[time-nuts] More on paralleling output gates

Robert Lutwak Lutwak at Alum.mit.edu
Sun Apr 9 16:04:49 EDT 2006


I suspect you're just measuring the bandwidth of your 400 MHz oscilloscope.


-RL

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----- Original Message ----- 
From: "John Ackermann N8UR" <jra at febo.com>
To: "John Ackermann N8UR" <jra at febo.com>
Cc: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>
Sent: Sunday, April 09, 2006 4:00 PM
Subject: Re: [time-nuts] More on paralleling output gates


> 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.
>


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