ZTEC Instruments - Test and Measurement Blog

One of the strengths of using modular instruments is the ability to write programs to perform an automated set of tasks. Generally this is done though a programming environment and a standard programming language like ANSI-C or COM. Though this may be the norm; there are also other ways to communicate and even quickly program using the standard SCPI string interface.

Using ZTEC's ZFind command interface, it is easy to type in simple commands and queries:

Figure 1: ID Query

Some more efficiency can be gained using “Tree-Walking” this lets you send multiple commands/queries on the same line:

Figure 2: Tree Walking

Since the interface saves the command history, it is simple to change between a few commands using the up/down arrows on your keyboard. However, this starts to be less efficient with more than a few commands.

The command history can also be saved in its entirety. With the Save History option, you can save all of the commands that have been sent to a simple .txt file.

Figure 3: Command History

More advanced functionality is available through the command scripting. This allows the upload of a set of commands in a text file. This can either replay a previously saved command history, or can run though a manually generated list of commands.

Figure 4: Command Scripting

Figure 5: Command Scripting Results

Now the instrument can have complex programs run against it, using only a text file instead of compiling a program. The commands are standard SCPI, are provided in the instrument manual, are not case sensitive, and either the full or abbreviated command can be used. 

In today’s electronic battlefield, the radar support system is crucial for success. In order to ensure that these systems are prepared for real-world encounters, the system must be thoroughly tested. Threat simulation and response measurements can be accomplished using a RF synthesizer and arbitrary waveform control source.

The goal of threat simulation and response measurements is to replicate the target radar’s characteristics, and to test the target radar susceptibility to foreign signals. There are three main specifications that need to be taken into account when generating control signals:

1. Pulse Repetition Interval (PRI) - Time between output pulses
2. Pulse Width (PW)
3. Scan Rate - The amplitude modulation of the output which simulates the rotation and beam shape of the target radar antenna.

Figure 1. Example of PRI and PW
Figure 2. Example of an amplitude modulated signal.

The scan rate output modulation is independent from the output pulse. This requires a minimum of two output channels to properly stimulate the RF synthesizer. The ZT5210 waveform generator can be used to generate the two signals required from a single instrument. This channel density simplifies the development of software as well as hardware requirements and physical layout.

Scan rate is derived from the antenna beam shape and the rotation of the radar antenna. The simplest example of this is a standard AM modulated waveform (see Figure 2). As the beam shape and rotation of the antenna changes, the amount of time at the highest peak decreases, and the nulls become longer. This can be generated using an uploaded waveform based on actual antenna performance.

In addition, the ZT5210 has a specific output mode that is extremely useful for radar simulation. Burst output mode makes puslse generation at a specific width and repetition interval easy to accomplish. This will minimize the development time required for simple radar pulse simulation.

Threat emulation is also needed to thoroughly test a radar system. A common threat is a walking target which simulates an object traveling to or from the target radar. This signal can be generated internally on the ZT5210 waveform generator using the built in Serial Data functionality. Serial Data allows the user to send a specific serial digital word (up to 64 bits) to the output. Sixty-four discrete locations for the target can be defined by adjusting any bit up or down. Serial Data can also be used to generate several targets by having 2 or more bits enabled at the same time. In addition, mathematical bit manipulations can simulate multiple targets traveling to and from the radar source.

Additionally, the user can upload an arbitrary waveform and generate any specialized signal that may be required. This upload functionality can also be combined with the waveform sequencer to create specific test patterns and scenarios that need to be tested. For example, a more natural threat path (smoother steps) can be generated. This can be extremely useful when the target radar is using advanced signal processing.

Most conventional digital storage oscilloscopes (DSO) offer 8 bit ADC resolution.  The 8-bit resolution may be sufficient for a visual waveform display, but will not be adequate for many applications that need to detect small changes in large signals.  With 8-bit resolution, the DSO cannot distinguish a signal level less than 0.39% of full-scale (see table). Consequently, an 8-bit DSO is unsuitable for many applications including transient signal capture, communication signal reception, frequency domain analysis, and semiconductor testing. As an alternative, high resolution DSOs offer more than 8 bit resolution to address these applications.

