[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. 


----- 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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