Applications, Electronics & Scientific Guide, How To

How to Measure Ripple Voltage with Oscilloscope

Narrow power supply tolerances on CPU, DDR, and FPGA components overwhelm many oscilloscopes and probes. The power rail probes from Rohde & Schwarz promise relief.

Modern electronic designs can in extreme cases contain up to a hundred power supply networks with different DC voltages. Each power supply network has its requirements for the accuracy of the DC voltage and the maximum permissible interference signals. Unwanted noise on the DC power grid can cause EMC problems or affect the operation of a circuit.

Particularly critical are power grids for CPUs, memory modules or FPGAs. While the required DC supply voltages continue to drop, the tolerances allowed for the DC voltage become ever narrower. These must be fulfilled in all load conditions in order not to endanger the function of the circuit. A typical example is a DDR4 memory. Compared to memory chips of the type DDR3, the maximum data transfer rate increases from 2133 to 3200 MBit / s.

One clock interval is more 313 ps in DDR4 memory, half of which is often needed for clock and data jitter. Unwanted spurious signals on the supply voltage increase the clock and data jitter and lead to errors in data transmission. At the same time, the maximum toleranced ripple of the DC supply voltage for DDR4 memories is reduced to 60 mV (U SS ) at 1.2 V DC supply voltage.

The oscilloscope is the meter of choice

With an oscilloscope, the Power Integrity can be measured and also check the level and quality of supply voltages. At the same time, it is possible to analyze interference signals on the power supply in the time and frequency domain and to identify possible causes. So far passive probes have been used which, however, have some disadvantages in the verification of power supply networks:

  • High sensitivity can only be achieved with 1: 1 probes. However, the bandwidth is limited to <40 MHz. Short-term voltage dips due to rapid load changes or high-frequency interference signals do not become visible.
  • Higher bandwidth is possible with standard passive 10: 1 probe heads, but with significantly reduced sensitivity. Narrow tolerance limits for electronic components cannot be verified.
  • Offset for passive probes is specified by the oscilloscope and is in the most sensitive setting max. 1 V. This cannot compensate for the DC component of supply voltage networks. The DC component must be eliminated with AC coupling or blocking capacitors. But then it can not be said whether the measured ripple voltage lies within the tolerance window or whether the drift of the supply voltage violates the required tolerances on one side.
  • The DC accuracy of measurements with standard probes is limited by the oscilloscope with two percent and thus in the maximum allowable tolerance of the supply voltage of modern FPGAs.

A combination of high sensitivity, high measurement bandwidth, high offset and high DC accuracy can only be achieved with specially designed probes. The R & S RT-ZPR20 / 40 power rail probes from Rohde & Schwarz are ideal for this. They combine a low-frequency active circuit part with a high-frequency passive circuit part:

Key parameters of the probe

Sensitivity and Noise: The measurement signal is transmitted 1: 1. Therefore, the resolution is one mV/div on the oscilloscope. The R & S RTE1000 achieves a value of 500 μV / div. The probe increases the inherent noise of the oscilloscope by ten percent.

Measurement bandwidth up to 4 GHz: Designs, especially for IoT applications, often combine RF transmission and reception paths in a confined space with CPU and memory components. An unfavorable layout or insufficient decoupling of the power supply network can lead to the coupling of broadband noise on the power grid. This can lead to sporadic or regular malfunctions. With bandwidths of 2 GHz, most high-frequency interference signals can be reliably detected. Rohde & Schwarz has developed the R & S RT-ZPR40 probe for embedded components for mobile and IoT applications. Its bandwidth is 4 GHz, and it can detect coupled signals in the ISM band at 2.4 GHz and LTE band at 3 GHz and in higher RF bands up to 4 GHz.

DC offset of ± 60 V: Most digital power grids operate with voltages <5 V. For analog power grids, significantly higher offset voltages are required. Here, high voltages are required, at the same time interference signals on the power supply network of analog components play a major role.

DC accuracy of 0.1 percent with an integrated 16-bit voltmeter: In addition to the total signal, the power rail probes with integrated 16-bit voltmeter accurately measure the DC component of the supply voltage with ± 0.1 percent accuracy. The measured value is transmitted to the oscilloscope via the probe interface. The solution for automated tests is interesting because no additional DC voltmeter is needed.

Numerous connection variants: A power rail probe can be connected differently to a power supply. If several power supplies are to be checked quickly, the included 350 MHz browser, a probe tip with SMA connection for the probe adapter, is suitable. With the mass springs also contained earth loops can be minimized. For broadband and low-noise measurements, the supplied 50 ohms SMA coaxial cable is recommended.

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