> Chris wrote:
>Från: Chris Albertson
>Skickat: torsdag den 13 februari 2014 07:31
>Till: [email protected]
>
> 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
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