). Direct spectral method is not suitable for high performance crystal or direct frequency test of lower noise.
(2) Measurement results are combined noise
Whether using the analyzer's phase noise option or spectral analysis function, the analyzer cannot separate the modulation noise and the frequency modulation noise zone, so the measurement results are the sum of the amplitude and frequency modulation noise. In order to accurately measure the phase noise, the modem noise of the measured signal is generally required to be much smaller than the modulated frequency noise (less than 10 dB), and the measurement results are substantially phase noise.
(3) Pay attention to the influence of the dynamic range of the analyzer
The dynamic range represents the measurement range of the analyzer, which depends on the analyzer's own sensitivity and phase noise, and the upper limit depends on the 1DB compression point. The lower limit of the dynamic range of the deviation of the carrier is mainly depends primarily on the phase noise of the analyzer itself, where the phase noise of the analyzer itself is low, and the lower limit of the dynamic range is mainly depends on the sensitivity of the analyzer. Since the analyzer has no carrier suppression function, the measured dynamic range is limited, especially when measuring the distortion of the carrier, it is necessary to determine whether the measurement is limited to the dynamic range of the analyzer, so as to avoid errors for the measurement result.
(4) Reduce the effects of external environment on measurement results
The spectral drift of the signal will bring a lot of error to the results of the noise, and even the measurement is not measured. The measured equipment and measuring instruments need to be fully preheated to achieve a stable working condition before measurement, and the preheating time of the analyzer usually requires greater than 10 minutes. The instrument is securely connected to avoid vibration. It is best to place the instrument on an anti-vibration pad capable of absorbing vibration, reducing or eliminating salvation noise. In order to reduce the impact of the external environment on the measurement results, there is a conditional place to be measured in the interior.
(5) Pay attention to the correction of measurement results
Most old-fashioned spectrum analyzers with analog intermediate frequency use logarithmic amplifiers and peak detectors. The magnification of the logarithmic amplifier on the peak value of the noise signal is smaller than the other noise signals, so that the measured value is low; when the noise is measured by the sampling detection or peak detector, the measurement error is introduced. The error introduced to the logarithmic amplifier and the detector is typically 2.5 dB, that is, 2.5 dB is added over the original measurement results. When the analyzer is in the Fast Fourier Transform (FFT) mode of operation, its measurement results are the frequency domain transformation values of the discrete time domain peak response, not affected by factors such as peak detectors, video filters, noise bandwidth, measurement The results do not need to be corrected. In a high-grade signal / spectrum analyzer, as long as the "noise tag" function, the detection mode and the error correction coefficient will be automatically coupled, the tag read is the corrected normalized phase noise measurement result.
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