[time-nuts] Looking for GPSDO for home use

[Chris Albertson]

On Tue, Apr 15, 2014 at 10:06 PM, David J Taylor <
david-taylor at blueyonder.co.uk> wrote:

>
> Looking forward to seeing the circuit diagram and downloading the code,
> Chris!


I'm using (most of) the circuit posted a few weeks ago to this list by Lars
Walenius.  I posted the code here a week or so ago.   I was impressed by
how simple his GPSDO was and have sense simplified his design even more.
 The goal was not state of the art performance but an ultra simple design
and low cost.   Cost is about $8 another is about two dozen lines of code.

I plan to make it even less complex but I want a few days of test data
before I change anything.

Here is how to build one:

1) get a GPS receiver that has a "PPS" or pulse per second output.    Any
receiver will do but you really want a "timing mode" receiver.  You will
need an antenna that can see most of the sky and a power supply.  Get this
running reliably.

2) get a decent "OCXO" that is a crystal oscillator that is inside a
temperature controlled can and that has a pin where you apply a voltage to
fine tune the frequency.   I'm using a Bliley 10MHz square wave OCXO I
bought on eBay for $11.  There are better ones for $25.  Any of them will
work.  You will need to get a power supply to run it.  Get this working and
verify the output at least reasonable.

3) Next step is to connect the above two with a controller.   The
controller hardware is simply an Arduino development board.   You can buy a
"real" one that is made in Italy for about $30 or a much smaller version on
eBay for about $3.  They are functional the same.  I used one iof these:
pro-mini<http://www.ebay.com/itm/New-Pro-Mini-atmega328-5V-16M-Replace-ATmega128-Arduino-Compatible-Nano-TF-/371041988612?pt=LH_DefaultDomain_0&hash=item5663d37004>
but
yu need a "FIDDI" USB to serial adaptor to program it.  The self contained
version is easier to use like this one:
nano<http://www.ebay.com/itm/Arduino-Compatible-Nano-V3-0-ATmega328-5V-Micro-controller-Board-Mini-USB-/331178319577?pt=LH_DefaultDomain_0&hash=item4d1bc41ed9>
this kind has the usb/erialon board.   The made in Ital version is a bit
larger see this:  http://www.adafruit.com/products/50

The schematic is easy (1) use 74HC390 to divide 10MHz by 2 and send 5MHz to
Arduino pin-5, (2) connect PPS from GPS to Aruino pin-2, (3) 10M resister
to pin-1, 3.9k to pin7.  connect bothresiters together then to "RC filter"
then the OCXO's control pin.  That's it.

One thing.  It is looking like you can skip the 74HC390 chip.  connect the
10MHz directly to the Arduino.  The arduino has a fast analog comparer that
can handle small amplitude sine waves.

This is the simplest GPSDO  later we can improve performance with software
anymore parts.   I'd build this on a solderless breadboard and then change
to perfboard.
-- 

Chris Albertson
Redondo Beach, California