[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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