Comparison of specifications related to oscilloscope bit resolution:

Analog specifications such as noise, distortion and accuracy are more relevant for a high resolution DSO. Analog front end signal conditioning must have sufficiently low noise and distortion to not engulf the extra bits of ADC resolution.  Accuracy may also be important for some applications. A high resolution DSO with high accuracy can enable multimeter-like measurements at high speed.

The following lists example applications for high resolution digital oscilloscopes:

1. Transient signal capture:  Transient events range from electronic signals to physical phenomena.  Often, transient signals have signal components and structures that must be resolved in the presence of large signals.  For example, a high resolution DSO can be used to simultaneously capture the turn-on transient and the steady-state ripple of a DC power supply.
   
   
2. Communication signal reception:  Modern communication signals often encode digital data into small amplitude and phase changes within a sinusoidal carrier.  A high resolution DSO can be used to resolve the small changes between different symbols in the encoded signal transmission.
   
3. Frequency domain analysis:  A Fast Fourier Transform (FFT) is a common mathematic algorithm used to transform a time domain DSO signal into the frequency domain for spectral analysis.  The quantization noise of an 8-bit DSO would limit the ability to resolve signals beyond its 50 dB dynamic range.  High resolution DSOs provide the additional dynamic range necessary for spectral analysis of sound, vibration, noise, audio, video, etc.
   
   
4. Semiconductor testing:  When testing a semiconductor device such as a high-speed DAC, the DSO must have more resolution than the semiconductor device under test.  A high resolution DSO can be used to characterize the dynamic performance of high dynamic range semiconductor devices.
   
   

The Agilent / HP E1429B VXIbus oscilloscope is a differential oscilloscope that has been used for years in many ATE systems. Differential inputs are especially useful when input signals have undesirable common-mode components that must be rejected. The E1429B has been obsolete for years and a replacement is not available from Agilent. Fortunately, ZTEC has extensive experience with VXIbus oscilloscopes and the discontinued Agilent instruments in particular. The ZTEC ZT412-50 VXI is a modern VXI oscilloscope that provides extensive functionality, in most cases beyond that of the E1429B. Whereas the Agilent E1429B is a low-sample rate differential 12-bit oscilloscope, the ZT412-50 is a high sample rate pseudo-differential 16-bit oscilloscope. The E1429B has two separate input paths for each of its two ADCs: a single-ended path and a differential path. The ZT412-50 provides pseudo-differential inputs by using a math channel to difference a pair of its four single-ended inputs (using four ADCs and a DSP processor). The resulting differential waveforms for both oscilloscopes will be similar, but the internal architectures are different. Note that the ZT412-50 has an input offset zero self-calibration function that is particularly useful to zero out the common mode error for the pseudo-differential configuration. The table below provides a complete list of the differences between the E1429B and the ZT412-50. Note that the other specifications not listed below will be comparable for both instruments.

Because the instrument functionality, command set, and I/O ranges are different, there is some effort involved with the integration of the ZT412-50 in place of the E1429B. ZTEC’s application engineering team has significant experience with the functional differences between the E1429B and the ZT412-50, as well as the application details of using VXI oscilloscopes in ATE test programs. This experience is invaluable when assisting a test system integrator in successfully modifying an existing ATE system or TPS. Having successfully assisted customers with instrument replacement issues, the team at ZTEC appreciates the effort involved in additional test software development, validation and support required to replace an obsolete instrument.

Sometimes it seems that the problem with modern digital storage oscilloscopes is that they have too much information. With our latest M-class DSOs we provide a whopping 40 on-board measurements that can be performed on up to 12 channels. With so much it can be a challenge to read it all. To deal with this we came up with a great, but simple, feature that I find inordinately useful: measurement lists.

A measurement list is a set of measurements that can be enabled and read as a group. If lists are enabled during run-time, the measurements are performed automatically, rather than waiting until the user requests them. The lists can also be disabled so that the measurement setup remains, but the measurements are not performed. I think it's great that you can set up groups, and it's even more useful with multiple lists; I like setting up different lists for different tests. For example, one list could have frequency-based measurements on all the capture channels, while another could have voltage-based measurements. Alternately, multiple lists could be set to do the same measurements, but on different channels. This system also works great in a visual interface, where lists can be set up as tabs. Quickly tabbing between the lists allows for a very quick look at the measurements.

