[time-nuts] FE5680 missing PPS soln

d.seiter at comcast.net d.seiter at comcast.net
Sun Feb 5 02:23:43 UTC 2012


I was hoping that pulling the lock LED from my setup would clear up my PPS signal, but no luck. I'm now getting a 4.545Khz sawtooth with 10Mhz noise. I need to peek at the input to the 74AC240. 

-Dave 

----- Original Message -----
From: "Attila Kinali" <attila at kinali.ch> 
To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com> 
Sent: Friday, February 3, 2012 4:55:20 AM 
Subject: Re: [time-nuts] FE5680 missing PPS soln 

On Fri, 3 Feb 2012 09:50:52 -0000 
"Rob Kimberley" <robkimberley at btinternet.com> wrote: 

> Mine was a green LED out of the junk box, but I did use 330R in series. 1k8 
> seems rather large. 

Then i'm not surprised that your FE-5680 refused to work :-) 

Just calculate: 5V supply, approx 2V LED voltage -> 3V over the resistor. 
3V/330R = 90mA. As i wrote already, the 74AC240 has it's absolute 
maximum rating at 50mA. For a general logic output, i wouldn't recomend 
to draw more than 10mA (as Bob Grant wrote, the lock signal doesn't 
seem to pass trough the 74AC240). Hence, i would say that 330R seems 
rather small :-) 

Keep in mind that you drove circuit at over current for quite a long 
time. The circuit is most likely damaged (if they dont have an over 
current limit, which i doubt). If for an unknown reason the lock signal 
fails in the future, it might be due to this. 

Attila Kinali 
-- 
The trouble with you, Shev, is you don't say anything until you've saved 
up a whole truckload of damned heavy brick arguments and then you dump 
them all out and never look at the bleeding body mangled beneath the heap 
-- Tirin, The Dispossessed, U. Le Guin 

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