FIR filter configuration of older CMG-6TD instruments
This document describes the sequence of FIR filters used by the CMG-6TD instruments manufactured before the introduction of the CMG-CD24 digitiser.
Data from the modulator is decimated within the DSP 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 CMG-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.