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14 December 2020

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.

Figure 2. Reflections on the edges of the DQS (left) and DQ (right) signals
due to probing mid-bus instead of at the device pins.

Reflections occurring on signal edges at the measurement threshold can cause measurement errors and wreak havoc with eye diagrams. Luckily, there are several tools that can be applied to reduce or remove reflections.

Figure 3. VP@Rcvr builds a model of the transmission
path between the probing point and the device pins.

The first is an optional math function called VirtualProbe@Receiver (VP@Rcvr). The VP@Rcvr math operator is enabled on Teledyne LeCroy oscilloscopes by the Eye Doctor II optional software package. It is designed to quickly compensate for signal reflections due to a termination impairment.

As shown in Figure 3, VP@Rcvr builds a transmission line model to virtually move the probing point closer to the receiver. The user enters the termination and transmission line parameters for the model -- the time delay, resistance, inductance and capacitance. The output of the model simulates the output signal at the receiver by emulating a lumped component termination that sits between the actual probed point and the receiver. The simulation mode is used to verify the model before applying it to the acquired signals in the termination mode. The results of applying VP@Rcvr are quite good, as seen in Figure 4.

Figure 4. DQ and DQS signals before (top) and after (bottom) applying VP@Rcvr function.

The upper trace is the original condition showing the reflections on the DQ and DQS signals. After VP@Rcvr is applied, the reflections have been eliminated.

Figure 1 above shows an actual measurement where VP@Rcvr is used to clean up reflections due to mid-bus probing and also a measurement probed at the memory receiver (Rx) pins. The VP@Rcvr results (center) show a good correspondence with the signal obtained from probing at the device (right) compared to the uncorrected signal probed mid-bus (left).

The second and more sophisticated method of virtual probing is to use the optional VirtualProbe software. This more advanced tool applies S-parameter models of the transmission path to the input signals. The S-parameter models describe the signal path from the probing point to the ideal probing location and corrects the acquired waveforms. This is an innovative approach that may be used to represent more complex S-parameters, such as interposer models.

To learn more about using virtual probing techniques for DDR testing, watch the on-demand webinar, DDR4/5 & LPDDR4/5 Probing and Debug Solutions.

Also see:

Which Virtual Probing Method to Use?

Isolating DDR Read and Write Operations

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