FIR filter configuration of older 6TD instruments

This document describes the sequence of FIRFinite Impulse Response - see Wikipedia's article on FIR filters for more information filters used by the 6TD instruments manufactured before the introduction of the CD24 digitiser.

Data from the modulator is decimated within the DSPDigital Signal Processor - see Wikipedia's article on DSPs for more information to 1000 Hz. This is done using cascaded ÷5 and ÷4 filters with the coefficients given below.

After this, four further sets of FIR filters are used to decimate the data. ÷2, ÷4 and ÷5 filters are available; however, up to two FIR filters may be applied at each tap, allowing you to choose decimation factors up to 10.

• Coefficients for decimation by 2
• Coefficients for decimation by 4
• Coefficients for decimation by 5
• Decimation by 8 = decimation by 4 then decimation by 2
• Decimation by 10 = decimation by 5 then decimation by 2

The 6TD allows the user to select a decimation factor for all four taps. However, the hardware does not support a 500 samples/s output rate, so the first tap must be set to a factor of ÷4 or ÷5.

Other information, such as bit weights (in μV/count), normalisation factor, and poles and zeros, can be found on the instrument’s calibration sheet.