Good evening,

I was playing about with some low frequency crystal filters at the weekend
(60KHz - it's the ongoing epic that is my MSF receiver :) and I was just
wondering if anybody out there has any good sources of information that they
can point me to - specifically LF crystal filters and their nuances.  There's
tons of info on the 'net about MHZ region stuff, including Wes Hayward's
articles - all very useful if I was building a higher frequency filter.

I've noticed a few things about my filter that I'm specifically interested in
learning more about/fixing.  The filter itself is basically a DC-blocking
capacitor, a crystal and resistor in series, sandwiched between two
non-inverting op-amp stages (with resistor to ground on the + input to help
allieviate DC offset problems).  It seems to work well in terms of filtering,
but:

* It is susceptible to pick-up due to the high impedances involved; would it
  be wise to shield this section of the circuit in in some way, such as by
  enclosing the thing in a grounded bit of metal like a bit of copper clad
  board ?  The pick-up tends to happen if I touch the crystal or a resistor,
  which is unlikely to happen when it's in a project box (eventually).

* I believe the Q of the filter is very high - perhaps the bandwidth is too
  narrow as it seems to increase the high-low and low-high transition times
  of the carrier significantly.  I get a nice sine wave from the output, but
  it's no good for decoding if it takes tens of milliseconds to decay; without
  the crystal the carrier switches in the blink of an eye, so I know I'm not
  hitting any op-amp slew limitations, etc.  Any suggestions for increasing
  the bandwidth ?

I knocked up some SPICE simulations and did a lot of trial-and-error to get
something working, but would rather like some rules-of-thumb or more concrete
methods for designing something robust.

As I said, any pointers to LF crystal filter information would be very much
appreciated - or perhaps some words of wisdom from one who has been there
before !

Regards,

Pete Restall
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