[time-nuts] Egg on Face (was EPE - GPS Frequency Reference Project)

Joe McElvenney ximac at btinternet.com
Sun Sep 27 10:00:18 UTC 2009

>There's a subtle difference between 3rd phase detectors in the 74HC4046A
>(Philips/NXP, ON, TI) and the 74HC4046 (Fairchild, NS)
>The 3rd Phase detector in the 74HC4046A works as the designer intended
>in the GPSDO circuit, the 3rd phase detector in the 74HC4046 does not.

>The 74HC4046 uses an RS flipflop for the 3rd phase detector and requires
>narrow pulses on the R and S inputs.
>The 74HC4046A uses extra internal gates to ensure that only narrow
>pulses are seen by the RS flipflop in the 3rd phase detector.


You are perfectly correct. I hadn't noticed that there was a 74HC4046A
fitted to the board in the article but just a plain 74HC4046 shown in
the parts list. However, it is something to watch out for as this will
be a popular project among those, like myself, who do not need to slice
time quite so finely.

Cheers - Joe G3LLV

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