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,
no crystal.
David N1HAC
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.
Regards
Paul.
On Thu, Jun 27, 2013 at 5:37 PM, ed breya <[email protected]> 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
have
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
works.
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.
Ed
______________________________**_________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/**
mailman/listinfo/time-nuts<https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts>
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
