Why are PPS pulses so narrow? (was: 53132A triggering)
Can I ask why PPS pulses are so narrow? It makes them difficult to see on
a 'scope, and difficult to detect on a PC. And, as Bob said, far less
obvious if you trigger off the wrong edge.
Peter
On 16 September 2016 at 23:55, Bob kb8tq kb8tq@n1k.org wrote:
Hi
Most PPS signals these days are very low duty cycle. If you AC couple
them, you can easily be triggering on the wrong edge. With the narrow pulse
it may not be very obvious.
Bob
Hi
It is sort of an " everybody does it " sort of thing. Various justifications:
Less power is used / less heat in the drivers and terminations.
Transformer coupling works better ( lower delay ) with narrow pulses
Anything over 1 us has been "really long" in terms of logic speeds since the 1960's
A definite duty cycle "bias" let's you detect an inverted pulse.
The only real reason is "that's the way it's done". Big Customers ask for it that way. Suppliers deliver what is asked for. Nobody complains, nothing changes.
Bob
On Sep 16, 2016, at 7:44 PM, Peter Vince petervince1952@gmail.com wrote:
Can I ask why PPS pulses are so narrow? It makes them difficult to see on
a 'scope, and difficult to detect on a PC. And, as Bob said, far less
obvious if you trigger off the wrong edge.
Peter
On 16 September 2016 at 23:55, Bob kb8tq kb8tq@n1k.org wrote:
Hi
Most PPS signals these days are very low duty cycle. If you AC couple
them, you can easily be triggering on the wrong edge. With the narrow pulse
it may not be very obvious.
Bob
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On Fri, Sep 16, 2016 at 4:44 PM, Peter Vince petervince1952@gmail.com
wrote:
Can I ask why PPS pulses are so narrow? It makes them difficult to see on
a 'scope, and difficult to detect on a PC. And, as Bob said, far less
obvious if you trigger off the wrong edge.
None of us can guess the original designer's motivation but...
I think you answered your own question "far less obvious if you trigger off
the wrong edge.". For most cases, except for time nuts, a few
microseconds does not matter. So the GPS PPS works almost as good if you
invert the signal and trip off the wrong edge. Yes it makes such a
mistake hard to find but, I think that was the point.
--
Chris Albertson
Redondo Beach, California
Thanks guys - interesting answers all!
Peter
On 17 September 2016 at 18:00, Chris Albertson albertson.chris@gmail.com
wrote:
On Fri, Sep 16, 2016 at 4:44 PM, Peter Vince petervince1952@gmail.com
wrote:
Can I ask why PPS pulses are so narrow? It makes them difficult to see
on
a 'scope, and difficult to detect on a PC. And, as Bob said, far less
obvious if you trigger off the wrong edge.
None of us can guess the original designer's motivation but...
I think you answered your own question "far less obvious if you trigger off
the wrong edge.". For most cases, except for time nuts, a few
microseconds does not matter. So the GPS PPS works almost as good if you
invert the signal and trip off the wrong edge. Yes it makes such a
mistake hard to find but, I think that was the point.
--
Chris Albertson
Redondo Beach, California
time-nuts mailing list -- time-nuts@febo.com
To unsubscribe, go to https://www.febo.com/cgi-bin/
mailman/listinfo/time-nuts
and follow the instructions there.
I would not use such a narrow pulse for any of those reasons except
for power if that was an issue. I would and have however used narrow
pulses simply because it allows for a lower volt*time product on a
transformer.
On Fri, 16 Sep 2016 21:03:12 -0400, you wrote:
Hi
It is sort of an " everybody does it " sort of thing. Various justifications:
Less power is used / less heat in the drivers and terminations.
Transformer coupling works better ( lower delay ) with narrow pulses
Anything over 1 us has been "really long" in terms of logic speeds since the 1960's
A definite duty cycle "bias" let's you detect an inverted pulse.
The only real reason is "that's the way it's done". Big Customers ask for it that way. Suppliers deliver what is asked for. Nobody complains, nothing changes.
Bob
On Sep 16, 2016, at 7:44 PM, Peter Vince petervince1952@gmail.com wrote:
Can I ask why PPS pulses are so narrow? It makes them difficult to see on
a 'scope, and difficult to detect on a PC. And, as Bob said, far less
obvious if you trigger off the wrong edge.
Peter