[time-nuts] 4046 experiment for gpsdo

[Can Altineller]

Hello All,

I got a CD74HC4046 and started to experiment. It seems that this is a newer
version, which has another phase comparator instead of the zener diode.
There has been some changes in the chip basically. PC1out is a xor gate,
and PC3out is an RS flip flop. It will give pulses if one source is behind,
and it will give reverse pulses if one source is forward. PP pin gives the
reverse of every phase differences corresponding to a rising edge, so it is
half the xor pulses in frequency.

I made the following circuit on breadboard:


​
1PPS pulses from both RTC and SYNC source gets inverted, (because we are
interested in falling edges) - Signal and comparator pins of 4046 is fed
with those pulses. The PC1 out is a XOR phase pulses, for both rising and
falling edges, so we are only measuring it for display purposes on the
following images.

P3Out gives a pulse, if source is behind it will give a sharp pulse from 0
to logic 1 voltage, and if source is forward it will be at logic 1 voltage,
and give a sharp drop. (On the circuit above the 4046 has no zener, but PC3
instead).

The PP output from 4046 inverted gives as the phase pulse, no matter if it
is lacking or forward it will give the same normal pulse. By using and
gates and getting
!PP & PC3Out gives us forward pulse, and !PP && !PC3Out gives us reverse
pulse.

Here is a logic analyzer capture of XOR, Forward pulse and reverse pulse,
when the RTC is behind.


And here Source is behind



So in summary, we get pulses from one pin saying that the RTC is behind,
and if RTC is forward it will pulse from the other pin. I will now proceed
to feed these pulses into the RC network.

I got this far tru experimentation with the 4046. The VCO can also be
inhibited for low power consumption, (since we are only using the phase
comparators)
I tried a lot of logic circuits to cover a generalized case of the phase
difference between clock pulses, but now I want to make a hybrid algorithm
which will work with interrupts until within certain range steering the
clock in to the zone, and then it might tune from the RC networks. (I would
use two of them, for increasing and decreasing the oscillator)

I would love to hear your comments for the above circuit because I might
have overlooked something major, that will screw up the operation.

Best Regards,
Can

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