| 摘要 |
In electronic systems involving both analogue and digital components, noise may be random or systematic. One major source of systematic noise is the system clock. Ultrasonic gas metering can use mixed signal ASICs, which are particularly sensitive to systematic noise. Components include a pair of ultrasonic transducers (132,134) which in turn transmit and receive signal pulses; a fast main clock (124) and a slower sub-clock (122). Signal pulses are transmitted to coincide with sub-clock edges. Systematic interference occurs when the arrival of a signal pulse coincides with another sub-clock edge. To prevent coincidence, the signal pulse transmission may be delayed by a variable number of main clock periods. In one embodiment, the delay is first incremented in steps of one main clock period then decremented in steps of one main clock period. In another embodiment, the delay is controlled by a microcontroller and the knowledge of the delay is used to avoid coincidence with sub-clock edges. |
