[time-nuts] Determination of the placement of the first pps

J. L. Trantham jltran at att.net
Wed Jan 25 14:04:50 EST 2012


Magnus,

Thanks for the research.  Now that I have read your information, I am going
to have to go back and 'measure' the PPS output on my unit.  I just
'eyeballed' the '1 PPS' and it seemed close to 1 second.

It would be nice to find a serial command that would allow placing the 1 PPS
at any arbitrary point in phase to match GPS.

My impression of reading the posts about the FE-5680A is that the 1 PPS is
likely there, just not easily documented.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bob Camp
Sent: Wednesday, January 25, 2012 11:43 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Determination of the placement of the first pps

Hi

Based on the number of units that come in with the "1 pps output" missing,
I'd bet the $40 FE5680's were used only for the 10 MHz output.

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Magnus Danielson
Sent: Tuesday, January 24, 2012 10:14 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Determination of the placement of the first pps

On 01/25/2012 02:41 AM, J. L. Trantham wrote:
> Darn.
>
> I was hoping for that feature.  I still think it should be there.

Indeed. Should be in there somewhere...

Didn't see these link hit the list:
http://pastebin.com/S8UcnCMZ

http://www.dd1us.de/Downloads/precise%20reference%20frequency%20rev%200_7.pd
f

http://vk2xv.djirra.com/tech_rubidium.htm

Looking at the last one it says:

"NOTE: Although this unit is marked with both 10MHz and 1pps, research on
the 'net seems to indicate that the '1pps' output has only a period of
exactly 1 second when the frequency is set to 223 Hz (8.388608Mhz). 
According to those sources the '1pps' will have a period of 0.8388608
seconds when the output frequency is set to 10MHz. This should be easy to
verify. In any case I have no need for a 1pps output - I use a GPS module to
get a 1pps signal which also has the advantage of being in-phase with real
time seconds. "

Now... to speed-adjust the PPS phase, use the DDS and steer it intentionally
of frequence with sufficient delta frequence for suitable time and you
should home in pretty quickly.

Hunting some more:
http://www.qsl.net/zl1bpu/PROJ/Ruby.htm

"Without modification the units have just one output - 1pps (1 Hz). 
There is a simple modification to extract 8388.608 kHz, which is of course
223 Hz, and this frequency is used, through binary division, to generate the
1pps output. The 8388.608 kHz output is generated by a 32-bit Direct Digital
Synthesizer chip (AD9830). This output can be steered to any other frequency
within the operating range, by interacting with the controlling
microcontroller, with three provisos:

1. The unit has a peaked filter at the synthesizer output, and so the level
at other frequencies varies wildly. This can be corrected with minor
modifications.
2. When operating at any other frequency than 8388.608kHz, the 1pps output
is of course incorrect.
3. The synthesizer operating frequency can be set to within ±5 mHz
(milliHertz) of the requested frequency,
         - but ONLY if the calculations, on which the command sent to it is
based, correctly use 32-bit maths. "

Which is a confirmation...

http://www.leapsecond.com/museum/fei5650a/

http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:rubidium_oscillat
ors

So, this would work for the 50,255+ MHz based FE 5680A. For 60 MHz variants
it works a little different, but it has two MCUs sitting there, so some use
for theme should there be.

Cheers,
Magnus

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