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Digital Pattern Generator

What’s a Digital Pattern Generator?

Digital Pattern Generators, also called Data Pattern Generators or Logic Sources, are electronic test equipment devices that are used to generate electrical digital signals or patterns to stimulate a device under test. These patterns can be either repetitive or single-shot (once only) in which case some kind of triggering source is required (internal or external).

Programmable Level Digital Pattern Generators

Unlike standard Pattern Generators that typically output TTL level signals, the programmable level variants are able to generate signals with different logic levels e.g. ECL, PECL, TTL, LVDS, LVTTL, CMOS or LVCMOS.. The user is able to configure a group of output bits (channels) with a programmable ‘low’ voltage level and a corresponding ‘high’ level. With the UF2e-7200 series of Programmable Level Digital Pattern Generators from Strategic Test it’s possible on every group of 4 bits to independently set the programmable levels between -2V and +10V while also having the option to produce differential signals.

Compared to High-Speed Digital I/O Instruments

High-speed Digital Input and Output instruments can also operate as Digital Pattern Generators. The UF2e-7000 series of high-speed Digital I/O cards from Strategic Test are able to generate TTL levels signals.

Applications

Digital Pattern Generators are used to create stimulus signals to electronic systems or components under test. One example could include a new Digital-to-Analog Converter integrated circuit where the digital pattern could stimulate the DAC to output a sinusoidal waveform. The developer could then use a combination of oscilloscope and FFT analyser to measure the dynamic parameters of the DAC, for example Signal-to-Noise Ratio, Total Harmonic Distortion, etc.

How do Digital Pattern Generators work?

The output rate for Strategic Test Pattern Generators can be configured between 1 kHz and 40 MHz.

The patterns to be generated can be derived from previously recorded signals, or can be created by defining these as a series of ‘0’s and ‘1’s on a time reference – in the same way as we constructed simple x/y graphs at school. Today the most popular method to create new waveforms is to use computer software such as MATLAB or LabVIEW.

A Digital Pattern Generator consists of five main sections:

  • A digital section to generate logic level signals. If a Programmable Level Digital Pattern Generator the output bits are driven by two DACs, one producing the ‘low’ level and the other responsible for the ‘high’ level. Different Strategic Test cards are available with 16 or 32 bits.
  • A memory section used to temporarily store outgoing digital pattern data. To generate digital output signals the pattern data file is transferred from the host PC into the Generator card memory and can then be output once, or continuously until stopped. When the data transfer to the card is made when the card is generating this is referred to as the Standard Mode of operation on the Strategic Test cards. Clearly, the amount of memory on the card determines the maximum duration for generation. However, longer signals can be generated if the signal data is continuously transferred while the generation process is ongoing. This method is known as the FIFO Mode or Streaming Mode on the Strategic Test Digital Pattern generator cards.
  • A bus section that is the electrical interface to the host computer. Its purpose is to transfer commands to the Generator card to set up the card for generation. The bus also transfers the data to be generated from the PC to the card.
  • A clock section that generates a clock signal that determines the pattern generation speed. It also generates internal signals for internal bus synchronization and refreshing the onboard memory devices.
  • A control section. This is used to control the previous four sections and transfer the data from bus to the onboard memory.
Digital Pattern Generators – Modular Instruments

Digital Pattern Generators are available as standalone instruments and as electronic cards that can be integrated with standard PC’s or cards that require an external industrial chassis based on the PXI, CompactPCI, VXI, VME or CAMAC bus standards.

Strategic Test supplies instrument cards based on the PCI Express, PCI-X, PXI and CompactPCI standards.

What about Multiple Instrument Card Synchronization?

Pattern Generator cards, such as those from Strategic Test are available with a choice of 16 or 32 bits (channels). Each group of 16 bits is fed with the internal sample clock to ensure that all of the outputs operate synchronously i.e. without time-skew errors between the bit outputs. Of course, each bit is able to generate totally different signal but the clock rate must be identical.

What would be the situation if more bits were required to test a certain system and multiple high-speed Digital Pattern Generator cards had to be used? If the Pattern Generators are PXI cards then these have the natural advantage that clock and trigger synchronization signals are contained on the bus for multiple card synchronization. However, the PCI Express, PCI-X and CompactPCI buses were not originally designed for instrumentation applications and so don’t have this feature.

Strategic Test solves this problem with the Star Hub option. This consists of a small daughterboard that is added to one of the PCIe, PCI-X or cPCI cards, which feeds the master clock and trigger synchronization signals to all the other cards via small flat cables. Star Hub enables up to 16 cards to be perfectly synchronized in one PC or chassis, while the System Star Hub option that is only available on the PCIe and PCI-X cards can also synchronize up to 17 Star Hub enabled PC’s equipped with up to 271 instrument cards.

What Are the Advantages of PCI Express, PCI, PXI or CompactPCI Cards over Standalone Instruments?

If you need the highest sample rates possible, then only standalone Pattern Generators have the capability to operate at clock rates to 2 GHz and beyond. At slower rates up to 125 MHz modular instrument cards based on one of the standard bus formats will usually be more cost-effective or the preferred choice when flexibility, memory depth, number of channels and system size are determining factors.

For example, consider configuring a mixed-signal system consisting of a Digitiser with 4 channels, an Arbitrary Waveform Generator also having 4 channels and a 32 bit Digital Pattern Generator. Strategic test could supply three PCIe cards that were synchronized with the Star Hub option that would fit in any low-cost good performance PC that can also be used to analysis and report the measured data.

The alternative standalone system would consist of three large chassis that may have synchronization difficulties and would most likely cost significantly more and have less memory depth to record or generate signals. In addition, standalone instrument tend to use older embedded processors that offer no possibility to upgrade and have a limited use compared to the versatility of a PC.