[time-nuts] More on paralleling output gates

John Ackermann N8UR jra at febo.com
Sun Apr 9 16:00:54 EDT 2006


Here are the revised screen shots; no big difference but I hate having
(obviously) bad data out there.  I also switched from 2ns/division to
1ns/division for better accuracy.  The test setup is the same as before.*

This time, using a different chip, a single gate had a rise time of
2.70ns, while the three paralleled gates measured 2.48ns.  Since the
last go round had the single gate being faster, I think this is showing
variability among chips.  The conclusion I draw is that the risetime
(10-90%) is less than 3ns, and there's no significant difference between
a single gate and paralleled gates, other than the increased output
voltage of the parallel combination.

Since the rise time is about the same, but the voltage is greater, I
guess you could conclude that the slew rate of the parallel combination
is greater than that of a single gate.

For what it's worth, the risetime of the MAX232 output is about 2.5us,
in a very smooth ramp -- it's spec'd at 4 v/us.  I have heard that the
risetime is deliberately slow to match the RS-232 spec; I don't know
whether that's true.  The later version MAX232A is spec'd at 12 v/us;
still not all that speedy.

John

* An HP 5359A time synthesizer (transition time spec'd at <5ns) set to
generate a 1MHz, 50% duty cycle square wave through a 4 foot RG-58 cable
driving a TADD-3 board set for 50 ohm input termination.  The TADD-3
output went through a 4 foot HP 50 ohm test cable (the ~RG-58 thickness,
highly flexible, gray jacket type) into a Tek 2465B 400MHz scope with 50
ohm internal termination.
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 74AC04_1_gate_scaled.jpg
Type: image/jpeg
Size: 40124 bytes
Desc: not available
Url : http://www.febo.com/pipermail/time-nuts/attachments/20060409/b4553558/attachment-0012.jpg 
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 74AC04_3_gates_scaled.jpg
Type: image/jpeg
Size: 40529 bytes
Desc: not available
Url : http://www.febo.com/pipermail/time-nuts/attachments/20060409/b4553558/attachment-0013.jpg 


More information about the time-nuts mailing list