Method and apparatus for interference reduction

An apparatus comprising signal processing circuitry for processing a periodic data-carrying signal produced by an electromagnetic flowmeter to reject periodic interference from the data carrying signal at a first fundamental frequency and a second fundamental frequency, the second fundamental frequency being different from the first fundamental frequency,

• the periodic data-carrying signal being derived from an excitation signal having an excitation frequency, the excitation frequency being a sub-multiple of the first fundamental frequency,
• the apparatus comprising:
• comb filter circuitry configured to reject interference from the data-carrying signal at the first fundamental frequency and harmonics of the first fundamental frequency;
• wherein the comb filter circuitry has a comb filter delay period related to the first fundamental frequency;
• wherein the comb filter operation is configured to reject interference from the data-carrying signal at only the first fundamental frequency and harmonics of the first fundamental frequency;
• averaging or integrating filter circuitry in series with the comb filter circuitry, the averaging or integrating circuitry being configured to reject interference at the second fundamental frequency and harmonics of the second fundamental frequency;
• and
• wherein the averaging or integrating filter circuitry is configured to average or integrate over a period related to the second fundamental frequency.