One other thing that makes these lists extremely useful is that they are included in the system configuration settings. That means that the measurement list setup can be saved and recalled. The instrument can even boot up with the desired measurements already enabled! With 4 lists of 8 measurements and 30 possible saved configuration states, it is possible to have a setup ready for dozens of tests.

Support of obsolete instruments in Automatic Test Equipment (ATE) systems is an important concern for the US military and its allies. The US Department of Defense (DoD) has significant investments in ATE hardware and Test Program Sets (TPS) software based upon older pieces of test equipment. See Legacy TPS support and instrument compatibility keeps VXI in Military ATE for a discussion on legacy concerns in the DoD.

When a hardware instrument is discontinued by its manufacturer, the ATE system integrator is forced to upgrade to a newer instrument. In order to successfully migrate to a new instrument, comprehensive knowledge of the instrument functionality and the ATE system is critical. Applying new hardware to existing applications may involve some instrument integration and test modifications. One class of instruments that is particularly prevalent in long-lifespan ATE test sets are the discontinued line of HP/Agilent oscilloscopes including the E1426A, E1428A and E1429B. This discussion addresses the difficulties and tradeoffs associated with using newer VXI oscilloscope replacements for obsolete Agilent VXI oscilloscopes.

ZTEC has approached instrument replacement in two ways:

1. by providing a nearly-identical drop-in replacement for a discontinued instrument, or
2. by providing a state-of-the-art replacement with a good software interface and comprehensive application support to assist customers with the translation of old TPSs to new hardware.

Here are two examples that illustrate the two different approaches.

1. Replacing the E1428A using the ZT1428 VXI
2. Replacing the E1426A using the ZT452-01 VXI

The Automated Test Systems (ATS) Executive Directorate of the US Department of Defense (DoD) is responsible for the automated testing of electronics systems and components used in DoD weapons systems. An Automatic Test System (ATS) includes Automatic Test Equipment (ATE) hardware and Test Program Sets (TPS) software. The ATE hardware and TPS software assets must be supported for decades to match the long lifecycles of the weapons systems that they are used to test.

Backward compatibility requirements have kept the VXIbus (VXI) platform at the forefront of military & aerospace ATE systems. Unlike other bus platforms (PXI, PCI, PXIe, etc.) which follow the lifecycle market trends of personal computers, VXI has remained stable for decades. Although minor enhancements have occurred over time, the VXI platform has remained backward-compatible since the original specification was published in 1987. VXI enables the continued use of the existing investments in hardware ATE and software TPS assets. Other features of the VXI architecture, such as ±24V backplane supply voltages, high-level message-based command protocols, and GPIB common command support allow VXI to maintain a strong position in the marketplace. Although ZTEC continues to offer cross-platform instruments that have these same features in VXI, PXI, PCI and LXI, most DoD ATS programs still prefer VXI.

Finding a suitable replacement for an obsolete test instrument is challenging. Matching the instrument functionality required to run legacy TPS software may be nearly impossible, requiring additional test software development, validation and support. This process is always expensive. According to the DoD, the average cost to migrate a legacy TPS is $250,000 USD. Understanding this, ZTEC prioritizes the backward-compatibility needs of its military and aerospace customers. As new instruments and features are introduced, ZTEC’s oscilloscope and waveform generator products also maintain the legacy functionality required for ATS programs such as CASS, RT-CASS, TETS, and IFTE. As new technologies emerge, current and legacy test requirements remain equally important and cannot be overlooked.

One of the key features of the recently released ZT5210 M-class Waveform Generator is the arbitrary waveform library. The 8MSample volatile block of memory is completely user definable allowing a maximum of 4096 waveforms of independent sizes to be uploaded to the library. The arbitrary waveform library uses an allocation table of 512 sample sectors to provide simple and efficient control of the library space.

