Refinements Revitalize Variable Delay Lines

Feb. 28, 2005
Windows-based software and Ethernet and serial interfaces make these precision delay lines easier to configure for both time-domain and phase-shift applications.

Delay lines in the PDL Series from Colby Instruments (Bellevue, WA) brought precision delays to analog and digital testing from DC to 18 GHz almost two decades ago. Although capable of accurately adjusting delay lengths as long as 625 ps, this first generation of instruments was at times difficult to program and operate. Fortunately, the 2005 version of the PDL series is easier to use, with increased precision and range, a Windows-based user interface, TCP/IP Ethernet capability, and an upgraded communications controller.

The PDL Series programmable delay lines are used in a diverse array of applications ranging from synchronizing independent clock sources in bit-error-rate (BER) test systems to providing phase shift in phase-noise-analysis systems. They are also employed in shaping and timing laser pulses, and in development and testing of phased-array radars.

In every application, the success of the PDL Series delays is a result of their ability to provide extremely precise, repeatable values of delay over a wide range and over many thousands of operations. They are based on so-called "trombone" delay structures, which were conceived in the 1940s. Although providing excellent performance, trombone and other mechanical delays suffered from rapid wear at several key interfaces that reduced their utility to occasional use. However, Colby Instruments' founder Dr. Siegfried Knorr discovered a way to dramatically reduce wear at these interfaces, and developed (and subsequently patented) the technique (see sidebar). Although he knew his approach would make mechanical variable delays much more robust and usable in automated environments, it was only after years of service that it was possible to positively quantify just how great the improvement was. There are currently many PDL Series delay instruments in service that have performed over 500,000 operations without need for repair.

The most important enhancement to the PDL family is programmability via a PC using Windows-based software that is included with the instruments. The software controls the amount of delay and other parameters via commands over RS-232C or optionally Ethernet connections. The original GPIB control interface is still available, but the new serial and Ethernet interfaces reduce complexity and eliminate the need to buy a GPIB card and cable, saving several hundreds of dollars and freeing up a slot in the computer. The Ethernet TCP/IP interface allows the delays to be used in the latest ATE systems, and provides control over a local-area network (LAN) and supports the industry-standard VISA interface protocol.

In addition to remote control over a data bus, the PDL Series has always allowed the user to control the delay characteristics locally with its optional MT 1A microterminal that connects directly to the instrument. The handheld unit shows delay times and status information on its LCD display and performs a status test as well as self-calibration over the range of delay values.

However, the new software offers these functions as well as many more via a simple "virtual instrument" control panel on the PC display (see figure). For example, when entering delay values, the user simply enters the desired delay value via the numeric keypad and clicks on the "set delay" button. Delay settings can be stored in memory along with a short description, and each setting can be recalled by pressing a button. The step size dialog box allows pre-set and user defined step sizes to be entered. Step sizes are extremely useful when incremental or decrementing delay across a range of delays in "step" increments. The "position" dialog box uses similar slider controls to set delay and offers the ability to adjust the delay setting by simply moving a slider up or down to increment or decrement any one digit of delay setting.

Another tool should prove extremely handy for designers performing phase-noise analysis. Unlike most applications in which time-domain values of delay are required, phase noise analysis and other applications that employ delay lines to be used as phase shifters require values in the frequency domain (electrical degrees). While converting phase shift in degrees to delay in nanoseconds or picoseconds is a simple calculation, it is nevertheless time-consuming and formerly had to be performed manually by the user. The new software performs the calculation automatically. The user now clicks on the "phase" dialog box selection, enters the operating signal frequency, and can "dial-in" the amount of degrees of phase shift desired. The application program calculates and sets the corresponding delay value automatically.

The new PDL-100A employs the trombone technique to provide a total delay range of up to 625 ps with resolution of 0.5 ps, absolute accuracy of 0.1 percent 0.25 ps, and repeatability of 0.1 ps (see table). Delay extension options are available to add additional programmable and variable delay up to 100 ns. Most common options are 20 ns or 40 ns total delay. The PDL-100A with delay extension employs the trombone system to provide delay up to 625 ps along with relay-switched banks of coaxial delay lines to extend delay up to 100 ns in binary steps. The PDL-100A can be highly customized to meet specific delay requirements and will handle RF input power of 10 W CW and 50 W peak power. The delay lines are inherently free of jitter, noise, and phase noise, and have very linear phase characteristics over their operating frequency range. Colby Instruments, 1715 114th Avenue SE, Woodridge Building, Suite 112, Bellevue, WA 98004, (425) 452-8889, FAX: (425) 452-8802, e-mail: [email protected],

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