Situational Awareness: RF Noise in the Lab

Fig. 1. Time domain (top) and spectral (bottom) views of
signal shown in SPECTRUM-1 on a WaveSurfer 4000HD. 

Laboratories have multiple sources of RF that can affect measurements, like computers, cell phones, routers, local radio and TV stations, even a nearby airport. Knowing the RF background of your lab is another part of Situational Awareness. Only by knowing the background can you know what is actually due to the device under test.

One way to read RF is through spectral analysis of Fourier transforms (DFT and FFT). FFTs take a time domain view of a signal (e.g, amplitude versus time trace) and change it into a spectrum of amplitude plotted as a function of frequency. Frequency spectrums are great for observing signals than are asynchronous with the process being measured. They have a lower noise floor and offer better dynamic range than do time domain plots.  Consider the views of the same signal shown in the time domain and frequency domain in Fig. 1.

Situational Awareness: The Impact of the Interconnect

Fig 1. Coaxial cable has little effect on signal rise time,
but that's not true for every connection method.

How you connect a signal to your oscilloscope affects your measurements, and knowing the impact of different connection methods is an important part of your situational awareness.  

Using the same 40 ps fast edge signal we used for the risetime measurements in the last post, we’ll compare connections made using coaxial cable and a 10x passive probe, with and without different accessories.

Situational Awareness: Testing Oscilloscope Outer Limits

Fig 1. 40 ps signal measured full bandwidth on a
1 GHz oscilloscope shows visible over/undershoot.

Nothing is perfect. Every test instrument has its limits, and knowing the limits to your oscilloscope’s bandwidth in response to real-world signals helps to develop situational awareness when making measurements. This is especially true when testing signals that are at or very near the specified bandwidth limit of the instrument.

The measurements we’ll demonstrate were made on a WaveSurfer 4104HD, a 12-bit, 4-channel, 1 GHz bandwidth oscilloscope that samples at up to 5 GS/s.

Four Measurement Best Practices

To start the New Year right, we’re going to talk about four measurement "best practices", which will help you get the most out of any oscilloscope you have. These are important when doing any type of measurement—and you can get a good start on them simply by asking yourself the four questions in the sidebar.

1. Anticipate the results

Those who are familiar with Dr. Eric Bogatin’s Rule #9 will know this one. Before you do any measurement, anticipate what you expect the result to be, because that is the most important way of identifying if there is a potential problem. 

Decision Feedback Equalization in DDR

Figure 1. A transmitted rectangular pulse suffers
distortion by the time it reaches the receiver. 
Broadening and reflections from previous
transmitted bits add to the pulse response, 
creating inter-symbol interference.

High-speed serial links such as those used in DDR4 and DDR5 are subject to a variety of signal degradation challenges.  Insertion losses, frequency dependent attenuation and inter-symbol interference (ISI), as well as others, are among the most commonly encountered sources of signal degradation. 

Figure 1 shows how reflections can cause ISI on a rectangular pulse. When a rectangular pulse is transmitted, it suffers distortion which is apparent when it reaches the receiver.  It may be broadened due to group delay dispersion because different frequency components of the signal propagate along the signal path at differing velocities. In addition, there may be echo pulses, due to impedance mismatches in the channel.  These mismatches cause reflections that propagate back and forth over the channel and appear as these echoes where subsequent bits should be.

Removing Reflections from DDR Signals Probed Mid-Bus

Figure 1. Virtual probing methods like VP@Rcvr can help
remove reflections from signals probed mid-bus.

Probing DDR signals can present some interesting challenges. The JEDEC specification indicates that all measurements should be made at the output pins of the memory chip. The challenge comes because sometimes the pins of the memory chip are not accessible. You may be able to use an interposer, but even that requires some spatial displacement from the probing point to the Ball Grid Array (BGA) pins of the memory chip. 

If the board has already been populated, there is an even greater problem because the interposer can’t be used, so probes may have to be placed in the middle of the bus in order to make a measurement. In this situation, the probe picks up signals reflected from the memory controller and the memory chip, as well as the desired signals. Reflections appear as non-monotonic ripples on the edges of DQ and DQS signals, as shown in Figure 2.

Isolating DDR Read and Write Operations

Figure 1. DDR DQ and DQS signals are
in phase during a Read operation and
out of phase during a Write operation.

Whether you are debugging or running compliance tests on Double Data Rate (DDR) or Low Power Double Data Rate (LPDDR) memory, the analysis process requires the separation of Read and Write operations to enable measurements on each distinct operational mode. 

The phase relationship between the Data (DQ) signal and the Data Strobe (DQS) signal indicates the type of operation, as shown in Figure 1.

The DQ and DQS signals are phase aligned with edges overlapping in Read mode. In Write mode, they are out of phase, and the DQS edge overlaps the center of the DQ eye.  In the lower speed versions of DDR memory devices, the measuring instrument could be triggered on this phase difference, enabling the isolation of the desired operation for testing.