Hi CMOS is what you want rather than TTL in this day and age. Propagation delay and jitter are both much better than any of the TTL families.
NC7SZ125: Prop delay 2.6 ns Output high 50 ma max 32 ma rated Output low 50 ma max 32 ma rated (rated = ~0.5V off rail at 4.5V supply) 74S140 Prop delay 6 ns Output high 10 ma Output low 24 ma 74AC04 Prop delay 1 to 10 ns (call it 5 ns) Output +/- 50 ma max, +/- 24 ma rated (all above are ok on 5V) and we could go on and on and on….. Anytime you do this, you run into odd things on spec sheets so comparisons are never perfect. I would run things at as high a voltage as possible without creating smoke. The chips are normally faster and quieter at the higher voltage. ( = you can see a phase noise improvement going from 5.0 to 5.5V …). I would go with a “simple” gate rather than a Schmitt trigger. Phase noise on the simple gates has measured lower each time I’ve checked them. Having the switch points at different levels also tends to mess up duty cycle just a bit. (Good luck measuring that with fast edges …..). Bob > On Mar 18, 2019, at 2:43 AM, Julien Goodwin <[email protected]> > wrote: > > The 74S140 (dual 4-input NAND) is the classic TI TTL 50-ohm driver. > They're still available in DIP & SOIC. > > I'm currently waiting on some boards to arrive so I can try SN74LVC1G17 > as a more modern replacement that's much cheaper and smaller. > > Once I've proven it I'll do a separate thread to the list about the > whole project, since it's on-topic. > > On 18/3/19 1:27 pm, Bob kb8tq wrote: >> Hi >> >> CMOS is good. The faster the better. Go to your favorite vendor and find >> whatever is “king” this month >> and that’s what you go with. HC has been around a *long* time and is about >> as slow as it gets in terms >> of 5V CMOS ( yes, 4000 series is slower still …). Faster means less delay >> in the gate (so lower variation >> in delay). It also turns out that faster means lower phase noise so lower >> jitter. >> >> Buffers are better than inverters, simply because they have higher output >> drive capability. One “favorite” >> are the x125 and x126 tri-state buffers. NC7SZ125 and NC7SZ126 are two >> examples. >> >> If you want to drive 50 ohms, you likely will need to parallel gates in >> order to handle the current. There is >> an ongoing debate about source termination vs load termination vs “both >> termination” vs “don’t bother”. >> I would suggest that terminating one end or the other is a … errr …. really >> good idea. >> >> Lots more information in the archives. >> >> Bob >> >>> On Mar 17, 2019, at 6:39 PM, Taka Kamiya via time-nuts >>> <[email protected]> wrote: >>> >>> I'm very curious, too. Especially when I *just* built one using 74HC04N. >>> >>> --------------------------------------- >>> (Mr.) Taka Kamiya >>> I'm stuck in a wormhole.... Hello, worms! >>> >>> On Sunday, March 17, 2019, 4:01:26 PM EDT, Hal Murray >>> <[email protected]> wrote: >>> >>> >>>> For 1PPs the main goal is jitter and the selection of the ICs is very >>>> tricky >>> >>> Could you please say more? Do you mean logic family selection, or chip >>> selection within a family? >>> >>> Most modern digital chips are CMOS with an input threshold of VCC/2. I'd >>> expect power supply noise to be important and families with faster >>> switching >>> times probably make more noise. >>> >>> >>> -- >>> These are my opinions. I hate spam. >>> >>> >>> >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >>> and follow the instructions there. >> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. >> _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
