[time-nuts] Building a GPSDO & trouble using Jupiter-T
rayxu123 at gmail.com
Sun Jan 29 01:50:35 EST 2012
I'm planning to build a GPSDO to use as a frequency ref for my GHz ventures.
I've done research in what other people have built - but I have no
experience working with long-term precision/stability products (nor do I
have the equipment to do so -- I think).
I'm using the Jupiter T (the one with the 10-pin header) GPS and its 10KHz
output in an analog PLL that controls a 10MHz VCXO. It would be ideal for
me that if I were to multiply the 10MHz output up to 10GHz, I would get
about a few Hertz or so of inaccuracy. It would also be ideal if I can
have a PLL lock time of a few minutes while maintaining accuracy.
However, right now as I am trying to design my project, I can't find enough
information on the web regarding the VCXO's (or the PLL oscillator, in this
case) short term accuracy effect on output frequency. I know that its
short term accuracy depends on the "response time" of the PLL, which also
depends on the amount of jitter from the Jupiter-T's 10KHz output... I
know that the longer the time constant for the PLL, the better accuracy,
but I do not want to wait, literally, hours for it to lock...
Also, what is the advantage of using a OCXO instead of a VCXO in terms of
short-term accuracy? If the PLL time constant is only a few seconds, then
a crystal shouldn't deviate in frequency by too much within a few seconds,
assuming I'm using a crystal bought from a well-known manufacturer...or
could it? I am inclined towards using oscillators that do not require any
significant warm up time...
For those who have experience using the Jupiter T GPS:
I have bought this
I could not get it to communicate via serial. So far, I do not have
antenna available and so the antenna port is just left disconnected. When
I turn it on, there is a 1pps and 10KHz signal and the TX line is at logic
high. However, I cannot get it to communicate to anything else, even a
dumb terminal. It does not respond to when I send a message ID to it.
(Because of computer difficulties, I am using the PicKit 2's UART tool).
Is my device a dud or am I doing something wrong?
Many inputs is highly appreciated. Thanks
73, Ray Xu
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