On Sun, 31 May 2009 11:03:38 -0600, Clint Turner wrote:

> - The use of a quartz crystal as the bandpass element.  These crystals 
> are cheap (available from Digikey and others) and have passband 
> responses that will easily pass the WWVB signal and its modulated 
> sidebands while providing an extremely narrow bandwidth that will reject 
> a great deal of "garbage" signals from adjacent frequencies.

Clint, thanks for the input.  I would've acknowledged you sooner, but I've
been looking at what you suggested for the last couple of days - I had no
idea you could use regular crystals (as opposed to specially constructed ones)
in this way.  It raises an interesting possibility - I already have a couple
of 60KHz crystals I bought for an earlier receiver attempt.

I would like to create a single-pole crystal filter, but am having trouble
finding any decent resources - design notes for ladder filters with >= 3 poles
don't seem to be a problem though.  The closest I've come to what I want so
far is Intersil's app note AN9815, but the equations don't seem to work for
me.  Also, the filters people use are always several MHz; nothing on the
nuances of using lower frequency crystals.  Or matching networks for them
(don't want to use transformers like some examples I've seen).


> Looking at the datasheets for the chips that use these crystals can be 
> informative, as it gives one a general idea how to work with these 
> crystals to deal with their usually-high series resistance as well as 
> appropriately load them for the bandwidth appropriate for a WWVB 
> signal.  Even if going to a simple TRF scheme is anathema to one bent on 
> using DSP-type techniques, the lessons learned (AGC techniques, quartz 
> bandpass filter, etc.) from these data sheets are still useful.

Have you got any datasheets in particular in mind ?  I've looked at some
(generally CMAX) and they are drawn as black boxes.  I've noticed that the
ESR is very high as you said (20K according to my datasheet), and the drive
level is very low (1uW).  I dare say that I will need to measure some crystal
parameters rather than rely on the (poor) datasheet - my crystal type is
MC-306 and it basically gives very wide ranges for *groups* of crystals.
Have you any recommendations on such measurement equipment/techniques ?

After Olin pointed out that FIR filters were not the way to go and pointed me
to the simple lowpass recursive filters, it didn't take long to get my DSP
simulation working with less computations, so that's the way I'd still like
to take the project - but 'bent on it' might be a bit strong !  I also shelled
out for some dsPICs and a nice DSP book, so I'd at least like to use the shiny
new techniques I've learned !

I reckon that using a crystal as a bandpass filter will make a nice addition
and should hopefully clean things up for the amplifier and ADC; and they're
small and pretty cheap.

The problem is finding decent information though - 'fish waving' seems quite
the norm.  Design equations are harder to come by; and when you do they tend
to be incomplete (ie. in the Intersil app note I mentioned, is the V_t
used in their equation meant to be the p-p or RMS signal voltage, or can it
mean the maximum voltage of any out-of-band signals too ?)

I've come across various references to Wes Hayward's works in old QST
magazines and some programs (like AADE filter designer) that do the
calculations (but crash on single-pole designs !), but the 'net is rather
scant in this area :(  Any pointers to material to get me going ?

And thanks again for the suggestions Clint - it's raised more questions
than provided answers though !

Cheers,

Pete Restall
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