[time-nuts] wwvb 60 khz tuning fork crystals Some insights
n1hac at alum.dartmouth.org
Thu Jun 27 18:41:59 EDT 2013
Lower gain is better as long as it oscillates. The 74HCU04 is unlikely
to drive spurious responses. The 74HC04 is OK as long as you keep the
feedback gain low - sometimes a series resistor from the output to the
resonant circuit is required. A 74HC14 is the WRONG part for the job as
it can and will oscillate without the crystal controlling it - just try
it with a resistor for feedback and a capacitor to ground at the input,
On 6/27/13 6:30 PM, paul swed wrote:
> I will say the fact is the 74hc14 is a bit of a power pig we are talking 12
> ma. The rcvr is something much less like 100 ua. At least for the moment it
> all works but 12 ma is a pig.
> Especially when you take the signal out and knock it down to 100-200 uv.
> On Thu, Jun 27, 2013 at 5:37 PM, ed breya <eb at telight.com> wrote:
>> Still having email problems - here we go again. This is second try, please
>> excuse if both show up.
>> Hal Murray said:
>>> They make 74xU04 for many values of x. The U is for Unbuffered. They
>> lower gain in the linear region.
>> I thought they were intended to be used for things like this, but I don't
>> understand that area. Can anybody give me a quick lesson or point me at a
>> good URL?<
>> I always thought the unbuffered "U" versions were preferred for ring
>> oscillators mostly to save power - you don't want the high-drive output
>> stages to be cooking away in linear mode if not needed. The propagation
>> delay can also be less since the U ones have only one stage instead of
>> three (the building block is the totem-pole inverter stage), but they can't
>> drive very much load anyway. I think that most MSI and LSI parts that have
>> built-in ring/crystal oscillator sections use the U topology, but I don't
>> think there's anything special about it - it's the simplest thing that
>> I've made quite a few CD4000 and 74HC oscillators, and never worried too
>> much about U versions or not, except for battery-run items where power is
>> critical (or you can run the oscillator at lower voltage). Often they are
>> made from inverting gates that are part of a shared package, where you
>> wouldn't want puny drive capability in the other gates anyway. They are
>> relative power hogs though, whenever linear biasing is needed. Except in
>> the 4000 series, I don't know if U versions are available in anything but
>> the '04 hex inverter, but I suppose it's possible. I think the
>> Schmitt-trigger types like HC14 are necessarily buffered, so have three
>> stages, since you need a non-inverted version of the signal for the
>> positive feedback to the input.
>> I've never tried making one in 74AC - I don't know if it's even possible
>> to bias one up that way without it burning up. I'm working on some related
>> circuits now, so maybe I'll set up an experiment to see how much current it
>> would take for one inverter - I've often wondered about this.
>> I read about this years ago in various CMOS application notes, so I may be
>> missing some key points - there should be plenty of info online. The older
>> generation (when CMOS was fairly new) info may provide more detail about
>> the guts than that related to the newer, higher performance families.
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