Hi Peter,
Not quite an answer to your question but I've just been reading this
article <http://www.eix.co.uk/Articles/Radio/Welcome.htm> where he has
an idea for using a 60kHz ceramic filter in a super-regen circuit for
receiving time signals.
May be worth a look.

RP

2009/9/17 Peter Restall <pete@restall.net>:
>
> 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 t=
hey
> can point me to - specifically LF crystal filters and their nuances. =A0T=
here'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 intereste=
d in
> learning more about/fixing. =A0The filter itself is basically a DC-blocki=
ng
> capacitor, a crystal and resistor in series, sandwiched between two
> non-inverting op-amp stages (with resistor to ground on the + input to he=
lp
> allieviate DC offset problems). =A0It seems to work well in terms of filt=
ering,
> but:
>
> * It is susceptible to pick-up due to the high impedances involved; would=
 it
> =A0be wise to shield this section of the circuit in in some way, such as =
by
> =A0enclosing the thing in a grounded bit of metal like a bit of copper cl=
ad
> =A0board ? =A0The pick-up tends to happen if I touch the crystal or a res=
istor,
> =A0which 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 t=
oo
> =A0narrow as it seems to increase the high-low and low-high transition ti=
mes
> =A0of the carrier significantly. =A0I get a nice sine wave from the outpu=
t, but
> =A0it's no good for decoding if it takes tens of milliseconds to decay; w=
ithout
> =A0the crystal the carrier switches in the blink of an eye, so I know I'm=
 not
> =A0hitting any op-amp slew limitations, etc. =A0Any suggestions for incre=
asing
> =A0the bandwidth ?
>
> I knocked up some SPICE simulations and did a lot of trial-and-error to g=
et
> something working, but would rather like some rules-of-thumb or more conc=
rete
> methods for designing something robust.
>
> As I said, any pointers to LF crystal filter information would be very mu=
ch
> appreciated - or perhaps some words of wisdom from one who has been there
> before !
>
> Regards,
>
> Pete Restall
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