[time-nuts] Arduino GPSDO with 1ns res TIC

[Lars Walenius]

> Chris wrote:

>Från: Chris Albertson
>Skickat: ‎torsdag‎ den ‎13‎ ‎februari‎ ‎2014 ‎07‎:‎31
>Till: time-nuts at febo.com

>



> 1) what connects to analog pins A1, A2, A3, A4, and A5.  The schematic
>shows only A0 used for the TIC capacitor.





A1 and A2 is connected to temperature sensors like the LM35 or an NTC with pullup. Note that the program sets all ADC to read 0-1.1Volt. If you don´t use the temperature sensor it might be wise to put a pulldown of say 100kohm on the A1 and/or A2. On my board A2 goes to the 4x2 pinheader that is used for the external inputs of 10MHz, 1PPS, temperature (A2) and output of control voltage.

A3 and A4 is fed from 4 dipswitches each via 100k, 200k, 390k and 820 k from the +5Volt. On both A3 and A4 I have a 12kohm pulldown. So actually this makes a 4bit DAC.  This was a way to get 8 digital inputs from 2 pins. Probably an I/O expander would be a better choice in a commercial product (or another processor :) . This arrangement requires a fixed relationship between the +5V and +1.1V. If the board is only powered with USB it might be problem even if I haven´t seen this with my computers yet. The TIC also need a fixed relationship in the same way as the HC4046 is powered by +5V and the ADC has it´s internal +1.1V.

A5 is connected with a 20kohm pot + 82kohm resistor from the +5V if I remember correct. My intention was to have an input for different “DAC ranges” to be able to switch between different oscillator without changing a value in the program. It is not implemented in my program.


> One chance is that my Rb oscillator is controled via serial port, not a
> voltage.  So I'll want the program to use serial commands rather than a
> DAC.    You say your unit is pressure sensitive, well there are cheap and
> very easy pressure sensors that are easy to use.
> It might be fun to see what I can do with a cheap $1 10Mhz crystal rather
> then using an OCXO.





I would select the best 10MHz oscillator you have. Even with a Rb it is possible to set low time constants to get quick loop response for test. With low time constants you will get more noise on the output frequency but that will mainly be because of the 1PPS jitter if you have a good oscillator. If you have a noisy oscillator it will only make it worse. Probably a cheap VCTCXO will give more noise than even a position type GPS receiver at time contants longer than 50 seconds. But if the best oscillator you have is a VCTCXO try it. One thing to remember, that my gpsdo shares with the Shera controller I think,  is the narrow capture range of the TIC. My is even worse than Sheras as he uses a divide by 32 before the HC4046 and I use divide by 10 (I used divide by 100 from the beginning and 10 times higher RC-net time contant that gave 10nS resolution). So it is important to manually adjust the frequency of the loop in hold mode before activating the loop. Using a working timer1 together with the TIC would give the possibility to a much wider capture range.


Changing the program to output a serial value to the Rb would be a great idea.


I think it is a good idea to first set the controller in hold mode and set the DAC values to mid, min and max and have a look on the TIC values and learn something from that before closing the loop (setting normal mode and fast locking).




Lars