I have often wondered about all this sawtooth correction stuff, and I
think I've asked here too, but never got a definitive answer. Every time
this comes up, there are all sorts of explanations of the
characteristic, and inevitably someone mentions the T-bolt having its
internal GPS clock synchronized with the desired ideal 10 MHz output,
either eliminating or greatly reducing the sawtooth effect. But, to my
knowledge, nobody has said for sure if this is indeed the case, or
whether that's all it takes to achieve perfection, or if more magic is
needed besides synchronizing. I'm also aware that some people have come
up with external sawtooth correction, using programmable digital delay
lines and special control circuits. But, why do all that if you can just
fix the clock instead?
Many years ago I looked at my Motorola Oncore VP (or whatever model is
used in the HPZ3801A), to see what its clock was, and if it was
reasonably possible to synthesize it from the 10 MHz. I think the clock
is a Motorola brand odd looking TCXO, labeled "19096" or something like
that, probably 19.096 MHz, as I recall. That's about as far as I got.
Since then I've just wondered what would happen if it was synthesized
from the 10 MHz, or if it was even worth trying, or if it would result
in other problems. Maybe a certain amount of dither is necessary for
proper operation.
So, here are some questions, that if answered, may go a long way toward
possible improvements in our GPS stuff.
1. If the GPS RX module's internal clock is synthesized (to the same
nominal frequency) from the 10 MHz output of the GPSDO, can that alone
eliminate or substantially reduce the sawtooth effect?
2. Does the T-bolt actually do this, and if so, is that all it takes?
If either answer is yes, then I would think the GPS RX makers would have
provisions for external clock reference, at least for certain high-grade
timing type models.
3. Do any GPS RX modules have such provision?
That's all I can think of for now.
Ed
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