Projects/EchotekHardware/EchotekHardware/GC4016Filter

Graychip 4016 Digital Filter Design for Echotek

The script outputs CSV files for the cfir and pfir coefficients. It is intended to be used with the resampler bypassed as we don't have a good model for that unit. When using this script the filters are specified as half band (i.e. x2 for full RF bandwidth). Additionally if the stop band is beyond the nyquist rate the results on the actual hardware may not match the simulations. This is the likely cause of the passband shape seen with the Echotek provided filter. To achieve steep rejection it is necessary to have the final RF bandwidth less than the final sample bandwidth. You can push it a little bit and the point of transition to a problem situation isn't clear. RF bandwidths significantly narrower than the sampling bandwidth tend to work very well with the card.

The script is essentially derived intact from the Mathworks design example for their filter design toolkit. They saved a lot of time by figuring out all the config parameters for their tool for each Graychip stage.

Example scripts, config files, simulations, and data include :

• ion_line_script.m

  config file

  simulation

  data

  plasma_line_script.m

  config file

  simulation

  data

Note the data examples are for 10 second integrations through our MIDAS-M UHF tuner using the on board noise source. The notch at zero frequency is from "dc offset" removal which is done in the plotting software. Dynamic range is limited by the low strength of the noise diode. Larger sampling bandwidths show lower relative dynamic range for the same integration time.

The Ion Line filter has been optimized for a 200 kHz sample rate with very smooth RF to 100 kHz and usable bandwidth to 150 kHz. Passband ripple for this filter is less than 0.1 dB peak to peak. Filter roll off is extremely smooth and leads to a close rejection of better than 93 dB (< 1 MHz offset) and wide rejection better than 120 dB (> 1 MHz offset).

The Plasma Line filter has been optimized to provide wider fractional bandwidth at the expense of rejection performance. It has been implemented to use a 500 kHz sample rate with flat bandwidth to 400 kHz and usable bandwidth to 450 kHz. Passband ripple for this filter is less than 0.05 dB peak to peak. Filter roll off is extremely smooth and leads to a close rejection of better than 81 dB (< 1 MHz offset) and a wide rejection of better than 100 dB (> 2 MHz offset).

For reference the Echotek supplied filter we have been using :

• config file

  data