[time-nuts] Brooks Shera GPS Controller Help

Jason Rabel jason at extremeoverclocking.com
Thu Jan 18 11:38:07 EST 2007


Checked both U1-2 & U2-1, I am getting the 5 MHz / 0 to 5V square wave. It's
not the nicest looking, but my counter reads it fine. Maybe this could be a
problem?

http://www.rabel.org/archives/Images/Misc/5MHz_Square.jpg

Yes, U3-15 has the GPS PPS pulse, I can't see it on my scope but I can see
the trigger light every second. Also if I unplug the GPS input then the
board goes haywire. We are talking about the PPS pulse going from 0V rising
to ~5V for the ttl in, correct? (just making sure it's not supposed to be
inverted or some other weird thing, I'll try anything at this point.)

The EFC voltage is about 3.35V or so when I adjust it to my other GPSDO
reference.

As soon as I flip from mode 0 to mode 1, the voltage will jump to around
1.1V and go up and down in that range, the EFC jumps to about 3.42V. This
throws the frequency off noticeably, I've even tried to re-center the
frequency while it's in mode 1 (but nothing different really happens). The
phase reading never rises above ~155. It will reach that then fall back to
about 10 and work its way slowly back up.

I'm stumped...

Jason


> Hi Jason:
> 
> 1)    The oscillator needs to go into the VCXO In jack and 
> then if you 
> look at U1-2 (same as U2-1) you should see a zero to +5 
> square wave at 
> the frequency of you oscillator.
> 
> 2)    The GPS 1 PPS goes into GPS In and of  you look at U3-15  you 
> should see a zero to +5 pulse once per second.
> 
> If (1) and (2) above are correct then the inputs are OK.
> 
> Do you know what the control voltage right at the input pin 
> of the VCXO 
> should be when you are locked?
> (3) Measure the voltage at the input pin with a scope while the 
> Controller is ramping to see if it goes past the required voltage.
> 
> Have Fun,
> 
> Brooke Clarke




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