[time-nuts] New to list and GPSDO questions

M. Simon msimon6808 at yahoo.com
Thu Jun 13 20:31:26 EDT 2013

I like the LPC1114 chip (50 MHz). I am well along on the design of such a board using a 40MHz TCXO (good to about .1 PPM when calibrated and operated at room temperature) divided down to 10 MHz as the master clock. It has a 10 MHz  real 50 ohm out with a +5V/0V nominal signal levels. You can put that in your counter to tweak it. It has a tweaking  trimmer. 

The LPC1114 is available for about $3 in low quantities. Or free if you can get NXP to send you samples.

There is probably another month or three of work to do to complete the project. The board will have a 32 bit fig-FORTH to go with it (how retro) making it easy to program. Of course if you insist on "C" you can do that too. 

The interfaces include - I2C (LCD display, extra control lines [slow], real time clock,  etc). Real DB9 RS-232. TXD, RXD, installed. CTS, RTS, DSR, DTR, available. Output levels of +/- 10V nominal (about +/- 8V actual). Top expected speed of 115,300 Hz. Jumperable for DCE or DTE (header and jumper shorts). Male and Female DB9s.  And of course the capture pins are on a nominal parallel port. 

I can post to the list when the project is completed if anyone is interested. Bare boards will be available. It is almost all Surface Mount (nothing smaller than 0603 case size), but designed for hand soldering. I'm an old man (68), my hands shake and my eyes are bad. If I can do it so can you. To help with that I have designed a low cost hot air iron. 


I am open to modifying the MCU board design for TIME NUTS. Let me know what you want. 


Date: Thu, 13 Jun 2013 22:39:01 +0200
From: Attila Kinali <attila at kinali.ch>
To: Bob Stewart <bob at evoria.net>, Discussion of precise time and
    frequency    measurement <time-nuts at febo.com>
Subject: Re: [time-nuts] New to list and GPSDO questions
Message-ID: <20130613223901.26b487eed384b6cc8a8607f7 at kinali.ch>
Content-Type: text/plain; charset=UTF-8

On Thu, 13 Jun 2013 12:17:53 -0700 (PDT)
Bob Stewart <bob at evoria.net> wrote:

> I want this to be as low budget as is reasonably possible, barring wastage,
> so I think a 1PPS source and Bert's FLL are the right choice.? Is there
> actually a PLL circuit that works with a 1PPS reference?? I only found a
> very few GPSDO projects during my search, and Bert's was the only one using
> the 1PPS signal.? And, I just like the way he did it, right down to giving
> the possibility of powering the output op-amp from the VREF output of the
> 34310-T.? I'm assuming that won't overload it, of course, so any comments on
> that would be appreciated.

[1] is probably the most cited GPSDO design. It's simple, it works and
you can build it quite cheaply

[2] is similar to what Shera did, with slight modifications on how to
measure the "phase".

[3] Describes how to use a clock synchronizer to build a GPSDO. Probably
not the easiest and not the cheapest way, but definitly one with a very
low parts count.

I think the cheapest way how to do a GPSDO that is fairly well accurate
is to use a uC with a high clock rate and a 32bit capture/compare unit
(eg STM32F4 which can do 78MHz on their c/c unit, thus giving ~14ns
resolution for ~15USD for the naked chip or an eval board).
Feed the 10MHz from your OCXO to the uC's clock input. Use the internal
PLL to upconvert it to what you need (i.e. as high a frequency as possible).
Feed the 1PPS to one of the c/c pins and measure when it enters. 

If your OCXO is fairly stable, you can integrate over a minute or two
(or even longer) and get down to uncertainties below 10^-9.

If you tweak your control loop such, that the mean of your PPS pulse
is exactly between two bins of your c/c, then you can get even better
than the resolution of your c/c, if your GPS generates a symetrical
distribution of the PPS pulse.


            Attila Kinali

[1] A GPS Based Frequency Standard, by Shera, 1998

[2] Building a GPSDO controller, by Pedersen, 2011

[3] The AD9548 as a GPS Disciplined Stratum 2 Clock, by Gentile, 2009

The people on 4chan are like brilliant psychologists
who also happen to be insane and gross.
        -- unknown


Engineering is the art of making what you want from what you can get at a profit.

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