Tuesday, January 5, 2016

Prj 141 - DDC Filter Design (Part 5)

Following on in the set of posts covering the digital down converter (part 1part 2, part 3 and part 4), this post comments on the design of the filters.

The Altera Application Note 455 “Understanding CIC Compensation Filters” provides a good overview of both CIC filters and the issues associated with designing compensation FIR filters for them.  It includes basic Matlab (or Octave) code to design the compensation filter coefficients.  This was used and expanded upon to plot different views of the data to help familiarize myself with the issues and get a feeling for the trade-offs.  The script used to produce all of the figures is available here

With multi-rate filters you have to be careful and understand at what frequency the operation is being applied at and how it translates to the up-stream and down-stream stages. The analysis is all based on the 0603 firmware version parameters.  Those are:
  • Fs = ADC sampling rate = 10MHz
  •  N = number of CIC stages = 4
  •  M = CIC differential delay = 1
  •  R = CIC rate change = 50,
  •  Final bandwidth = 100 kSPS (limited by SPI interface)
  •  FIR filter sample rate = 200kSPS (or 2x the final for a FIR filter decimation rate of 2),
  •  L = number of FIR filter taps or order = 96,
Said differently, Fs/R = 10MHz/50 = 0.2MHz = 200kHz.  The CFIR decimates by 2 to yield a final sample rate of 100kHz.  The cut off frequency of the CFIR is chosen to be slightly less than half of [ (Fs/R) = CIC rate] / [ 2=CFIR Decimation ] or 45kHz to allow for a transition band which yields a Fc = 45kHz/200kHz = 0.225 when normalized.

The number of CIC stages had to be limited due to the number of DSPs available in the Spartan6 LX9.  CIC filters without DSPs higher than order 4 cause problems with the map phase of the FPGA tools (i.e. they weren’t completing within an hour indicating layout issues).  The number of FIR filter taps was limited to 96 to avoid requiring additional DSPs to implement this filter.  Recall, that since the DDC is implemented discretely within the FPGA, there are two chains of CIC/CFIR one for I and one for Q.

To get a feeling for the response across the entire ban d, the CIC is plotted in three reference frames shown below.

CIC Filter Response 
Note that the current implementation uses a 10MSP ADC, however, the target is 60MSPS.  The difference is whether R is 50 or 300 and does not impact the rest of the design.  There is an antialiasing hardware filter prior to the ADC which limits input signals above Fs/2.  Using the Matlab or Octave fir2() function, a FIR filter with a response to normalize the CIC response close to unity gain over the first CIC Nyquist interval is designed.  The response of that filter looks like the following.
Compensation FIR (CFIR) Desired Response
In this example Fs/R = 200kHz, and half of that (or the first Nyquist region of that filter) is 0.5*Fs/R.  Since this will be decimated by 2, the true cut off frequency is 0.5 of this (slightly less in this case for some margin).  The following compares the CIC response over the target range along with the CFIR and the total response of the cascaded filters.
CIC, CFIR and Composite Response

The above graphs only show the response over the CIC first Nyquist which is the final goal for evaluating passband, however, we need to consider the full response over multiple intervals due to the periodicity of both filters. 
CIC, CFIR and Composite Response Over Multiple Nyquist Intervals of Final Sample Rate
 There is an increase in the composite filter response in the 0.5 to 1.0 Fs/R interval where the CIC filter is still decreasing toward the first null of its response but the CFIR has wrapped into its second Nyquist region and has a passband response.  We could address this by various filter parameters (e.g. M = differential delay of CIC or the decimation rate of the CFIR reducing the passband at the Fs/R rate).  The above figure also provides a feel for the relative gains of each filter in an absolute sense.  The following figure provides all of the gains normalized on a single graph similar to the Altera application note.
Normalized Composite and Constituent Filter Responses
Based on this figure the response is very clean up to around 0.78 Fs/R where the composite picks up to around -43dB based on the CFIR wrapping and the CIC not having rolled off very far.  The following provide two detailed views of the passband focusing on transition region and passband ripple.
Passband to Stopband Transition
Passband Ripple Evaluation
In short this filter works well for the intended purposes and fits within the LX9.

Related:
Prj141 Schematic
Prj141 Overview
Prj141 Digital Down Converter
Prj141 Digital Interface
Prj141 Software
Prj141 Filter Design
Prj141 Filter Evaluation
Prj141 LX9 Utilization
Prj141 Higher Sampling Rates

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