Rahul VU3WJM has been scratch building a µBitx and experimenting with the 12 MHz QER filter. He notes the following:
1) With the set of xtals that I have with 100pF caps and 200Z I/O filter the bandwidth is around 1.85Khz for HC49S crystals and 3.3Khz for HC49U crystals.
2) With a filter based on HC49U crystals, a lower bandwidth is achieved using 150pF caps but impedance drops down to 150-160 ohms. Lowering the
bandwidth also degrades the shape factor to around 1:2. This can be well understood considering that HC49U crystals have a lower motional inductance Lm.
3) As suggested by Allison KB1GMX, 82pF is working just fine in the filter and achieves a bandwidth of around 2.2Khz. Capacitor value is bit touchy between 82 to 100pf.
4) The Q of capacitors used has a major impact on the filter response. Parallelling up two values to arrive at a desired overall capacitance value results in a better response.
To see sweeps of a few different filter configurations select the reference on the list. Rahul would love to hear from others of their experiences on the 12MHz crystal filter.