[time-nuts] Isolated 1PPS-input for distribution-amp?

[Charles Steinmetz]

Anders wrote:

> I'm looking for a solution for galvanic isolation of a 1PPS signal-input to
> a distribution amplifier.
>      *   *   *
> Does anyone have experience with a circuit with good rise-time and low
> jitter?

Sometimes the simplest solutions can be best.  In this case, capacitive 
coupling of the source into the load may be appropriate (see attached 
diagram).

If the two chassis can be connected safely, all you need is to 
cap-couple the signal line (upper diagram).  While there may still be a 
ground loop between the two chassis, it will not affect the PPS signal 
because the small capacitor used to couple the signal line (probably 
around 100pF, depending on the load impedance and the PPS pulse width) 
will not carry any significant current at line and line harmonic 
frequencies.

If the two chassis cannot be connected safely, then cap-couple the 
common line as well (lower diagram).  The common cap must be rated for 
the maximum possible voltage between the two chassis.  The two caps 
should have approximately the same value.

The capacitive isolation can be at either end (source or load).  I 
prefer putting the AC coupling at the source end, so the interconnect 
cable is shielded at all frequencies to the load.

This scheme will only work if the PPS output is robust enough to drive 
the load impedance without further buffering.  Also, there is no DC 
reference at the load end.  This may not be a problem if the PPS pulse 
is short (tens of uS).  Just make sure there is a DC path to ground on 
the signal line (input termination resistor).  Otherwise, you would need 
to use a clamp to DC-restore the PPS signal.

Best regards,

Charles


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