MODULAR COMPONENTS afford a great deal of flexibility in high-frequency circuits and systems, especially when there is some level of compatibility among components.

Common pin assignments and power requirements, for example, can go a long way to helping an engineer retrofit a function module from one manufacturer with the product from another supplier. Although function modules may not always live up to the expression "plug and play," it is useful to have them at least match an existing footprint.

The Special Report on microwave modules (p. 33) provides a brief look at recent developments at some technologies affecting module design, and a sampling of some products available in module packaging. What the story does not do is address test instruments modules. Once an area associated with "plug-ins" for high-performance oscilloscopes, or frequency cards for swept signal generators (such as the venerable HP 8350 series), card-and module-based instruments have gained a certain level of "respectability" relative to their larger and heavier rack-mount counterparts.

Performance earns respect, and the latest generation of test modules provide measurement capabilities comparable to "rackandstack" instruments. National Instruments (www.ni.com), for example, offers the NI PXI-5670 vector signal generator with a 22-MHz-wide realtime modulation bandwidth and tunable frequency range from 250 kHz to 2.7 GHz. Based on the PXI modular instrument format, it has an onboard 10-MHz oven-controlled crystal oscillator (OCXO) for frequency stability of ±20 PPB and frequency accuracy of ±50 PPB. LeCroy Corp. offers the 1-GHz PXD1022 PXI digitizer, with 2 GSamples/s sampling rate.

But all test modules are not created equal. While some test companies offer modules with PCI or PXI compatibility, others have adopted the LXI standard. The PCI (or peripheral component interconnect) bus was developed by Intel in the early 1990s as a low-level synchronous bus based on 32-b data transfer at 33 MHz. PCI evolved into CompactPCI and then PXI (PCI with extensions), which is interoperable with CompactPCI. VXI emerged as a military-oriented, higher-performance instrument bus defined around the VMEbus architecture. The VXI format works with a fast backplane for speed, but includes four different module sizes, although all use a common 96-pin connector.

LXI is also a test module format, strongly endorsed by the military. While the other formats are based on instrument functions controlled by software, LXI has function modules defined by software. With the right modules, the software could flip a spectrum analyzer into an oscilloscope. Each module format has its supporters, and only time will tell if the variations increase or decrease.

See Associated Figure