You need to test, we're here to help.

You need to test, we're here to help.

22 January 2018

Setting the Stage for On-Die Power-Rail Measurements

Measuring on-die Vdd rail noise requires a suitably instrumented die and package
Figure 1: Measuring on-die Vdd rail noise requires
a suitably instrumented die and package
Armed with a suitable oscilloscope and active voltage-rail probe, you're now ready to make some power-rail measurements on a semiconductor die. Of course, making measurements on a die is a little different than making measurements on a printed-circuit board. This is where careful design-for-test at the die level comes in, because the chip, its packaging, and the board on which it will be mounted must be instrumented so as to make the on-die measurements possible.

Power-Rail Noise: Small Signal, Big DC Offset

Your scope's vertical adjust has its limits
Figure 1: Your
scope's vertical
adjust has its
limits
We've been working through the various challenges in making power-rail noise measurements. One of those challenges is RF pickup that can often swamp the noise signal, and the way around that is to ensure a coaxial connection from the oscilloscope's input down to the power rail itself. We want a high signal-to-noise ratio (SNR), so we're better off with a 1X probe than with a 10X attenuating probe. We want high bandwidth, so we want a 50-Ω termination at the oscilloscope input.

19 January 2018

Bandwidth vs. Current Load in Power-Rail Measurements

Connecting a 6" length of coaxial cable between a low-impedance DUT and a 1-MΩ produces ringing artifacts on your signal acquisition
Figure 1: Connecting a 6" length of coaxial cable between
a low-impedance power rail and a 1-MΩ produces
reflections and ringing artifacts on your signal acquisition
Among the various challenges we've discussed in measuring noise on power rails are RF pickup and signal-to-noise ratio (SNR). Here's another: how do you achieve high bandwidth in your measurements while also minimizing current load on your DUT? Given that your DUT is a power rail, you really don't want to draw too much current from it. But these two measurement criteria are at loggerheads with each other. It's a quandary, and it has to do with the fundamental nature of signals on interconnects.

18 January 2018

How 10X Attenuating Probes Kill Signal-to-Noise Ratio

Figure 1: Signal waveforms captured using a 10X attenuating
probe (top) and a BNC probe (bottom) with tips open
We've begun discussing things that can derail (see what we did there?) your power-rail measurements, such as the deleterious effects of RF interference. In the same context, one should always be mindful of certain characteristics of oscilloscope probes; namely, the 10X attenuating probes that are often lying around on the testbench.

Understand RF Pickup When Measuring Power Rails

Teledyne LeCroy's  HDO8108A sports a very low  noise floor of about 145 μV
Figure 1: Teledyne LeCroy's
HDO8108A sports a very low
noise floor of about 145 μV 
Measuring the noise on a power rail seems to be a straightforward task. However, there are some basic pitfalls that can cause incorrect, or even downright strange, results. Let's look at one of these challenges: RF pickup. We'll demonstrate the effect of RF pickup on a power-rail measurement, and then we'll show you an effective means of mitigating that effect.

17 January 2018

Some More PCIe 3.0 Test Examples (Part II)

This shows how a PeRT 3 state-machine log can be invaluable in diagnosing timeouts in requests for presets
Figure 1: This shows how a PeRT 3 state-machine log can be invaluable
in diagnosing timeouts in requests for presets
Continuing on from our last post, let's look at some more examples of common PCIe 3.0 test scenarios and how a well-equipped PCIe 3.0 testbench would approach them. Recall, if you will, that such a testbench would comprise a real-time digital oscilloscope of suitable bandwidth (such as Teledyne LeCroy's SDA830Zi-B oscilloscope), a protocol-enabled receiver tester (such as Teledyne LeCroy's PeRT 3 Phoenix System), and software that enables simultaneous, correlated views of the protocol and physical layers (such as Teledyne LeCroy's ProtoSync software).

Some PCIe 3.0 Test Examples (Part I)

Protocol and electrical views of  slow electrical response to a preset request
Figure 1: Protocol and electrical views of
slow electrical response to a preset request
We took a tour of a typical PCIe 3.0 testbench setup in a recent post. Now, let's see that testbench in action with some application examples of some common bad behavior one might encounter from a PCIe 3.0 channel. These include: slow electrical response, slow protocol response, and so on.