[time-nuts] pulse height

shalimr9 at gmail.com shalimr9 at gmail.com
Mon Nov 26 11:05:19 UTC 2012


This kind of discussion comes up regularly, so I wrote this age: 
http://ko4bb.com/Test_Equipment/CoaxCableMatching.php

Didier KO4BB


Sent from my Droid Razr 4G LTE wireless tracker.



-----Original Message-----
From: "M. Simon" <msimon6808 at yahoo.com>
To: "time-nuts at febo.com" <time-nuts at febo.com>
Sent: Sun, 25 Nov 2012 3:09 PM
Subject: [time-nuts] pulse height

Depends on which end of the cable you are looking at. At the source end it is 2X the cable "length". At the sink end only 1 cable length. 

One to two uS should be more than enough time for all the reflections to die out from cable losses alone. So even in a badly mismatched situation you should see  the source voltage at the sink with lab conditions and an "open circuit" at the sink. Assuming a 10 to 20uS pulse. 

Thought experiment: Launch a one ns rising edge down your 10 ns long cable. When that pulse encounters a discontinuity (open) it will rise to about 2X the launch voltage - assuming a launch from a 50 ohm source with a 50 ohm cable - that is it rises to the source voltage.  When the reflected pulse gets back to the source the source will then also be at the source voltage (which is to say the source resistance disappears).  If the source is unmatched the reflections can go on for a while. 

Modern ICs (LVC say) are designed with about a 25 to 50 ohm output impedance to absorb line reflections. The older stuff (TTL and especially F) were low impedance devices so you needed a terminator in the line to kill reflections. About 33 ohms at the source seemed to work well. You wanted the smallest resistor possible (to minimize losses) while wanting something as close to 50 to 100 ohms as possible to absorb reflections. 
 
I had the very good fortune to work with all this back in '67 at Raytheon Computer when we were characterizing TTL for wire wrap. We used a Tek dual channel sampling scope capable of  measuring time to better than .1 nS. 

The rule generally was that the wire should be shorter than the rise time for unterminated lines. i.e. about a foot long for 1nS rise times.  

=====

Message: 8
Date: Sun, 25 Nov 2012 10:03:46 -0800
From: Said Jackson <saidjack at aol.com>
To: "M. Simon" <msimon6808 at yahoo.com>,    Discussion of precise time and
    frequency measurement <time-nuts at febo.com>
Cc: "time-nuts at febo.com" <time-nuts at febo.com>
Subject: Re: [time-nuts] pulse height
Message-ID: <67F262EB-8E0B-4AC1-8BC6-56D696664669 at aol.com>
Content-Type: text/plain;    charset=us-ascii

Mark, Simon,

First,
 I can check the 1PPS on my Z3805A with a small white LED. The pulse 
time is long enough to see in dark light conditions, and it verifies 
that the output voltage is >2V, and the driver can supply some mA of 
current. Primitive but works really well. Don't use a resistor on the 
LED.

Second, the travel time through an open ended coax is back 
and forth, so 2x the time of your calculation. During that time the coax
 will look like 50 ohms, afterwards just like a cap. So 10 feet of coax 
would give you more like 30ns or more load time. This assumes the edge 
is much faster than the tpd, say it has a 2ns rise time.

Bye,
Said

Sent from my iPad

On Nov 25, 2012, at 2:03 AM, "M. Simon" <msimon6808 at yahoo.com> wrote:

>
 Unless your coax is unusually long it should only look like 50 ohms on 
the leading and trailing edges of the pulse. A rough guide is  1nS per 
foot. So 10 ft of coax will look like 50 ohms for 10 nS. Of course the 
velocity factor of the coax will lengthen that time by roughly 50% for 
most coax you will find around the shop. 
> 
> You might 
want to set up a triggered 555 (556) or some such fed into some high 
speed CMOS (LVC or AHC etc) to see if it is your set up or the eqpt. For
 starters just the 555 will give you an indication. Start with 10 or 
20uS pulses at around 100 Hz and slow the rep rate down. 
> 
> Dumb question: are you using a 10X probe? Have you accounted for that? 
> 
> Simon
> 
> =====
> Date: Sat, 24 Nov 2012 16:52:50 -0800 (PST)
> From: Mark Spencer <mspencer12345 at yahoo.ca>
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Question re 1pps output on the Z3805A
> Message-ID:
>     <1353804770.27074.YahooMailClassic at web121001.mail.ne1.yahoo.com>
> Content-Type: text/plain; charset=us-ascii
> 
> Thanks all for the responses.  
> 
> I
> am routing the signal via 50 ohm coax to the 1Meg ohm input of a 100 
> Mhz analog scope using dc coupling.  After some fiddling with the 
> trigger settings I am able to observe a brief pulse every second that 
> appears to be .15 volts above ground.   I'm not inclined to believe that
> the signal is actually .15 volts though.  I did try terminating the 
> scope end of the coax with a 50 ohm load and the signal as displayed on 
> the scope didn't change.   I recall making similar measurements in the 
> past and seeing more reasonable results.    
> I really don't want to 
> blow the inputs on my 5370B's hence my caution here. I did try 
> connecting the signal to one of my 5335B's and based on the trigger 
> settings I was able to use to get that counter to trigger I'm inclined 
> to believe it is  3.3 or 5 volt TTL level but I'm not 100 percent sure.
> I'll
> have to do some more experimenting with my analog with other 1pps 
> signal sources to gain more confidence in my measurements and or ask 
> Santa for a digital storage scope (:
> 
> Regards
> Mark Spencer
> 
> 
>  
> 
> 
> 
> Engineering is the art of making what you want from what you can get at a profit.
> _______________________________________________
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