[time-nuts] Value of parts for the RC network of "GPSDO with 1ns res TIC"

Jim Harman j99harman at gmail.com
Thu Sep 24 10:14:52 EDT 2015


Diode at HC4046 output pin 15 is 1N5711 (Schottky signal diode)
This is in series with 3900 ohm resistor.
Cap to ground is 1nf NPO ceramic
Resistor across the cap is 1 M

These values are suitable for the Arduino Uno (ATmega 328 processor) with
the analogReference set to INTERNAL. This gives the ADC a full-scale range
of 1.1 V. On the Micro (32u4 processor) the ADC range is 2.56 V so the cap
should be 220 pf.

Pin 3 of the HC4046 is fed by 1 MHz derived from the oscillator to be
controlled and pin 14 is the 1 pps from the GPS, which also drives the
interrupt that triggers the ADC.

The phase comparator is basically an R-S flip-flop. Once per second it
produces a 5 V pulse 0-1 microsec wide. The width of this pulse corresponds
to the time difference between the two inputs. When the pulse goes H, the
cap starts to charge through the diode and 3900 ohm resistor. The cap stops
charging when the pulse goes L. The diode prevents it from being discharged
by the LS4046. The ADC is triggered at the end of the pulse, reading the
peak value on the cap and thus the time difference. The 1 M resistor
discharges the cap over the remainder of the second before the next pulse.

The charging slope is not exactly linear, but for 0-1.1V it is pretty
close. It is noticeably curved for 0-2.56V, but works fine for this
application. It would be quite easy to linearize the response in software.

On Wed, Sep 23, 2015 at 7:07 PM, Can Altineller <altineller at gmail.com>
wrote:

> Hello,
>
> I got some 4046 plls and I decided to implement the schematic given on:
>
>
> https://www.febo.com/pipermail/time-nuts/attachments/20140211/8bdbc232/attachment-0004.jpg
>
> in the drawing phase comparator 3 output is connected to a diode, which
> then is fed to a network, and then read from an adc. I see a 1nF capacitor,
> but the other 2 parts I can not identify. (I am guessing one is a 1M
> resistor, and the other above is some inductor maybe?
>
> Best regards,
> C.A.
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-- 

--Jim Harman


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