On 01/04/16 16:51, Gordon tekscopes@... [TekScopes] wrote:
On 01/04/2016 15:37, Tom Gardner tggzzz@... [TekScopes] wrote:
At 5V and using a 350MHz scope (Tek 485) the risetime/falltime is fractionally
under 1ns. Looking at the spectrum leads me to believe the risetime is
600-650ns.
That's the scope and generator rise summed of course. The 485 is spec'd
at 1ns so one, the other or both are faster but that's part of what I'd
like to know. If you know one for definite you can calculate the other,
within reason. Rightly or wrongly I thought it would be easier to
'tighten up' the layout with a single chip and I wanted something that
could be put together by anyone with reasonably predictable results.
David Hess pointed me to what I think was the origin of the paralleled
7414 circuit and I did try it but the overshoot/ringing bothered me a
bit. OK for the tdr which it was originally designed for I think but not
really good enough for calibrating overshoot!
I understand and accept that you aren't making definitive claims; neither am I :)
Overshoot is usually associated with inductance in ground leads, especially when using scope probes. I use it as an example of why all circuits are analogue and why probes matter.
There shouldn't be any overshoot if the outputs are directly connected to the scope with a 50 ohm transmission line. Using SMD gates allows the trace on the PCB to be very short, and I couple the board directly to the scope with a 2" BNC male to BNC male adaptor, which should be OK to 1GHz, maybe more.
None of which constitutes proof, so I would like to test it on a decent scope - that's why I didn't offer to help you.
Without calibrating the 485, it looks like
<>
It works down to a supply voltage <1.5V, with correspondingly lower output
voltages and much slower transition times, of course.
In contrast, 3 NC74SZ04 have a marginally slower risetime of about 1ns.
I based my choice on the minimum tpd which is lower for the NC7...
assuming the rise time has to support the minimum tpd but I'm not that
experienced at these things so we'll see.
I didn't care about the propagation delay, which is usually how fast the output gets halfway (i.e. to the threshold voltage). I was more concerned about the ability to drive a 50 ohm load all the way to the supply rails. The LVC family seemed to have a marginally better current driving capability near the supply rails, and the load is further reduced by having 3 in parallel plus resistors.
"The Art of Electronics" indicates the LVC and SZ are both "nice" at 5V, and that you can get a lower tpd with ALVC at 3.3V and AUC at 2.5V. Unfortunately AUC's current driving capability is also lower . I'm tempted to spin another variant of my board, sometime.
The 485 also has a very nice calibrator with a 1ns or less risetime. It is used to calibrate the 485's front end. It is effectively an ECL output with a 450ohm impedance thus forming a 10:1 divider when attached to a 50ohm load. The only "surprise" is that voltages incident on the 20pF (plus 50ohm) scope input cause a small reflection, which is again reflected when that hits the 450ohm output and finally appears as a short glitch at twice the cable's propagation delay.
Its output levels are calibrated by removing the transistors and using a DMM to measure and tweak the output voltages. Sneaky.
