Delay lines are simple in concept but difficult to implement with precision and repeatability, especially over the broad frequency range needed for electronicwarfare (EW) and radar applications. But Colby Instruments has developed a patented electromechanical trombone mechanism that not only provides broad bandwidth and picosecond delay resolution, but makes delay adjustments over time and temperature with repeatability as good as 0.02 ps. The company offers several 18-GHz programmable delayline models using the trombone tuner alone or in combination with relays and semirigid coaxial delay lines.
Delay lines represent a difficult engineering challenge since they must affect the time-domain (delay) or frequency-domain (phase) characteristics of an RF/microwave signal, including pulsed waveforms, without altering its amplitude characteristics. And they must do so predictably over time and temperature.
The delay lines from Colby Instruments, such as the PDL-100A series instruments, build upon the total delay range available from an electromechanical trombone assembly (625 ns), and add precision calibrated semirigid cables for additional delay times from 1.25 ns to 100 ns. The coaxial cable assemblies are switched in and out by low-loss RF/microwave relays, with multiple assemblies used in series for longer delay times. Low-loss semirigid cables are used to minimize insertion loss even for long delay times. Insertion loss is a function of total delay time and frequency. For example, the typical insertion loss for a trombone-only 625-ps model PDL- 100A-625PS at 300 ps delay time is only 1.8 dB at 18 GHz. Obviously, the insertion loss increases as delays (and lengths of conductors) are added.
The PDL-100A series programmable delay lines are completely passive assemblies that are DC coupled with full 18-GHz bandwidth. For digital signal testing, this translates to the capability to add delays to signals at data rates from DC to 40 Gb/s. The programmable delay lines provide tuning precision of 0.5 ps per step or 0.18 deg. per 1 GHz in the frequency domain. Absolute accuracy is typically 0.1 percent per step, with worstcase repeatability of 0.02 ps for the 18-GHz models. The PDL-100A series delay lines can handle 10 W CW input power and 50 W peak power. They are specified for operating temperatures from +10 to +30C. The delays can be controlled by GPIB, Ethernet localarea-network (LAN), and RS-232C interfaces. The firm also offers the dual-channel, programmable PDL-200A delay-line series, with two trombones in a common housing.
In addition to standard delay lines, the company also offers a modular, original-equipment-manufacturer (OEM) version, model PDL-100A-OEM, for system integrators (see figure). The OEM version (see table) includes only the trombone assembly. The 18-GHz PDL-100A-OEM, as well as its longerdelay siblings, are designed for use with +5 and +28 VDC supplies and can be programmed by GPIB, Ethernet LAN, and RS-232C interfaces.
The company also offers several lower-frequency delay lines for applications through 3 GHz, which will be reviewed in Microwaves & RF. These 18-GHz units offer the frequency range, delay times, and precision in support of both EW and radar system and simulator applications. In addition, they can be used for adjusting the timing of digital data streams, for clock synchronization, and for adding phase offsets in phase-noise testing. Each of the units is rated for 500,000 delay adjustment operations before a recommended service interval. Colby Instruments, Inc., 1715 114th Avenue SE Woodridge Building, Suite 112, Bellevue, WA 98004; (425) 452-8889, e-mail: firstname.lastname@example.org, Internet: www.colbyinstruments.com.