Comparing the measured and computed bandpass shapes of the digital filters

sep 2002


The computed band passes.
Comparing the actual spectra with the computed band passes.


     The interim correlator input is filtered by  8 analog filters (50 Mhz wide at the 260 Mhz IF... one for each sub correlator).  It is then sampled at 100 Mhz using an 8 bit A/D converter. For bandwidths of 25 Mhz and below the  real data stream is digitally down converted to complex samples I, Q streams. Each I, Q stream is half band filtered 1 to 8 times (bandwidths 25,12.5,6.26,....195Mhz) and then digitally upconverted back to a real data stream. The resulting data is then converted to 3 or 9 levels and sent to the correlator chips for processing. The figure below shows 1 of the 8 paths:

  1. This is a 50 Mhz anti aliasing filter.
  2. The 260 IF is down converted to 0 to 50 MHz (real signal)
  3. The 8 bit digitizer real samples the 50 MHz bandwidth at 100 MHz sampling rate.
  4. The digital filters down convert to I, Q complex samples and then filter each of these to 25 Mhz.
  5. This is the half band filtering chain applied once for each octave of filtering requested. Every two stages the output bits are shifted by 1 bit to compensate for the reduction in bandwidth.
  6. The I, Q complex samples are filtered and then upconverted back to a real signal.
  7. This last step generates a data stream twice as fast by interpolating the real samples. It is needed for double nyquist sampling. It is done for all bandwidths less than 25 MHz. If double nyquist is not used then every other sample for the output is kept.

Computed band passes.

   A the computed band passes  are shown in the figure. On the low frequency side the green, dark blue, and purple lines overlay one another.
processing: x101/digfilter/

Comparing the computed band passes with real data.

    The  plots show the actual spectra with the computed band passes. Data was taken from a noise source in the downstairs IF/LO (so it will not include any ripples from the upstairs if/lo or standing waves from before the horn). The data from correlator board 1 (polA) was used. The low frequency left edge of all of the band passes is the same and matches the computed bandpasses pretty well. The 25 Mhz band falls off differently but this is probably because of the filter shape of the 50 Mhz analog filter. The right edge drop-off of the 12 Mhz bands follows the computed bandpass. The 25 Mhz drop is a bit steeper than the computed one (again probably because of the analog filter).

    All of the bands below 12 Mhz have bandpass shapes identical to the 12 Mhz shape.The interleaved band passes were odd because they jumped between the 12 Mhz and the 25 Mhz shapes for different boards at the same bandwidth  (see measured bandwidths less than 25 Mhz). This only happened for bandwidths less than 25 Mhz. It must be the extra interpolation stage at the end that is only done for bandwidths < 25 Mhz that is causing this.

    When data is taken without an off position, the computed digital filter band passes can be used to remove the digital filter shapes. This can extended the useable parts of the band pass.

processing: x101/digfilter/