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.ch...@gmail.com>
To: "Discussion of precise time and frequency measurement"
<time-nuts@febo.com>; "Magnus Danielson" <mag...@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 <
mag...@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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