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TruePosition GPSDO build question

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edpalmer42:

--- Quote from: Squiddaddy on January 10, 2019, 04:15:07 pm ---Something else I noticed reading all of the TruePosition discussion on here is a command to set the cable delay of the antenna.
I have 50ft of cable going to the outside timing antenna. Would it matter if the delay was added, or is it small enough to not bother? Does the cable delay affect the 10MHz output, or just the 1PPS signal? I'm not planning to use this as any time source, just a frequency reference.
I don't know how to get the delay figure anyway.

--- End quote ---

Typically, the antenna delay is used if you need your 1 PPS to be synchronized with UTC.  Since there's delay in the antenna cable, you have to compensate for it.  We don't care about such things, so it's almost never used.


--- Quote ---Also, the DAC value to the OCXO on it still changes. Once this thing locks to GPS, shouldn't the DAC stay put for long periods of time? Or is it always adjusting? These changes are in microvolts, so I believe it's normal. Currently at 2.071xxx volts, with the last 3 digits changing slightly every 15 seconds or so. And that's with the Atmega328 rounding it to 6 digits. Usually seeing 8 satellites in the sky.
So far, I'm impressed with this thing.
Been powered up a little over a week now. Unfortunately, I have to shut it down to put it on a UPS at some point.  :(

--- End quote ---

The OCXO will probably take weeks, if not longer, of continuous operating before it really settles down.  Aging will be high, there may be frequency and/or phase jumps.  During this period, the PLL circuitry will really be working hard and, as a result, the DAC value will bounce around.  How stable the DAC value eventually becomes will depend on the characteristics of your particular OCXO and the output of your GPS receiver.

Ed

cdev:
You need to know the velocity factor and length of your antenna's coax cable, it could be gotten either from the manufacturer - (or a chart like one at http://rfcafe.com/references/electrical/coax_chart.htm  ) if you know exactly what coax was used- (should be written on it) or it can also be determined experimentally using an oscilloscope



texaspyro:

--- Quote from: cdev on January 11, 2019, 02:42:28 am ---You need to know the velocity factor and length of your antenna's coax cable, it could be gotten either from the manufacturer - if you know exactly what coax was used, or determined experimentally.

--- End quote ---

Lady Heather's cable delay setting command can set the value directly in nanoseconds, or you can specify the value in feet/meters of cable with an optional velocity factor (default Vf is 0.66).  Also your antenna cable delay value should include the delay in the 1PPS wiring...  which probably has a different velocity factor.

One gotcha in setting cable delay values is the sign of the required value.  Some devices use positive numbers, others use negative numbers... and very few manufacturers document what their device expects.

cdev:
Negative VF?

That would really be an amazing feat wouldn't it be, if a signal could be made to emerge from a transmission line before it went in!

"Deja-vu"

Squiddaddy:
When I watch the clock output of the TruePosition and the computer, they both switch to the next minute as accurately as my eyes can determine. I'm not going to sweat over nanoseconds.  :-DD
I think I'll leave the settings alone.
Good info on determining the velocity factor though.  :)

Question for @texaspyro, how does LadyHeather know when to drop a satellite off of the list? Does the TruePosition tell you anything in the $SAT messages from the stream? I was going to include satellite data in an array for display on my AtMega328 LCD, but didn't bother because I didn't know how/when to remove them. And I didn't want to overcomplicate it since it's only software for me.

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