Oculus is a software suite for measurement and design. It comes in several configurations consisting of one or more of the following modules:
- Designer generates eye-pattern diagrams (EPD) for a transmission channel from S-parameter data. For more details see the section on Oculus Designer.
- tdVNA extracts vector S-parameter data from TDT data. Bandwidths of approximately 10 GHz are routinely achieved. This module turns a TDR/TDT instrument into an inexpensive alternative to either a two port or a four port VNA. For more details see the Oculus tdVNA section.
Oculus Designer assists in the design and evaluation of communication channels. In it's simplest form, this consists of assembling a series of two port components as shown below. These components can either be specified in terms of S-parameters or can be one of the built in component types (transmission line, lumped element, etc.)
Designer simulates the signal transmission through this circuit which can be displayed in several different ways. For example the simulated eye pattern diagram (EPD) of a 2 GB/s psuedo random bit sequence (PRBS) of length 2^10-1 passing throuhg the above circuit is shown below.
The simulated output can also be displayed as a 2-dimensional plot of the bit error rate (BER); this is similar to the EPD output, except that that instead of showing the trace density, the bit error rate at that location is shown instead. Another output format is the bathtub plot, which shows a slice of the BER plot through the center of the eye on an XY plot. Examples for each of these plots is shown below:
A powerful feature of Oculus Designer is the ability to automatically compute values for filter components so as to equalize the signal, improving the quality of the transmitted signal. The values for the RC filter in the section above were computed in this manner. Below, we show the EPD for an unequalized version of the circuit as well as the original, equalized EPD.
Multi Level Signals
Oculus Designer supports the simulations of multilevel (2, 3, 4 and 5-level) signalling. Shown below is the EPD for a 4-level signal propogating through the circuit shown earlier. Note how equalization is even more critical to maintaining an open eye for the 4-level case than for the 2-level case because of the reduced margins.
Oculus Designer also supports crosstalk components as shown in the following circuit. The crosstalk is represented using 4-port S-parameter data.
The epd can vary dramatically when there is crosstalk versus when there is not any crosstalk and shown in the following two images:
Oculus Designer is a powerful yet simple tool that allows the simulation and evaluation of a large class of communication channels. If you would like more information or have a specific question please write contact us at email@example.com
Oculus tdVNA measure the S-parameters of cables and other passive components using time domain transmission/reflection (TDR/TDT) data. Two types of measurements are available: for compact components (see below) full 2-port or 4-port S-parameter measurements are available. S21 measurements are available for both compact and extended component.
Compact components are those for which the time it take a signal to travel through the component is small relative to the duration of the signal being used to characterize the component. In other words, if the electrical length of the component is L and the the duration of the signal being used is T then the condition that must be satisfied is Lc≪T.
In practice this means that connectors, small circuit boards and short cables are appropriate for taking for full 2 or 4-port measurements, while long cables are appropriate for S21 measurements. In general, this works out well since cables typically have a well defined impedance and return loss (S11) is less of a concern.
The simplest measurement to take using tdVNA is a S21 measurement. The measurement procedure consists of two simple steps. First, a response calibration is performed by taking a through measurement, then the device under test (DUT) is connected and a second measurement is taken. An example of a through measurement, a DUT measurement and the resulting S21 data is shown below.
For 2-port measurements, a through-reflect-match (TRM) calibration is performed by successively connecting these three standards to the measurement ports and measuring the response. The resulting calibration is more accurate than the response calibration discussed above and corrects the return loss (S11/22) in addition to the insertion loss (S21/12). The DUT is then connected to the measurement ports, four time domain measurements are taken and the results are computed as shown.
For 4-port measurements, 2-port calibrations are performed at ports 1/2 and 3/4. Then, a single through measurement between ports 1 and 3 is performed to combine the two cablibrations into a single 4-port calibration. Once again, the DUT is then connected to the four measurement ports, sixteen time domain measurements are taken and the results are computed. Below we show a set of 4-port time domain measurements and the resulting S-parameter data.
Oculus tdVNA measures 2-port and 4-port S-parameter data using a time domain reflectometer. Because it uses a TDR rather than a VNA it is both more economical and more familiar to those with a time-domain rather than microwave background than a full fledged VNA. Although it's tdVNA is not appropriate for all uses, VNAs have a larger dynamic range for example, tdVNA can fulfill many common S-parameter measurement needs. If you would like more information or have a specific question please write contact us at firstname.lastname@example.org
Coming soon to the Apple App Store
SnPViewer is an iOS app that enables viewing of SnP (Touchstone) files (*.s1p and *.s2p) files on an iOS device. SnP files received via Mail, Dropbox or any other App that allow implements Apple's Open In functionality.