GPS disciplined OCXO project

V

VK2RK

Guest
Moving on into the next design, a GPS disciplined OCXO using a UBLOX GPS module along with a bunch of ripple counters and a PLL chip
The frequency divider part could be used as a standalone device to provide 10, 5, 1 MHz with just one 74HC390, using two will provide a 10KHz signal to compare with the 10KHz from the GPS module and provide a phase lock, at least in theory.. Stay tuned as I go down this rabbit hole.

GPS-OCXO.jpg

GPS .jpg
 
V

VK2RK

Guest
Further developed the design and created artwork for prototype

GPS-OCXO.jpg

GPS-OCXO_PCB.jpg
 
J

Josh

Guest
I remember seeing a rather inspired design that used a the 10Mhz through a by ten divider into the reset input of a HC4046 and i think the GPS 1PPS output was feed into the set, the result was a pulse which duration was the phase of the PPS relative to the 1MHz clock. The HC4046 output was feed into a external charge hold circuit for an microcontroller adc. I though that was a really clever way to get the phase drift information to a high degree at a high rate. Maybe you can use the idea.
 
V

VK2RK

Guest
I remember seeing a rather inspired design that used a the 10Mhz through a by ten divider into the reset input of a HC4046 and i think the GPS 1PPS output was feed into the set, the result was a pulse which duration was the phase of the PPS relative to the 1MHz clock. The HC4046 output was feed into a external charge hold circuit for an microcontroller adc. I though that was a really clever way to get the phase drift information to a high degree at a high rate. Maybe you can use the idea.
Been thinking on similar lines, hence the choice of phase comparator that include the two D type flipflops and not just exclusive OR's
The Jitter from the GPS module that is a function of both the internal divider based on a 38KHz internal clock and Satellite timing differences I estimate to be in the range of 400 pS to a worst case occasional of 5nS, I don't have any equipment to more accurately measure the jitter or evaluate in more detail, thus by making some assumptions and using integration technique that I can apply in the Low pass filter to the OCXO control voltage, so here I can average out any jitter causing no correction to the control voltage only that of actual frequency drift. At least that is the plan. Also note that now I can select either 100 or 10 KHz as a reference, this to test my assumption, if the integration averaging works I should be able to increase the reference frequency. Time will tell.

All the design so far has been theoretical, the board has been ordered along with the IC's, I did hope to test my theory on a PLL chip but I cant get one locally, all answers will be found on the prototype, hopefully emerge from the rabbit hole.

I am attaching the final drawing and PCB
GPS-OCXO_PCB.jpg

GPS-OCXO.jpg
 
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J

Josh

Guest
there is a program to use with gpsdo's called lady heather.
http://www.ke5fx.com/heather/readme.htm

strickly speaking it does not have to be a gpsdo, but just hooking up the serial nema stream to it can give you some very useful data about gps related uncertainly.
 
V

VK2RK

Guest
The project is coming to an end, only left is a suitable container (Box)
The Arduino code gave me some headaches, its been some time since I wrote any code in C particular to the Arduino, this with having forgotten of some of the rabbit holes to be found with millis() macro along with buffer overflow on the virtual com port, several days of hair loss took place.
Now with lock ups overcome here is the video showing the functionality of the device.

Only part I did not film is the GPS module hardware failure that shows as a message on the screen followed with a reboot after approximately 10 seconds.

 

VK2RK

New member
GPS Disciplined OCXO Parts List (Price listed are approximate)


Hardware
PCB 1 $10.00
GPS Module 1 eBay https://www.ebay.com.au/itm/402807251139 $20.00
OCXO 1 eBay https://www.ebay.com.au/itm/332389156868 $10.00
Project Box 1 eBay https://www.ebay.com.au/itm/363140568029 $20.00
Regulator 1 eBay https://www.ebay.com.au/itm/393378668946 $4.00
SMA socket 4 eBay $4.00
DC Socket 1 eBay or Jaycar $2.00
LED Red 3mm 1 Jaycar $0.50
LED Blue 3mm 1 Jaycar $0.50
Battery Holder 1 eBay $0.50

Integrated circuits Jaycar does not have all. Source eBay or others
74HC390, 2 $4.00
74HC4046 1 $2.00
74HC14 1 $1.00
CD4001 1 $1.00

Resistors Metal Film 1%
100K 2 Jaycar pack of 8 $0.85
1K5 1 Jaycar pack of 8 $0.85
1K0 2 Jaycar pack of 8 $0.85
R51 6 Jaycar pack of 8 $0.85

Capacitors
100uF 16V Tantalum 1 $0.20
10nF Ceramic 7 $2.50
100nF Ceramic 4 $1.50
270pF Ceramic 2 $1.50
470pF Ceramic 3 $1.50
1000pF Ceramic 1 $0.50
2700pF Ceramic 2 $1.00
4700pF Ceramic 1 $0.50

If you find any problems sourcing any parts please contact me, chances are I will have some on hand.

The Arduino controlled LCD requires the Nano and the LCD to be added to the list
LCD must be I2C type.(16x2)

Any issues just contact me.
GPS_)CXO.jpg


When you assemble this project the first thing to do is to install the DC socket and the regulator board.
Placing a voltmeter on the 100 uF electrolitic capacitor pads, adjust the regulator for 5 Volts

Failure will over-voltage the Oven Controlled Oascillator and damage it.
 
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BillC

New member
Hi Rob congratulations on your project, a very handy instrument to have in the workshop. I will start building one soon as I get a chance. Regards Bill.
 
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