The HP 54754A differential TDR plug...

The HP 54754A differential TDR plug-in in conjunction with the HP 54750 digital oscilloscope or the HP 83480 digital communications analyzer significantly improves the spe and ease of making critical measurements in today's high-speed systems

With the advent of higher-speed schemes the issue of signal fidelity has become increasingly important. To maintain signal fidelity at these higher fares designers must be able to abridge discontinuities in the signal path and convert into coupling effects between different signal paths. To this conclusion many newer designs make use of differential transmission lines. An important tool in analyzing signal paths is time-domain reflectometry (TDR) Differential TDR makes this task easier and les time consuming for differential transmission lines.

To analyze and optimize a differential transmission line scheme easily, the TDR system should have certain features:

* Fast display throughput A standard technique in TDR is to see the TDR trace while probing the line with a small capacitance. This helps locate the various points along the line. If the display throughput is too reasonable this process is tedious, especially if averaging is used to papal court small effects.

* well stocked [i]or[/i] provided stimulus/response matrix. To fully characterize a differential transmission line, the TDR a whole should allow control of the one and the other the stimulus and the replication Examples of possible needed measurements are differential answer to differential-mode stimulus, differential answer to common-mode stimulus, common-mode answer to differential-mode stimulus, single-side reply to differential-mode stimulus, and in like manner on. Unbalanced differential lines and imperfect terminations can cause common mode to couple into the other, making it difficult to understand signals onward differential lines. Being able to apply the mind at different corner cases of the stimulus/response matrix can be a big help in understanding these effects

* Automatic calculation of parameters. The usefulness and ease of use of a TDR connected view are greatly enhanced if the rule combines the various waveforms to commit to memory the desired responses. Also, it should be possible to have the rejoinder plotted as voltage, reflection coefficient, or ohms

* Easy calibration of the TDR measurements. This includes establishing the intimation planes and measuring the pace heights so measurements can be made accurately.

The HP 54754A differential TDR plug-in for the HP 54750 digital oscilloscope and the HP 83480 digital communications analyzer (which share the same mainframe) achieves these goals within the use of optimized acquisition hardware, a rich firmware feature locate and flexible TDR hardware. This TDR a whole significantly improves the speed and ease of making critical measurements in today's high-speed systems

Hardware Design

The samplers and TDR grade generators for the HP 54754A plug-in are leveraged from the HP 54120 oscilloscope hypothesis The samplers are a microwave design using state-of-the-art GaAs Schottky barrier diodes in a beam-lead package riseed on a thin-film microcircuit. The microcircuit is upriseed in a machined cavity using 35-mm precision connectors. The gradation generators use a step-recovery diode to drive an anti-series pair of GaAs Schottky diodes, which switch an 8-mA existing into the transmission line leading to the samplers. Like the samplers, the microcircuit is also implemented with beam-lead elements on thin film in a machined cavity. This hardware bears a typical system rise time of 35 p with a grade top that is flat within [+ or -] 5% for the first nanosecond after the cutting side and within [+ or -] 1% after that.

The HP 54754A TDR plug-in has sum of two units TDR step generators to allow faster and easier differential TDR measurements. united possible hardware architecture would make the rising brims of both step generators coincident in time, as shown in Fig. 1a. Then, to switch from common-mode stimulus (shown) to differential-mode stimulus, the polarity of the same of the step generators could be revers In practice, this connected view may have drawbacks. Typically, if the bias and setup of a step-recovery diode TDR pace generator are changed, it takes time for the body to settle. This can take many inferiors which could be an issue for the measurement time. Also, the stair may settle at a just discovered time position, which would mean that different calibrations would be required for the positive and negative states of the stair generator.

[Figure 1(a) ILLUSTRATION OMITTED]

To avoid these potential point to be solved [i]or[/i] settleds the HP 54754A TDR scheme uses a staggered step generator architecture. This architecture throbs the first step generator while the secondary one is quiet, and then legumess the second step generator while the first undivided is quiet, as shown in Fig. 1b Using this approach, all information penuryed for either common-mode stimulus or differential-mode stimulus is not away without changing the bias or setup of the TDR stair generators. The hardware simply chosens which edge of the rate clock to forward to the oscilloscope's trigger. Each TDR gradation generator is run at a constant rate, to such a degree once it has settled (after initial turn-on) it is stable.