Table of contents
Explanation of technical terms
Elimination of Phase Noise from a Common Clock Source
Elimination of utility power noise
overview
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The amplifier output is sent to an equalized mixer (phase detector). A phase detector mixes the two signals, producing a sum and difference product at its output. The sum product is filtered out by a low pass filter. The remaining difference product forms the 250 MHz output signal down converted to DC (phase noise). The LNA provides enough gain to overcome the noise floor limitations of the spectrum analyzer.
overall architecture process
Using the test setup shown in Figure 1, two DUTs with the same part number were clocked by a single 1 GHz clock source. The device is programmed to divide the clock frequency by four to produce an output of 250 MHz. In addition, the two output signals are phase shifted 90° relative to each other (in quadrature) to minimize down-converted signal levels appearing at dc. The DUT signal is amplified by a low-noise amplifier (LNA) to increase the dynamic range of the measurement system (the phase noise generated by the amplifier can be negligible).
Explanation of technical terms
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ADC:
Analog-to-digital converter, or A/D converter, or ADC for short, usually refers to an electronic component that converts an analog signal into a digital signal. A common analog-to-digital converter converts an input voltage signal into an output digital signal. Since the digital signal itself has no practical significance, it only represents a relative size. Therefore, any analog-to-digital converter needs a reference analog quantity as a conversion standard, and the more common reference standard is the largest convertible signal size. The output digital quantity represents the magnitude of the input signal relative to the reference signal
technical details
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Elimination of Phase Noise from a Common Clock Source
Figure 2 shows absolute phase noise measurements for two clock sources (with very different phase noise characteristics). In theory, neither clock source will affect the DUT's additive phase noise as measured by the residual phase noise setup. Figure 3 confirms this theory. It plots two separate residual phase noise measurements, one trace for each clock source. The two traces nearly overlap, proving that the common clock source noise is canceled by the residual phase noise device. In an absolute phase noise setup, this noise will not be cancelled. In fact, if the DUT were ideal (no additive phase noise), its absolute phase noise curve would match the curve in Figure 2 (but it would be 12dB lower due to the quarter-frequency translation). Clock Source 2 exhibits a phase noise of -92dBc/Hz (at 1kHz offset) when normalized to a 250-MHz carrier, while the measured DUT phase noise associated with Clock Source 2 is -135dBc/Hz (at 1kHz ). Therefore, the residual phase measurement rejects about 40dB of input clock phase noise
Elimination of utility power noise
The figure uses the same common power connection as in figure 1. The figure shows the effect of using separate noise supplies for each DUT. Uncorrelated power supply noise can cause a large increase in close-in phase noise.
Absolute phase noise measurements using low noise power supplies are shown. The absolute phase noise with the low noise power supply and the residual phase noise with the low noise power supply alone show good agreement. Power supply phase noise has been removed in residual phase noise measurements but not in absolute phase noise measurements
summary
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The residual phase noise measurement method is a very useful technique for identifying the phase noise generated by individual components as part of a system design. Using this approach, external noise sources, such as input clocks and power supplies, are correlated at the output of each DUT and thus effectively eliminated. In addition, it may account for the phase noise contributed by buffers or amplifiers used in the DUT residual noise measurement by making additional residual phase noise measurements of these components. Using a combination of residual and absolute phase noise measurements is a very effective way to identify major noise sources in a system design. Measurement data obtained on frequency dividers demonstrates the concept and utility of residual phase noise measurements and quantifies the effects of noisy input clock and power supplies. With this evaluation method, system designers can derive specifications for input clock sources and power supplies based on actual measurement data.