Good Practice Guide to Phase Noise Measurement ... - Sequanux

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Measurement Good Practice Guide No. 68 levels to the mixers need to be carefully controlled to avoid intermodulation products. This tends to limit the wide band signal to noise ratio of the spectrum analyzer, in turn limiting the noise measurements that can be performed far from the carrier. Figure 9: A phase noise measurement on a spectrum analyzer The spectrum analyzer filters further limit the capability of the measurement. The carrier is displayed as a large signal and on either side the filter response can be seen. It is not until the user gets to a considerable offset that the ultimate performance of the analyzer can be seen. This limits the capability of the spectrum analyzer to measure phase noise close to the carrier frequency. 13
Measurement Good Practice Guide No. 68 Figure 10: Limitations of phase noise measurements with high performance spectrum analyzer 8 The net effect of these limitations can be seen graphically on the above diagram for a high performance spectrum analyzer (lower performance spectrum analyzers will place even more limitations on the measurement capability). For frequencies close to the carrier, the measurement is limited by the spectrum analyzer filter response. Selecting the 100 Hz filter limits measurements to broadly those above 300 Hz offset. A narrower filter will improve this minimum offset frequency on a roughly pro-rota basis. At greater offsets, the performance is limited by the phase noise of the local oscillator system. These limitations will be frequency dependent, the higher the frequency the greater the limitations. Note that in this example the performance at 600 MHz is reasonably representative of a wide range of input frequencies, including relatively low frequencies. Until the spectrum analyzer starts to use harmonic input sampling, the performance is relatively frequency independent. At higher frequency offsets the phase noise limitation is imposed by the signal levels permitted at the mixers and the noise limit flattens out. As the input frequency is raised, the harmonic sampling further limits the spectrum analyzer performance. The performance of the system at the larger offsets can be improved by ensuring the input attenuator to the spectrum analyzer is correctly set, and the setting is optimized for dynamic range in preference to linearity. Those spectrum analyzers which perform narrow band measurements using digital techniques, can generally perform better close to carrier measurements than analogue versions and, with the right software, provide more reliable conversion of measurement results to phase noise. 3.1 Down converter and IF filter with a spectrum analyzer The major problem that a spectrum analyzer has in measuring phase noise is that, unlike some of the other methods, the carrier has not been removed from the signal. The spectrum analyzer is forced to make a high dynamic range measurement that exposes limitations in the performance. For some applications the noise limitations of spectrum analyzers can be overcome by the use of band-pass filters or band-stop filters. The method requires the use of a second reference RF or microwave signal source and a mixer to convert the signal to a convenient intermediate frequency (IF). The signal from the mixer is then passed through a low pass filter to reject the sum components from the mixer, a low noise amplifier to increase the signal level and a band-pass crystal (Xtal) or ceramic filter 8 Source. Agilent data sheet for PSA range of spectrum analyzers 14
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Good Practice Guide to Phase Noise Measurement ... - Sequanux

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