[time-nuts] FE5680A 1 PPS and FAQ

[beale]

>  From: Sam Reaves <sam.reaves at gmail.com>
>  I have a LED with a 330 ohm current limiting resistor on pin 3 (the 74ACT240 can sink up to 24mA and I am well below that) for the lock indicator. All of my units lock and produce 10MHz although the 1pps never  switches.

Pin 3 is not driven by the 74ACT240. It is an input to the '240, and your LED is pulling it up above the logic threshold....  see below.

http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#how_can_i_get_a_1_pps_output

How can I get a 1 PPS output?

A 1 microsecond wide, logic level 1 PPS signal appears on the DB-9 pin 6 whenever the unit is locked (hard to see on an analog scope, should be easy on a digital scope). The PPS signal does not appear until the unit is locked (pin 3 goes low). The current sink capability of pin 3 is weak, and if it is driving an LED + 1K resistor from +5V, that will leave pin 3 at 2.3V when locked, not enough to enable the 1 PPS signal. Advice: buffer the pin 3 lock signal before driving an indicator light. All being well, pin 1 of the 74ACT240 will drop low in step with the lock condition and the PPS signal will appear at the DB9.

Alternatively, it is easy to get a 1 pps signal from the 10 MHz output with a single picDIV chip (possibly with a 0.1uF cap and two 10k resistors to bias the sine wave to 1/2 the PIC power supply voltage). picDIV parts (based on PIC12F675 cpu) from Tom Van Baak take a 10 MHz clock input and generate a 1 PPS output. See also: http://www.leapsecond.com/pic/picdiv.htm

Thanks to Bob Grant for describing the pin 3 LED  pin 6 PPS problem.