Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios

As the power source of the Electronic system, the quality of the power network will directly affect whether the system can work normally. Therefore, power testing and debugging are an extremely important part of ensuring the normal operation of the system. There are many types of power supplies commonly used in the system, including SMPS (AC-DC, DC-DC) and LDO. For these power supplies, in addition to paying attention to the voltage amplitude, more and more attention has been paid to the power supply ripple signal, especially It is the power supply for RF IC and HSS IC. Because high-frequency noise in the power network may stream into these sensitive circuits, causing interference.

This article will further describe the application of Spectrum View through common test scenarios such as power network debugging and PLL fault diagnosis.

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 1. MSO64 features the new TEK049 platform and ultra-low noise front end TEK061

Application of Spectrum View in Power Debugging

As the power source of the electronic system, the quality of the power network will directly affect whether the system can work normally. Therefore, power testing and debugging are an extremely important part of ensuring the normal operation of the system. There are many types of power supplies commonly used in the system, including SMPS (AC-DC, DC-DC) and LDO. For these power supplies, in addition to paying attention to the voltage amplitude, more and more attention has been paid to the power supply ripple signal, especially It is the power supply for RF IC and HSS IC. Because high-frequency noise in the power network may stream into these sensitive circuits, causing interference.

Even if the power supply ripple is relatively weak, the impact on sensitive circuits cannot be ignored. There are two problems in the power supply ripple test: (1) Can a weak power supply ripple be measured? (2) How to quickly determine the power supply noise frequency? Both of these are challenges faced in ripple testing, and these problems are easily solved using Tektronix’s latest platform oscilloscope, the MSO64, with the help of very low noise, high bandwidth power rail probes.

The new platform oscilloscope MSO64 adopts the new TEK049 platform, which not only achieves a high sampling rate of 25GS/s when 4 channels are turned on at the same time, but also achieves a high vertical resolution of 12-bit. At the same time, due to the adoption of the new low-noise front-end amplifier ASIC TEK061, the noise level is greatly reduced. At 1mv/div, the measured RSM value of the noise floor is only 58uV, which is far lower than similar oscilloscopes in the market. These characteristics are the strong guarantee of high dynamics and low noise floor in MSO64 spectral mode – Spectrum View.

With the help of the TPR1000/4000 power rail probe, the MSO64 can reduce the system noise to 300uVpp, greatly improving the weak signal testing capability, which is far beyond the reach of ordinary probes. The TPR series probes have a bandwidth of up to 4GHz and an offset voltage range of ±60V, making them ideal for testing high frequency noise in conventional power supplies. In addition, the TPR probe also provides a wealth of detection front-ends and supports flexible detection connection methods, including point testing, soldering testing, snap connection, plug connection, etc.

During the power supply ripple test, the measured ripple may not be a sine wave in most cases. As shown in Figure 3, the time domain waveform of the ripple is relatively “chaotic”. The frequency content of the ripple is not known. When the Spectrum View is turned on, the spectrum of the noise on the power supply can be clearly seen, as shown in Figure 5. The power supply noise spectrum mainly includes two groups of frequency components: 8kHz and its harmonic components and a linear spectrum with a spacing of 200Hz. The line spectrum is mainly caused by the pulse with a period of 5ms as shown in Figure 3, while 8kHz and its harmonics are caused by the crosstalk of the clock signal on the circuit board, which provides a debugging basis for the fault diagnosis of the circuit board.

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 2. Tektronix offers high-performance power rail probes

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 3. Weak power supply ripple test waveform

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 4. Weak power supply ripple test waveform (partial zoom)

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 5. Power Supply Noise Waveform and Spectrum Analysis

Application of Spectrum View in PLL Debugging

With excellent phase noise performance and frequency stability, PLL-based frequency synthesizers have been widely used in RF/uW and HSS circuits. In the process of PLL development and debugging, in order to improve the efficiency of diagnosis and testing, multi-channel test equipment is required to observe multiple signals at the same time, including RF output, VCO DC voltage, VCO tuning voltage, etc., to facilitate linkage analysis. Because the stability and purity of the VCO power supply and tuning voltage will directly affect the performance of the PLL, and even cause the PLL to lose lock.

Figure 6 is the actual output signal spectrum of a PLL operating at a frequency of about 2.4GHz, when the PLL is in a stable state. However, when the RBW was reduced, it was found that there were many spurs in the signal, as shown in Figure 7. In the figure, the spectrum view of three channels is opened at the same time, Ch.1 observes the spectrum of the RF signal, and Ch.2/3 observes the spectrum of the tuning and DC voltage waveforms respectively.

The center frequency of the spectrum of the three channels can be set to be different, Ch1. is set to 2.42GHz, Ch.2/3 is set to 20MHz, and span is set to 50MHz. As can be seen from Figure 7, the time-domain waveforms of the tuning and DC voltages are not stable, but contain many spurious frequency components. Comparing the spectrums of the three channels, it is found that the spurs on the RF spectrum are basically directly modulated by the spurs on the tuning and DC voltage waveforms. Therefore, in order to improve the spectral purity of the radio frequency signal, it is necessary to filter the power supply and the tuning voltage.

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 6. Spectrum when PLL is stable

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 7. Spectrum of PLL RF and supply/tuning voltage separately observed using Spectrum View

The time-frequency domain linkage analysis function of Spectrum View also facilitates the PLL loss-of-lock analysis. Figure 8 captures the PLL’s loss-of-lock state. By analyzing the voltage waveform at this time, it can be seen that a problem with the tuning voltage causes the PLL to lose-lock. Causes instantaneous abnormality of DC voltage waveform. In addition to troubleshooting loss of lock failures, this feature of Spectrum View can also be used to test the stable response time of a PLL.

Application of Spectrum View in power network debugging and PLL fault diagnosis scenarios
Figure 8. Multi-Channel Testing Assists PLL Loss of Lock Failure Analysis

in conclusion

This article focuses on the application of Spectrum View, a new spectrum analysis function of Tektronix oscilloscopes, for power supply debugging and PLL troubleshooting. The actual measurement shows that the multi-channel time-frequency domain linkage analysis of Spectrum View is very convenient for the location of interference signals and the troubleshooting of circuit faults, which provides an important basis for development engineers to debug products.

The Links:   LB121S03-TL01 LQ065T5DG01

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