[time-nuts] pulse height

Said Jackson saidjack at aol.com
Sun Nov 25 13:03:46 EST 2012

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.


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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