12MHz xtals and QER Filter

Rahul Srivastava


I have been scratch building a uBitx and had been experimenting with the 12 MHz QER filter. I have noted few things and adopted some from various postings on group.

1) With the set of xtals that I have with 100pF caps and 200Z I/O filter BW is around 1.85Khz for HC49S xtals and 3.3Khz for HC49U xtals. 
    With filter based on HC49U xtals lower BW is achieved using 150pF caps but impedance drops down to 150-160 ohms. Lowering the
    BW also de grades the shape factor to around 1:2. This can be well understood considering that HC49U xtals have a lower motional inductance Lm.
2) As suggested by Alsion 82pF is working just fine in the filter. With BW of around 2.2Khz. Capacitor value is bit touchy between 82 to 100pf.
3) Q of capacitors used has a major impact on filter response. Parallelling 2 values to arrive at desired value results in a better response.

I am attaching sweeps of few different combination's on my SNA.

I would love to hear from others regarding their experience in this regard.


Rahul VU3WJM  

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