| 摘要 |
A processing system (10) includes a backplane bus (13) that defines a plurality of locations (20-31) implemented as backplane slots each for connecting any option module (41) thereto. Each slot includes contact pins (201a-d) that carry binary logic levels forming a different digital number at each slot. Option modules include their own source of timing signals (220) comprising an oscillator (221) and an identical thermally-sensitive crystal (222) for driving the oscillator. Each option module includes receptacles (207a-d) for the contact pins. The receptacles are connected to the digital input port of a D/A converter (225) whose output port is connected to a heater (224) mounted in physical proximity to the crystal in the crystal's case (223). Depending on which slot an option module is mounted in, the D/A converter receives a different digital input and hence generates a different level of output. The heater generates heat in proportion to the converter's output and hence raises the temperature of the crystal to a different level at each slot, causing the crystal to generate somewhat different fundamental frequency at each slot. Hence option modules connected to different slots generate different fundamental frequencies and therefore emit different harmonic frequencies. Hence the amplitudes of the harmonics emitted by different option modules are non-additive, and radiated emission levels of the processing system equipped with all option modules are no higher than those of the system equipped with only one option module.
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