When a user requests an upload to the waveform library the allocation table quickly determines available free space, allocates sectors, and assigns a waveform handle. Once this is done, uploading the waveform into the predefined handle marks the allocation table entries and handle as valid. The uploaded waveform can be any number of samples up to the total number of allocated sectors. i.e. A user requests a waveform upload of 1000 samples such that two sectors are allocated with the waveform handle and a total space of 1024 samples is available. If the user decides later that slightly more or fewer samples are required multiple uploads to the waveform handle will overwrite the existing data in the library without having to allocate more space.

Once a waveform is in the library it can be copied to the output channel directly or used as part of a sequence. Sequence waveforms on the M-class Waveform Generators allow a user to specify any waveform in the library as a stage in the sequence and loop that waveform up to 65,535 times before advancing to the next stage of a sequence. Generating a sequence waveform creates the waveform in the output memory. Once built in output memory a waveform sequence can be copied into the waveform library for use as an independent waveform.

Although the terminology for the different classes of generators is not standardized, there is a general market consensus on the naming conventions for signal, function and waveform generators.

A signal generator provides a high-fidelity sine wave signal ranging from low frequencies to many GHz. Attenuation, modulation, and sweeping are typical features of a signal generator.

A function generator is a lower-frequency instrument that typically provides sine, square, pulse, triangle and ramp waveforms. Function generators provide these standard functions from DC to a few MHz, and provide large voltage ranges.

An arbitrary waveform generator (AWG) is a highly flexible signal source that generates any arbitrary waveform that has been constructed in digital memory point-by-point. The constructed waveform is converted to an analog signal using a digital-to-analog converter (DAC) operating at clock rates up to a few GHz. The variety of waveforms that may be generated with an AWG include standard functions (sine, square, pulse, triangle, ramp), non-standard functions (sin(x)/x, exponential, cardiac, noise, etc.), compliance test waveforms (video color bar pattern, AM/FM radio tones, encoded communication test signals, etc.), a combination of signals (multi-tone, noisy sine wave, or digital pulse stream with transient spikes, etc.), or the playback of signals captured with digital oscilloscopes.

The following table summarizes some of the basic differences between the three generator types.

Many RF and communication applications use both a signal generator and an AWG. The AWG is used as a modulation source that is applied to the RF signal generator. This allows the highly flexible AWG to generate a complicated or lengthy modulation signal for the signal generator’s high-frequency sine wave output. Often a two-channel AWG is used to provide complex I/Q vector modulation for the RF signal generator.

All three classes of generators have relevant applications. In fact, some arbitrary waveform generators can operate as conventional function generators by using on-instrument algorithms to generate standard functions. These dual-mode sources can be applied as either arbitrary waveform or function generators, making them very useful laboratory instruments.

One of my first tasks at ZTEC was to design a Graphical User Interface (GUI) for release with the first M-Class product: the ZT4610 Digital Oscilloscope. I’d used many benchtop oscilloscopes before, and was familiar with our modular oscilloscope drivers, but learning how to combine the two was not as simple as I initially imagined. I found that the differences in the method of interaction can make the same functionality be perceived very differently.

The first place that I saw this difference was when I tried to convert our functions into knobs. I had to ask myself: "What does a scale knob do?”. Well, clearly it changes the waveform settings, but what parameters exactly do they affect? The more I thought about it the less obvious it became.

First I tackled the voltage amplitude knobs. Here I had to decide if turning the knob clockwise should make the amplitude larger or the waveform larger. I found that while programmer writing an automated test that wants to change the vertical scale will consider the peak-to-peak voltage that should be captured, a user interacting with a visual interface considers the visual waveform size. Both methods are, in essence, changing the window. However, a programmer will think in terms of the capture parameters (the window), but when a user is interacting with a visual interface, the 'window' is a fixed display screen and the sensation (if not the reality) is of changing the waveform to fit. While the results may be the same, the user experiences differ diametrically.

This is only one example, but there are many ways in which the user experience can affect GUI design in interesting and often unexpected ways. I am continuously learning ways to take the powerful tools provided by our programmatic interface and expose them in a way that makes sense visually.