Specifying Delay Lines

By: Paul Alessandrini
Submitted: 2007-01-17 16:26:59
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THERE HAS BEEN a tremendous growth in the computer, television and radar fields. As a result, the demand for components that give time control over pulse information has led to the development of a great variety of delay lines; delay lines that find use in systems that relate electrical information to time. Computers, television studios, telemetering systems, guided missiles, navigation systems, identification coders and decoders, radar systems and video tape recorders are typical systems that use delay lines.

The selection of the proper delay device for a given application is not only important from an economic standpoint, but it can also contribute to the ultimate success or failure of a system. In order to understand and compare the different types of delay lines, it is first necessary to define certain parameters.

Basically, there are two types of delay lines: electromagnetic lines (http://www.ElectromagneticDelayline.com) and sonic delay lines(http://www.SonicDelayline.com) There are important differences between these two types. The electromagnetic line is limited in its time delay to rise time ratio to 250 :1. This limits the upper frequency response but the passband starts at d-c and extends to its 3 dB cut-off point. For long delays, or where the required frequency response is in the Megahertz range, this 3 dB cut-off point imposes a severe limitation and makes the choice of a sonic line mandatory at these higher frequencies.

The sonic delay lines can be broken down into two basic types: magnetostrictive and solid (ultrasonic) lines (glass and quartz). Both types feature very high frequency response and excellent temperature stability. However, their attenuation (40 to 70 dB), narrow bandwidth, and poor pulse fidelity has lead to the popularity of electromagnetic delay lines. Electromagnetic Lines

The electromagnetic lines are divided into two groups: distributed and the lumped constant delay lines. The distributed delay line is further broken down into two categories The stick line closely approximates a transmission line. It is fabricated by winding a coil (either a solenoid or a multilayer) on a rod (glass, ceramic or phenolic) that has been covered with a silver or copper coating. This conductive coating is the ground conductor. Between the coil and the ground plane is a thin dielectric layer. The coil provides a continuous and uniform inductance along the rod. The coil of wire and the ground plane act as a capacitor. The higher the dielectric constant of the dielectric layer, the greater the capacity.

The delay of the line, Td, is a function of the total inductance and capacitance (Td = √LC), as is the impedance (Z = √L/C). The stick lines generally do not exceed 2 μsec of delay and are limited in their figure of merit, rarely exceeding a value of 10, which requires a rod six inches long. Not all impedances are possible for every delay, due to a limit on obtainable capacity. The attenuation of the small delays is generally very low since these are wound with heavy wire. When the delays approach 2 μsec, however, the attenuation increases greatly and reaches a maximum of about 3 dB. The temperature coefficient is generally about 150 ppm from -55° to + 105° C. Sizes of the stick lines usually range from about 3/8" x 3/8" x 2" to 1/2" x 1/2" x 6".

Paul Alessandrini is an experienced MRI-RFI filter technician. Additional information regarding Fil-coils MRI, Delay Lines, Tempest, and HEMP filters can be found at http://www.custompowersystem.com

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