| 词条 | Draft:Switching Noise Jitter (SNJ) |
| 释义 |
Switching Noise Jitter (SNJ) is the aggregation of variability of noise events in the time-domain on the supply bias of a system .SNJ is measurable using real-time spectral histogram analysis method and expressed as rate of occurrence in percentage. The discovery of SNJ was firstly demonstrated by TransSiP Inc in 2016, as well as 2017, at the [Https://www.apec-conf.org/about/history/2017-conference-photos/pageindex508/6 Power Electronics Conference (APEC)] and featured as a case study published by Tektronix[1] . The discovery of SNJ was also featured in multiple articles published by Planet Analog magazine[2] and EDN Network[3]. SNJ is detrimental to many modern digital communications creating an aggregation of variability of unwanted signals over time which will be digitized and stored in memory. Contents 1 Signal Noise Jitter 1.1 How does SNJ Arise 1.2 Issues associated with SNJ 2 Switching-Noise-Jitter (SNJ) Measurement How does SNJ Arise?In switch-mode power supplies (SMPS), noise present in the control loop circuitry of the supplies causes dislocation in up-slope and down-slope timing of the saw-tooth ripple waveform. The ripple waveform exhibits jitter and noise carried on the ripple also jitters. When this type of supply bias is used to power a system operating in power saving modes or pulsed applications as shown in Fig. 1 - the current drain is fluctuated in pules, typically a load enters high power stage (e.g. RX/TX On) for tens of micro- to mini-seconds and is then switched to low power or standby mode for hundreds of mini- or tens of seconds, this type of transient loads create further noise displacement or noise perturbation carried on the supply bias which results the occurrence of SNJ. Issues associated with SNJAt present, many modern switching DC-DC converters offer dual-mode operation by fixing the switching frequency (e.g. WM) at full-load to ease noise filtering. When light-loads are present, they switch to power-saving mode, typically referred as pulse-frequency, pulse-skipping or frequency-reduction mode for increased efficiency. Nevertheless, this has the disadvantages of creating additional transient noise and momentary fluctuations in output voltage, as well as reduced efficiency due to the increased quiescent current required by complex mode-switching circuitry. To make things more difficult, the power-saving modes create a wide spectrum of noise at the output voltage that is difficult to filter. The influence of SNJ is accumulative in many modern digital communications creating an aggregation of variability of unwanted signals over time which will be digitized and stored in memory. The SNJ signature therefore becomes a dominating influence on performance of noise-sensitive SoCs, circuit components and fidelity of weak signals after the ripple voltage has been suppressed. 2. SNJ Measurement Noise in the output of DC-DC converters is commonly determined by the output-voltage ripple, analytically expressed as peak-to-peak voltage or noise spectral density. Fig. 2 and Fig. 3 demonstrate the typical visual characteristics of the output from a switching regulator and a linear regulator (LDO) respectively. A saw-tooth ripple waveform is the consequence of LC filtering on the output of a switching regulator, while the output voltage fluctuations of a LDO are typically Accompanied with overshoots and undershoots in a pulsed application. Supply Bias Ripple – Choosing between LDO and a switching regulator The ripple present on the DC supply bias provides a major challenge for noise-sensitive circuitry design. Ripple changes due to load fluctuation must be allowed for and there are critical tradeoffs to be made between system performance, efficiency, space, cost, and time. As a result, LDO regulators are commonly used to power most RF applications and noise-sensitive circuitry because the supply bias noise is generally lower than switching regulators. However, LDO is much less efficient in power conversion than a switching regulator. Fig. 4 compares the supply bias noise (ripple) in a GPS application as measured by a spectrum analyzer. The primary bias voltage is provided respectively by a LDO and by a switching regulator. The frequency range of the noise spectrum for both is very similar, lying below 1MHz. However, the noise amplitude of the LDO is about 20dB lower than the switching regulator. So in general practice, the primary bias voltage provided by the LDO is expected to perform better and is generally used to build a golden reference unit. A switching regulator may be used to replace the LDO thereby achieving higher conversion efficiency but presenting tradeoffs in performance. The LDO is able to provide a low-noise bias, but only when the DC load is constant. In practise of course, the current draw of a load will fluctuate. Careful examinations of the supply bias of pulsed applications will show that transient noise is, in fact, present, as shown in Fig. 5, and this is difficult to filter. Powering a noise-sensitive SoC or circuit component in a pulsed application with an LDO may well turn out to be a suboptimal solution: in addition to low