[time-nuts] Question for my new GPSDO

[Tobias Pluess]

Dear colleagues

I was a bit lazy in the past, so it took way too long to finish the schematics for my new GPSDO.
I am currently making some first trials on the PCB layout; however, I still would like to show you guys my schematics and ask what you think about it. See the attachment.

Page 1 shows the microcontroller circuit. It uses an ordinary STM32F407, nothing special here. The microcontroller will be clocked directly by the OCXO and I don't plan to use the PLL, so everything runs on the 10MHz clock.
On the lower left is the GPS module I use; I plan to add a jumper which allows to select the voltage for an active antenna (3.3 volts or 5 volts, or nothing by completely removing the jumper).
On the lower right is the Nutt interpolator, as sauggested by Attila.
D2 works as a driver which shall generate nice and sharp 1PPS output pulses. Via the PPSEN signal, the software can disable the 1PPS output of the GPSDO as long as it not locked yet.
I also added a serial interface. It will be possible to operate the serial interface in two different modes:
a) transparent mode - the microcontroller forwards everything between the serial interface and the GPS module. I plan to use this module to communicate with the GPS module directly, e.g. by using U-Center.
b) normal mode - in this mode, it will be possible to output the time interval measurements, as well as the temperature and the measured VCC voltage. This will be handy to measure the OCXO's stability with the aid of Timelab or some other software.
The connector X2 is a FPC connector which is intended to connect a simple front panel, featuring a small LCD panel as well as a bunch of buttons and LEDs. On the LCD, I want to output useful information, as UTC time, lock status and so on. I don't know yet for what I will use the buttons, but maybe it could be handy to have them e.g. to adjust filter time constants and so on.

Page 2 contains the temperature sensor which I plan to mount close to the OCXO. It will come in handy to make some statistics about the temperature fluctuations around the OCXO, the knowledge of which could help in optimising the design (e.g. construct a better enclosure, or whatever).
We have already discussed about the DACs, so I skip this section.
Further, there is a current monitor for the OCXO. This will be interesting, since it maybe allows to determine the point at which the OCXO is warmed up.
D11 creates a square wave for the microcontroller, while N3 and N10 are used as buffers and amplifiers to have two independend 10MHz outputs. I have already used those OpAmps in my previous design and measured the isolation, which is pretty good. I can't tell much about the phase noise, yet, which may be added by those amplifiers, but at least I use "ultra low distortion" devices to prevent generation of excess harmonics.

Page 3 is my highly sophisticated power supply ;-) for which I use Traco switching regulators. I have a 15 volts wall-wart power supply. The OCXO voltage is filtered by two ferrite beads and a bunch of capacitors; however, if things get out of hand, I can still replace the switching regulator by a LM7815, for which the dropout voltage is maybe at its lower limit (need to test, I think).

What do you guys think about it. Should I add some more stuff to it?
It would be so awesome to have a RJ45 port which then allows to connecto to a web interface and time server! But my programming skills are way too low level to program such sophisticated things into a small uC, so I stick to the plain LCD panel and a few buttons.

When I have my PCBs made, I plan to let the OCXO free run with a fixed EFC voltage, and just log the TIC values on a PC and then analyse them using Timelab or Matlab to find out the allan deviation of my OCXO and determine the optimal filter time constant.

As soon as I have the layout finished, I will order some PCBs (at Eurocircuits). If there is interest in that, I can order a few more for you guys (however, I am based in Switzerland, so it is maybe not interesting for mose users on this list, but I ask anyway). The software will be on Github, then.



Thanks for your comments,
best
Tobias
HB9FSX




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