[time-nuts] First success with very simple, very low cost GPSDO, under $8

[Tom Miller]

Your oscillator is on its way. I set it right on 10.00000000 MHz against a 
GPSDRb house standard. You will get a better waveform out if it sees about a 
100 - 200 ohm termination, though the edge is nice and sharp with some 
peaking overshoot.

You might get a bit more stability if you can add to the thermal insulation 
around the oscillator. Also, consider using the internal reference output on 
the pin marked n/c. It runs near 4.0 volts and is stable to ambient temp 
changes. Maybe put the whole thing in a Styrofoam container that has about 
one inch wall thickness.

Regards,
Tom



----- Original Message ----- 
From: "Chris Albertson" <albertson.chris at gmail.com>
To: "Discussion of precise time and frequency measurement" 
<time-nuts at febo.com>; "Magnus Danielson" <magnus at rubidium.dyndns.org>
Sent: Sunday, April 13, 2014 12:30 AM
Subject: Re: [time-nuts] First success with very simple, very low cost 
GPSDO, under $8


> On Sat, Apr 12, 2014 at 9:48 AM, Magnus Danielson <
> magnus at rubidium.dyndns.org> wrote:
>
>>
>> In fact a 5 bit counter is enough, and then a '373 to sample it. The
>> enable to the 373 needs to be synchronous to the 5/10 MHz clock, so a 
>> pair
>> of DFFs ('74) is needed to synchronize the PPS and another pair to create
>> the single cycle enable.
>>
>>
> It turns out all of this is built into the AVR chip.   There is a counter
> and logic to copy the current counter value to a register on a PPS pulse
> raising edge.    The counter keeps running and every second its value is
> trapped.
>
> I can connect the OCXO and the PPS directly to the AVR pin.  The AVR has
> hardware (a fast comparator) to "square" a low amplitude sine wave and 
> trap
> the counter on a zero crossing.   So it looks like I can get rid of  ALL 
> of
> the external chips.   The built in DAC is working well also but it needs
> some external resisters and caps.
>
> No need for '74 FFs or '373' or counter chips.    I do get precision 
> timing
> with no time critical software, no 74xxx chips.
>
>
> --
> Chris Albertson
> Redondo Beach, California
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