conversion efficiency transient noise is present on the supply bias. When observed on an oscilloscope, the saw-tooth waveform of a Pulse Frequency Mode (PFM) Switch Mode Power Supply (SMPS) with SNJ will appear as shown in Fig. 6. Abnormalities occurring in the time-domain allow the switching events to “move”. Since the converter is PFM type, the switching frequency as shown on the oscilloscope moves with the fluctuation of the load. This makes it impossible to discriminate how much of the movement of the waveform in t1 and S1 (need to define these) is caused by the load and how much by the noise in the control loop circuitry. The Impact of SNJ Conditioning The waveform in Fig. 7 demonstrates the outcome from the same setup with SNJ conditioning in place: the variability of t0 and S0 (idem) are substantially reduced. Digital Phosphor (DPX) Measurement Thanks to the introduction of event density as an additional measurement dimension, a DPX plot can be used to discriminate between load-induced and jitter-induced movement of the waveform. DPX Spectrum display adds an additional dimension to frequency and amplitude. The color temperature spectrum (“Z” axis) shows the number of occurrences of signals (or ‘event density’) over a set period of time. Z-axis scaling adjustments for maximum and minimum occurrence values can be manipulated to discriminate visually between different signals beneath the maximum amplitude . Fig. 8 shows the DPX plot of the supply bias waveform shown in Fig. 8. The event density of interest can be determined by setting the boundary parameters of the sampling box, and the distortion of the intermediate (green/yellow) bands on the bitmap is clearly visible. Fig 8 DPX of Supply Noise with SNJ Present Measuring Noise and SNJ A measurement box, defined by the horizontal frequency range and vertical amplitude range, is used to contain and measure the average event density over a period of time. In this example, Fig. 9, Fig. 10, the measurement period is set to infinite (i.e. the system collects SNJ continuously on the DPX display until stop). This measurement technique enables the identification and measurement of SNJ in the supply bias provided by the switching regulator both qualitatively and quantitatively. Consider the signal density produced by the switching frequencies on the left hand side of the trace (i.e. the raised colored area). Ripple events at the maximum levels of amplitude are indicated as blue and are seen clearly at the lowest end of the signal frequency spectrum: in fact most analog circuits have effective PSRR (power supply rejection ratio) to suppress ripple at low frequencies. Ripple amplitude may be further reduced by increasing the capacitance value and lowering the ESR of the output capacitor. With the amplitude of the ripple suppressed, SNJ becomes the ‘last mile’ noise component and is the dominant influence on noise-sensitive applications or circuit components. It is SNJ that prevents a noise-sensitive application powered by a switching regulator to achieve equivalent, if not better performance to that achieved by a low-noise LDO (Des- the output capacitor forms an LC filter with the inductor exhibiting bandpass properties over a specific and narrow range of frequencies. Your statement is true in the case of PWM, but in PFM the frequency is changing all the time. So can we state that ripple is effectively attenuated in a PFM device?). The presence of SNJ produces abnormalities with lower dB amplitudes, but these have higher event density or frequency of occurrence -represented in the color bitmap from the green through yellow and into red, the highest event density. Events shown beyond the switching frequencies, i.e. further along the x-axis are not caused by SNJ but conventional noise due to well-known causes such as grounding, ringing, parasitics, and probe interference. The diagram of Fig. 9 zooms into the measurement box of Fig. 8 where the green and yellow zones are seen to be expanded and the event density is about 8%. This expansion indicates abnormality in the time-domain component of the saw-tooth waveform or ripple. This abnormality is caused by the variability of timing which occurs when a PFM controller changes frequency in response to a change of the load. This movement in the time-domain is ‘unveiled’ by the DPX plot as ‘rate of occurrence’ or event density. The crucial noise reduction, achieved when the SNJ is conditioned, can be seen clearly in Fig 10. The reading of the event density has been reduced by 50% to about 4% by SNJ conditioning. In summary, the unique supply bias noise profile provided by the Symphony A2(1) chipset enables replacement of low efficiency linear DC regulators by highly efficient switch-mode PFM-type DC-DC conversion, without compromising the performance of noise-sensitive pulsed circuitry in low-power remote, portable, wearable, IoT and UAV applications. In addition, the SNJ capabilities of the Harmony module offer opportunities for reduction/elimination of data convolution errors in high speed ADC applications. References{{Reflist}}External links[Http://www.transsip.com/ TransSiP, Inc.] |
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