In today's high-speed circuit world, embedding and de-embedding cables, probes, interconnects and fixtures is important to achieving accurate measurements. Teledyne LeCroy oscilloscopes offer multiple tools for accomplishing this, some standard on higher bandwidth models. This paper provides an overview of the available tools and highlight the different use cases for each.
Cable De-embedding Option
The Cable De-embedding Software option configures the oscilloscope input channels to account for cable attenuation. If you have attached cables to the inputs, but not yet characterized their S-parameters, this is the simplest method to use.
When the software is present on the oscilloscope and a cable is connected, a Cn Cable De-Embedding tab appears behind the input channel setup dialog. Open this and enter the cable properties, which can usually be found in the table of attenuation constants provided with the cables.
Each cable can be saved as a separate setup file and re-loaded whenever you are using that cable.
If you already have the cable S-parameters, the Eye Doctor II or VirtualProbe software options provide more sophisticated methods of de-embedding.
VirtualProbe@Rcvr Math Function
This oscilloscope math function is installed with the VirtualProbe or SDAIII-CompleteLinq software options. It is especially useful for removing the effects of probes that are not placed at the end of an interconnect, such as when probing DDR and other high-speed signals mid-bus.
A probe placed in this manner usually will observe reflections, but what is of interest is what the signal looks like at the end of the interconnect. This math function allows you to virtually “move” the probing point. This is illustrated in Figure 2. The red probe in the image represents where the probe is physically located. The grey probe shows where the probe would ideally be placed, were it possible to physically place a probe at that point. Any reflections present in the circuit when probing mid-bus are removed by using the math function to model the circuit to the left of the red probe, thereby de-embedding the problematic portion of the channel.
You apply this method by creating a math function that uses the input channel being de-embedded (e.g., C1) as the source and the VirtualProbe@Rcvr operator, entering the transmission line and LRC circuit parameters to create the model.
The math function models a transmission line and termination, as shown in Figure 4. The actual probe is at the point labelled “In”.
The waveforms Z6 and Z7 in the top two grids of Figure 5 show the physically probed signal. The waveforms Z5 and Z8 in the bottom two grids show the virtually probed signal after the application of the model. Notice that in Z5 and Z8, since the probe is virtually placed at the pins of the DRAM, there are no reflections. Z6 and Z7 which are probed mid-bus have reflections on the rising and falling edges.
Eye Doctor II Software
The Eye Doctor II Software can be used to emulate (embed) or de-embed cables, channels or fixtures that have already been characterized and for which you have the S-parameters. It can also be used to add or remove emphasis, to equalize a signal using CTLE, DFE or FFE, and to model PLL clock recovery and tracking. This enables you to view and measure a signal as it would appear following all the signal conditioning normally applied to the channel.
A limitation of Eye Doctor II is that you can only de-embed one fixture or emulate one channel per circuit at one time. To emulate and/or de-embed more structures, use the VirtualProbe Software.
Another option for emulation or de-embedding is the VirtualProbe Software. This software also requires S-parameters of the characterized structures. You can simultaneously emulate (embed) or de-embed up to six structures on a single circuit using an ideal thru, transmission line parameters or a Touchstone file to model each structure (one “block” of the flow). You can even model cross talk using eight- or 12-port S-parameter files. The signal can be viewed and measured at a different point in the signal path than where it is physically probed—hence the concept of “virtual probing.”
All the Eye Doctor II and VirtualProbe capabilities described above can be accessed through the SDAIII-CompleteLinq Software option. SDAIII-CompleteLinq acts as a wrapper around the Eye Doctor and VirtualProbe software, adding them to the signal processing path along with the SDAIII eye, jitter and noise measurements. You choose to use either Eye Doctor or VirtualProbe as the emulation/de-embedding tool. Each lane of analysis can use a different de-embedding method.
The Processing Web is a netlist-based approach that can be used to embed or de-embed structures from a signal path on a Teledyne LeCroy oscilloscope. Used in conjunction with VirtualProbe or SDAIII-CompleteLinq (which provides the required math operator), this is the only method that allows you to account for non-linear/parallel processes, such as when the oscilloscope is utilized with an interposer and the signal “branches” in multiple directions, represented by three- or six-port S-parameter files. By contrast, the Eye Doctor and VirtualProbe applications assume a linear system that progresses from left to right without any branching, using two- or four-port S-parameter files to model the structures.
Take for example the DDR configuration diagram in Figure 11, showing the SOC, circuit board interconnect, interposer and DRAM, represented by the colored blocks in the diagram. The grey blocks in the figure represent non-physical elements of the netlist model. The signal path shown here is for DQS. CLK would have a similar block diagram. DQ would have a single-ended signal path, so its block diagram would look slightly different. The interposer for the differential paths is characterized by a six-port S-parameter file. A probe is connected to the interposer at test points 5 and 6 (shown on the blue D4 Interposer block in the image). By using the VirtualProbe math processor within a processing web, you could virtually “move” the probe to any other point in the circuit. For example, you could view the differential signal between N3 and N4 which is the output of the SOC. This point might not be accessible by a physical probe, but by using this method, you are able to view and measure the signal at that point.
Figure 12 shows how the processing web is set up. The three VirtualProbe blocks refer to netlist files that in turn refer to the S-parameter files representing the circuit elements of the configuration. The three blue lines represent the three DDR input signals (DQS, DQ, and CLK). The top two signal paths are the differential signals. The bottom one is the single-ended DQ path. After processing the input signals based on the linked netlist files, the VirtualProbe processors will output the de-embedded results (green lines) to math functions F9 through F11.
Processing web setup can be intricate. For more information on using this method, contact your local Field Applications Engineer or Teledyne LeCroy Customer Support.