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

Erno Peres ernieperes at aol.com
Sun Jan 29 02:01:14 EST 2012


Hi Ray,

please find on this link the info what you are looking for.

         http://www.jrmiller.demon.co.uk/projects/ministd/frqstd.htm

Rgds Ernie.









-----Original Message-----
From: Ray Xu <rayxu123 at gmail.com>
To: time-nuts <time-nuts at febo.com>
Sent: Sun, Jan 29, 2012 7:51 am
Subject: [time-nuts] Building a GPSDO & trouble using Jupiter-T


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
xperience working with long-term precision/stability products (nor do I
ave the equipment to do so -- I think).
I'm using the Jupiter T (the one with the 10-pin header) GPS and its 10KHz
utput in an analog PLL that controls a 10MHz VCXO.  It would be ideal for
e that if I were to multiply the 10MHz output up to 10GHz, I would get
bout a few Hertz or so of inaccuracy.  It would also be ideal if I can
ave 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
nformation on the web regarding the VCXO's (or the PLL oscillator, in this
ase) short term accuracy effect on output frequency.  I know that its
hort term accuracy depends on the "response time" of the PLL, which also
epends on the amount of jitter from the Jupiter-T's 10KHz output...  I
now that the longer the time constant for the PLL, the better accuracy,
ut 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
hort-term accuracy?  If the PLL time constant is only a few seconds, then
 crystal shouldn't deviate in frequency by too much within a few seconds,
ssuming I'm using a crystal bought from a well-known manufacturer...or
ould it? I am inclined towards using oscillators that do not require any
ignificant warm up time...
For those who have experience using the Jupiter T GPS:
 have bought this
ttp://www.ebay.com/itm/Navman-jupiter-T-Tu60-GPS-Kit-1pps-10khz-GPS-Module-/260790984470but
 could not get it to communicate via serial.  So far, I do not have
n
ntenna available and so the antenna port is just left disconnected.  When
 turn it on, there is a 1pps and 10KHz signal and the TX line is at logic
igh.  However, I cannot get it to communicate to anything else, even a
umb 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).
s my device a dud or am I doing something wrong?
Many inputs is highly appreciated.  Thanks
-- 
_________
3, Ray Xu
F5LJO
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