[time-nuts] Building a GPSDO & trouble using Jupiter-T

Ray Xu rayxu123 at gmail.com
Sun Jan 29 01:50:35 EST 2012


Hi guys

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
http://www.ebay.com/itm/Navman-jupiter-T-Tu60-GPS-Kit-1pps-10khz-GPS-Module-/260790984470but
I could not get it to communicate via serial.  So far, I do not have
an
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
KF5LJO



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