[time-nuts] FE-5680A GPS discipline board on sale now

Attila Kinali attila at kinali.ch
Fri May 20 05:29:41 EDT 2016


Hoi Nick!

On Thu, 19 May 2016 10:08:10 -0700
Nick Sayer via time-nuts <time-nuts at febo.com> wrote:


> This is a mash-up of my breakout board and GPSDO. You give it 30+W 18-24
> VDC in (hack up a surplus laptop power supply) and it supplies up to 2A @ 15
> VDC and 500 mA @ 5 VDC. In my testing I see around 25 mV p-p of ripple on the
> 15V rail. The 5V rail is a bit noisier at around 35 mV. Functionally, the
> discipline system is the same as the latest OH300 units - it has the phase
> discriminator that Jim Harman recommended, but now there’s a JFET that acts
> to make the phase ramp more linear (useful if you want to gather stats).

Nice! Thanks for sharing!

Three comments:

The discharge resistor of the integrator (R13) is with 10M way too high.
The PCB resistance is usually in the same order of magnitude. Ie with
a resistance that high, the actual resistance highly depends on how
clean the board is (finger prints, dust) and the amount of humidity
it absorbed (both the epoxy and the solder resist are quite hygroscopic).
A resistance of 1M usally is the maximum recommended value. For systems
that should run reliably for a long time, i would even try to keep it lower.
Of course, you have to increase the capacitor accordingly, which also makes
the capacitor less dependent on parasitics. 

The FET Q2 makes the phase detector more linear, but also makes it more
dependent on temperature. Also the V_GSth has a variation of a factor of 3
between different transistors. But I have to admit that I don't know a
good solution for this. 

The output of the FE-5680 is exteremely noisy. A small VCXO locked
to the 10MHz output using a simple PLL (an XOR gate with RC filter
should be enough) is recommended to get at least rid of the spurs
above 100Hz-1kHz


				Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson


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