Recently, one of my friends asked about the preferred way of probing a power distribution network (PDN): “Which probe should I use to measure power plane noise? Assuming I am planning to measure PDN noise in time domain, I have two choices of probes: 1) a 50-ohm passive coax cable, or 2) an active high-impedance probe which is normally used for high speed signal measurement.”
Although, as usual, the correct answer starts with “It depends,” in this case the generic answer is more clear-cut: For many PDN measurements, a simple passive coaxial cable is better.
To determine which probe is better suited for time-domain PDN measurements, passive or active, low-impedance or high impedance, we have to consider three things: a) the signals we want to measure, b) the typical characteristics of oscilloscopes and c) the characteristics of passive and active probes.
First, let us look at the signals. As opposed to signal integrity measurements, where our signals tend to be much wider band and have a relatively large swing, typical PDN noise on a PCB is more band-limited and should be much smaller in magnitude. High-speed signal voltage swing is on the order of 1 Vpp at the transmitter, and it may be around 100 mVpp at the receiver.
In contrast, PDN noise on sensitive low-voltage power rails is measured in millivolts, at most a few times 10 millivolts peak to peak. Though newer signaling standards allow for very low voltage swing at the receiver, the contrast is clear: The magnitude range of the typical rail noise is lower than the typical range of signal swings. Luckily enough, the difference in bandwidth requirement helps us to make up for the differences in levels. Even after traveling through a lossy channel, the bandwidth of our SerDes signals has to extend up to at least the Nyquist frequency of signaling: half of the bit rate value.
For instance, the Nyquist frequency of a 5 Gbps PCIe Gen2 signal is 2.5 GHz. The majority of our PDN noise, on the other hand, goes through a chain of low-pass filters as it propagates between the board, package and silicon, resulting in a relatively band-limited signature on the board. The series connection impedance, together with the PDN impedances, creates a low-pass filtering on the order of 1 MHz for many of the memory sockets and high-power core rails, and tens, maybe hundreds of MHz cutoff frequency for lower-current power rails.
Unless we want to measure PDN crosstalk (noise coupled from one PDN rail to an adjacent PDN rail), which can be tricky in the time domain, or we want to measure high-frequency deterministic components, such as clock leakage to PDN (which can be better measured with a spectrum analyzer and not with an oscilloscope), we are better off using a lower-bandwidth oscilloscope for board PDN noise measurements.
Next, let us look at today’s oscilloscopes. We usually want to measure PDN noise in “real time,” without averaging. A big part of the PDN noise could be uncorrelated and random, and averaging would reduce both its peak and RMS value, which is exactly what we don’t want. We need real-time oscilloscopes, and we should possibly set them to infinite persistence mode to get the peak noise over time.
In recent years, the available bandwidth of real-time oscilloscopes has gone up significantly. Today, 30 GHz and 60 GHz real-time oscilloscopes are available , , . However, to achieve such high real-time bandwidth, we have to accept lower vertical resolution and higher trace noise. The front-end raw quantization is limited to 8 bits. Moreover, the true number of bits of quantization, called the Effective Number of Bits (ENOB), drops in most cases to around 5 as the signal bandwidth extends to or beyond 10 GHz.