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Very basic crystal question

When I go looking for information on crystal filters, everything I can find seems to be aimed at using a bunch of crystals and capacitors to widen the bandwidth, or else it's marketing for people who would be happy to sell me crystal filters of the above sort. I don't want to widen the bandwidth...

What I'm presently interested in is an exceedingly narrow-band filter and/or amplifier. 60Khz for the monent, as I'm playing with WWVB - and while I can buy an off-the-shelf amplifier or even amplifier and digital buffer chip for $3-4 (CME6005 and CME8000) which uses an external crystal to get very narrow bandwidth, there's no real discussion of what, precisely, that's doing. As a practical person, I may end up buying one of these units as a major building block, but I want a better general understanding going into it, as I've really done very little with radio, and this is as much about learning what's what as the actual end result.

All the DIY WWVB receivers I've found are using LC filters, which is classic, but seems to be more prone to noise than a very narrow bandwidth filter would be, given that in this case the frequency of the signal is _very_ well defined - in fact, given that I can get "the correct time" rather easily from any network attached computer these days, the frequency reference is as much or more of a driving reason as the time codes are. I don't have any logical reason to spend money sending my elderly ovenized crystal reference out for calibration, but I'd still like to be able to maintain it on spec as much as possible - which is, in theory, possible from the WWVB signal. The units mentioned above claim to get about a 10HZ bandwidth, though I'd call the datasheet "sketchy" at best.

Is it as simple as "slap a 60KHz crystal in line with the input (or the feedback path of an amplifier), and only 60KHZ will get through" (perhaps with some bypassing to block any multiples: 120KHZ, 180KHz, etc.)? That seems a bit too simplistic, but other than a reference to Walter Cady's 1922 paper (which I have not tried to go track down in person) using single crystals for very narrow bandwidth coupling (in this article:

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), I've somehow managed to miss any discussion of very narrowband crystal filters at the most basic (or too basic to be discussed) level.

I need to clear the paper and clutter off my bench and get to playing with this, having finally gotten a sweep generator.

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Reply to
Ecnerwal
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I'm not familiar with WWVB, but a word of caution is advised: If you wish to recover modulation, the filter bandpass must be wide enough to at least match the highest modulating frequency; an L/C circuit or a multiple crystal network will do well. If all you want is to recover the carrier, then a single crystal will do an admirable job.

Here are a couple links to look at:

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The Bode Plot of figure 2 is valuable in showing that the pass and reject frequencies are different, and you need to deal with that by tuning the network.

And

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This has an assortment of filter design insight, but mainly, it has a simple, generalized schematic for a single crystal filter network, that might be helpful.

Assuming you will build a TRF receiver, the filter would be at 60 kHz., and the transformer would be fairly simple to build or buy.

Reply to
Don Bowey

Try searching for "crystal ladder filter". If that doesn't work, go to the ARRL site

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and get this year's Handbook or one of their radio circuits design books. They may even have a book specifically on crystal filters.

At any rate, it can be done. When you're building a cheap crystal ladder filter you often find that the center frequency isn't exactly the marked crystal frequency -- this would be a problem if you want a 60kHz filter, 'cause you'd have to buy custom-made, low-frequency crystals.

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Tim Wescott
Control systems and communications consulting
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Reply to
Tim Wescott

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