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Hybrid microelectronics at Hughes

Most of the chips that Hughes Microelectronics was making were of a special sort called "hybrids." Hybrid chips combine two different kinds of semiconductor devices on a common substrate. For use in the high-stress world of weaponry, these hybrid chips are then hermetically sealed in metal or ceramic packages so they are protected from environmental stress. The circuitry is thereby isolated in an inert gas atmosphere of helium and nitrogen. This sealing protects the chips from corrosion and other environmental damages.

Figure 1 Simple, Un-Sealed Hybrid Microcircuit

Figure 2: Examples of Hybrid Packages from the Mid-80’s

There were over 70 programs for which Hughes Microelectronics was manufacturing hybrid chips from 1985 to 1987. The chip for each program was different. Because of military secrecy as well as company secrecy, exact specifications of the chips are unavailable. But we provide here an example of a common hybrid that was likely among those Hughes was manufacturing at the time: the Analog-to-Digital (A/D) converter.

Analog-to-Digital Conversion basics

One purpose of Analog-to-Digital conversion (A/D conversion) is to change a continuously variable analog signal into discrete digital signals that can serve as input to a computer.

An analog signal is a continuously variable physical signal. It can take many forms: radio wave, cellular phone transmission, radar signal etc. Usually, in A/D conversion the analog signal takes the form of an electrical current. An electrical current has a continuously changing voltage, and, when illustrated, usually takes on a sinusoidal wave form. The complex sine wave you see here is a combination of several waves, some of which might be "true" signal and other noise.

Figure 3: An example of a sine wave

A digital signal is a representation of this wave form as a discrete set of numerical values. All data that can be 7understood by central processing units (CPU) are in this discrete binary form. To understand how hybrid A/D converters function, it is important to note how A/D conversion works on the circuits. The circuit receives an analog signal (usually an amount of voltage) and simultaneously compares it to a set of reference voltages.

Figure 4: An analog to digital comparator

The number of comparators vary by the speed of the hybrid microcircuit (8-bit, 12-bit, 16-bit etc.). Each comparator is one Least Significant Bit (LSB) higher than the comparator immediately below it. If the input voltage level is above the reference voltage, the comparator takes on a "1" value. If the input voltage is below the reference voltage, then the comparator remains a "0" value. The output is thus in binary code as a sequence of zeros and ones that show where the voltage was at that instant in time. The process described above occurs at a specific sampling rate measured in Megahertz (megahertz is a million cycles of electromagnetic currency alternation per second). The input voltage level continues to change and is regularly sampled a certain amount of times per second. These discrete samples are then run through the comparators which produce the binary code.

When the binary code switches from 0 to 1, the actual voltage level is somewhere between the fixed levels of the two comparators. So, the more comparators there are in the circuit, the smaller the differences between the fixed voltage levels in the comparators can be. The closer the comparators fixed levels are, the better the computer’s guess is to the original input voltage level.

Figure 5: Computer representaation of an analog singal at a sampling rate of five

Figure 6: Computer representaation of an analog singal at a sampling rate of thirteen


In addition, the more frequently the samples are taken, the closer the digitized representation of the voltage wave is to its actual form. You can see this easily by thinking of sampling occurring every time there is a dot in this figure vs. every other time there is a dot. The faster sampling rate gives a much better picture of the incoming signal.

So, increasing the density of sampling and increasing the density of comparators are the two factors that can make A/D conversion more accurate.

So, a hybrid A/D chip has two kinds of technology on it. There has to be technology to receive, amplify, and relay analog signals. There also has to be technology to sample the signals and run the samples through comparators in order to change them to binary code.

These complex chips were then sealed in metal or plastic, so that the extreme variation in battlefield environment would not damage them. If they leaked when they were shipped form the factory, they could become damaged more easily in the extremes of battlefield environment. And if they became damaged (e.g. by heat, or cycling from heat to cold, or by shock), they could fail in a variety of ways. They might give a wrong signal or they might give no signal at all. If they gave a wrong signal, this might not be detected, and the missile might be mis-targeted or the airplane guidance system might give incorrect readings. In the heat of battle, or even in training runs, this could have lethal consequences.