Instrumentation and Test Expert

RF Switching for Test Instrumentation (page 2)

The RF switching, traditionally co-located in an RFIU, is perhaps the most challenging element of the platform to make common among applications. We have designed and constructed an RFIU for our RF testing platform, but the reality may be that a Common RF Test Platform includes a special-purpose RFIU. With this in mind, we have designed the control software with a layer of abstraction above the RFIU control. For example, the software does not include the following pseudo-code, “connect RFIU port 2 to PNA port 1.” Rather the pseudo-code reads, “connect UUT i, port j to instrument k, port l.” Configuration data is kept separate from the software source code. In this way the RFIU, and even the instrumentation, can change with only changes to the configuration data and instrument drivers without requiring changes to the software source code.

The digital and analog control subsystem interfaces with the UUT, the RFIU, the instruments and any fixtures or auxiliary equipment. Our design for the digital and analog control subsystem included the following high-level requirements.
     • UUT digital communication of at least 140 MHz
     • Real-time, deterministic control
     • Automatic purge of customer data at power down
We also had requirements as to the number and types of communication signals based on a survey of potential users of the common platform. These are reflected in the design shown below in Figures 4. and 5.
In many ways our choice of RF instrument architecture drove our choice of control architecture. There was no PXI or VXI mainframe in the system and adding one to accommodate the controller would have been a significant penalty in terms of initial cost and rack space (rack space is important as foot-print in the laboratory or on the manufacturing floor is a driver of total cost-of-ownership). Thus, we chose a machine controller that is monolithic and highly-integrated. If a synthetic instrument architecture is feasible in terms of RF performance, the choice of machine control architecture and instrument architecture are linked. This is because the controller can be put on a common backplane with the RF instruments. This allows for very high speed (especially in the case of PXIe) communication between the controller and the RF instruments. Because there is already a mainframe in the system, there is no additional rack space required and no additional cost (other than the cost of the control cards). The ability to integrate the controller and the RF instruments on a common backplane is an advantage often heard from proponents of synthetic instruments.

LXI is an Ethernet-based standard that adds an interesting element to the discussion of architecture. The designers of LXI attempted to include the best elements of PXI without the constraints on physical form factor and the reliance on personal computer standards. With the advent of Giga-bit bandwidth and deterministic protocols, Ethernet becomes a very attractive “backplane.” LXI can allow a collection of traditional instruments to start looking “synthetic.” AIXe is a newer standard that is gaining traction and surely has the potential to further blur the lines between traditional and synthetic RF instruments.
The controller we designed and implemented for this automatic test equipment allows 140 MHz communication with the UUT, implements real-time and deterministic control, and loses all user-specific data at power down. It is extremely versatile because it is FPGA-based and includes a very wide-variety and large number of digital, analog and open-collector signals.

Following is a list of the types and number of signals implemented in our general-purpose controller. 
     • LVDS – 24 each
     • TTL - 24 each
     • RS422 - 24 each
     • DAC - 12 each
     • ADC – 12 each
     • RS485 - 16 each
     • O/C Relay Drivers – 120 each
     • SPI - 2 each
     • Clock I/O - 2 each
Note that our controller is 2U high and can communicate with the system computer either over Ethernet or USB. 

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