On 6/17/2017 4:01 AM, Hal Murray wrote: >> Q3: It's only a 1Hz frequency, but is low inductance a desired trait of the >> chosen resistors? > It's a 1 Hz repetition rate, but the bandwidth depends upon the rise time. > If the rise time is ballpark of 1 ns, the bandwidth will be ballpark of 1 > GHz. So, yes, you want low inductance. That includes the power to the chip > as well as the resistors. Surface mount is your friend. So are ground/power > planes. > > How good is your scope? Not nearly good enough, as I don't have one. Have to borrow. Thank you for pointing me in the right direction on bandwidth...
>> Q: Why does everyone pick FIVE x 100 Ohm resistors? That's 20 Ohm out, not >> counting the gate impedance on the hex inverter... > The FIVE is 6-1. The one is for isolation. The 5 is the rest of the > package. You might as well use them all as drivers. You don't want to use > them for another signal (even if it is supposed to be identical) or you will > get minor crosstalk when you do things like plug or unplug a cable. Yes, I got this part... At least I generally get something. ;-) > I don't know why you are saying "gate impedance". That's over on the input > side. I would have said "output impedance" or "driver impedance" of the chip > or section. Apologize, was parroting the term used in the archived Time Nuts post I found referencing the need to address the "driver impedance" in selecting series resistors when trying to set the output impedance. > I'm not sure why they picked 100 ohms. Assume the net source impedance is 25 > ohms. Suppose the far end is terminated with 50 ohms. There won't be any > reflections so the source value doesn't matter. 25 ohms will provide a > higher voltage at the far end than 50. If you have a CMOS driver and a CMOS > receiver, 1/2 the voltage at the receiver is nasty. It might be OK if you > have HT type receivers. OK... I'll have to try and study this some more, because for a simpleton like me, and impedance mismatch between the PPS out port on the Nortel and the device to which it is connected of 25:50 ohms, reduces to 1:2, flipped around is a 2:1 VSWR, which means a fairly significant power loss... Maybe it doesn't matter much with a short cable.. but the cable is also tiny/lossy. > I would suggest a bit of lab work. What are you going to use on the far end? > Lots of gear has 1000 ohms rather than 50 so a 50 ohm source impedance takes > care of the reflections and leaves (almost) the full voltage at the receiver. Yes, I intend to do extensive testing. I was just trying to trim away some of the waste up front... time, materials, et al. :-) >> thus I can only include that I need to use something slightly more than 250 >> Ohms on a 5 gate parallel setup) > More than 250 divided by 5 and rounded up a bit for the output impedance of > the chip will be more than 50 ohms. I'd do some experiments. Yep... gonna need some more instruments asap... I may just have to wing it and accept "good enough" until I can round up the rest of the required instrumentation. Thanks for your assistance and not brutalizing me for my lack of knowledge. I'm trying to learn. - C. Autery _______________________________________________ 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.
