One is the upper capacitor in the schematic, three are paralleled to form (I'm saying "on" and "off" for simplicity the signal will be a sine wave.)Ī large part of the integrated circuit is occupied by five capacitors. The bias current is set midway between the drive transistor's on and off currents so the current flowing in and out of the capacitors balances out. The bias voltage sets the drive transistor's gate midway between "on" and "off", so the voltage oscillations on the crystal will turn it on and off. The bias voltage and current circuits are an important part of this circuit. Thus, the feedback from the drive transistor strengthens the crystal's oscillations to keep them going. The capacitors, reducing the voltage across the crystal. I couldn't find any information on the part number SC380003.Īs the voltage across the crystal increases, the transistor turns on, feeding current into the capacitors and boosting the voltage across the capacitors (and thus the crystal).īut as the voltage across the crystal decreases, the transistor turns off and the current sink (circle with arrow) pulls current out of The chip was manufactured by Motorola, with a 1986 date. Two pads (select and disable) are left unconnected. The brownish-green regions are the silicon that forms the integrated circuit.Ī metal layer (yellowish white) wires up the components of the IC.īelow the metal, reddish polysilicon implements transistors, but it is mostly obscured by the metal layer.Īround the outside of the chip, bond wires are connected to pads, wiring the chip to the rest of the oscillator module. The photo below shows the tiny integrated circuit die under a microscope, with the pads and main functional blocks labeled. It is mounted on the ceramic substrate and connected to theĪ surface-mount capacitor (3 nF) and a film resistor (10Ω) on the substrate filter out noise from the power pin. On the right side of the module is a tiny CMOS integrated circuit die. ![]() ![]() Inside the oscillator package, showing the components mounted on the ceramic substrate. This module produced a 4.7174 MHz clock signal, as indicated by the text on the package. (It is the "Rasco Plus" rectangular can on the right, not the square IBM integrated circuit.) The module is packaged in a rectangular 4-pin metal can that protects the circuitry from electrical noise. I examined the oscillator module from an IBM PC card. There's more happening in the module than I expected, so I hope you find it interesting. In this blog post, I discuss how the module works and examine the tiny CMOS integrated circuit that runs the oscillator. In the 1970s, crystal oscillator modules became popular, combining the quartz crystal, an integrated circuit, and discreteĬomponents into a compact, easy-to-use module.Ĭurious about the contents of these modules, I opened one up and reverse-engineered the chip inside. 1Ī quartz crystal requires additional circuitry to make it oscillate, and this analog circuitry can be tricky to design. Wristwatches were revolutionized in the 1970s by the use of highly-accurate quartz oscillators.Ĭomputers use quartz oscillators to generate their clock signals, from ENIAC in the 1940s to modern computers. Quartz oscillators were introduced in the 1920s and provided accurate frequencies for radio stations. ![]() Since a crystal can be cut to vibrate at a very precise frequency, quartz oscillators are useful for many applications. The quartz oscillator is an important electronic circuit, providing highly-accurate timing signals at a low cost.Ī quartz crystal has the special property of piezoelectricity, changing its electrical properties as it vibrates.
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