In the past, IEC 60050-446 (Part 446 of the IEV) listed the terminology for all types of electrical relays. In order to improve the precision of terms and definitions as well as the general visibility of relay terminology, it had been decided to split that part of the IEV into three separate parts optimised for the specific relay types covered.
Therefore, this part of IEC 60050 gives terms and definitions for elementary relays (nonspecified time all-or-nothing relays) superseding the terminology of IEC 60050-446 for such relays. The terminology for time relays is contained in IEC 60050-445 established in parallel with this part. A new part for measuring relays and protection equipment (IEC 60050-447) is also under preparation. When all these three parts are published, IEC 60050-446 shall be withdrawn from the IEV.
The first edition of IEC 60050-444 was published in 1973. It includes physical and electrical specifications for a PI that approximates the purely resistive equivalent circuit of Figure 1 (the capacitance Cp is ignored). The construction technique described uses disk and rod type resistors. Two trimmers per Figure 2 are recommended for adjusting the phase response that is introduced by the network (which is largest when the network is short circuited) to be flat within +/-0.5 degrees over a frequency range of 1 to 125MHz. The PI is used to measure Zero-Phase frequency and resistance of crystals.
The second edition of IEC 60050-444 was split into several parts, each referring to different aspects of crystal measurement. Only Part 1 addresses the electrical specifications of the PI, while the remaining parts cover application and adaptation of the PI to various crystal measurements (see Reference 1). Part 1 of the second edition was published in 1980. One major difference compared to the original specifications is an extended frequency range to 200 MHz. The rod and disk construction and the use of trimmer capacitors per figure 2 are maintained. The performance specifications are again aimed at keeping the phase response flat within narrow limits, so that the PI Network is assumed to be approximately resistive as shown in Figure 1. As in the original specifications, the PI is used to measure the crystal's zero-phase frequency and resistance.
In summary, the IEC 60050-444-1 specifications cover the physical construction, electrical performance, and measurement purpose of PI Networks:
The construction is based in the use of rod-and-disk type resistors and trimmer capacitors, which makes it unsuitable for miniaturization.
The performance specifications are designed to reduce stray capacitance effects, such that for calculation purposes, the PI can be considered to be purely resistive. The measurement purpose is to determine a crystal's zero-phase frequency and resistance from 1 to 200 MHz.
The advantage of the IEC 60050-444 specification is that a calibration at only one frequency suffices for measuring over the whole frequency range, and that a relatively accurate evaluation of the crystal parameters can be based on a simple equivalent circuit.
This advantage was all-important before the advent of affordable process computers, but it is no longer sufficient for modern applications with requirements such as small size, extended frequency range, and increased accuracy.
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