Two locked GPSDO and phase difference

[tkamiya]

Hi,

I have two identical GPDSO (Nortel GPSTM) that are connected to their own antennas.  Antennas and cable length are not the same.  One has less than 5 feet of RG58 and the other has about 50 feet of it.  Antennas come from different manufacturers and located at different parts of my house.

Feeding 10Mhz outputs to each channel of 2 channel scope, they stabilized with some phase difference.  They are not changing relative to each other.

Is this an expected result?  If I make everything the same, say feed off of one antenna, would phase sync exactly as well?  (power injection would be little tricky….)

[Circlotron]

Is there any phase difference between the two 1pps signals?

[edpalmer42]

Hi,

I have two identical GPDSO (Nortel GPSTM) that are connected to their own antennas.  Antennas and cable length are not the same.  One has less than 5 feet of RG58 and the other has about 50 feet of it.  Antennas come from different manufacturers and located at different parts of my house.

Feeding 10Mhz outputs to each channel of 2 channel scope, they stabilized with some phase difference.  They are not changing relative to each other.

Is this an expected result?  If I make everything the same, say feed off of one antenna, would phase sync exactly as well?  (power injection would be little tricky….)

Yes.  Since they're both locked to GPS they will tend to be the same.  If you measure over a period of hours you'll see some drift back and forth due to noise.  If you had two different GPSDOs, they would still track each other, but not as well due to differences in the tracking & disciplining code as well as differing responses to environmental factors such as temperature.

I've attached a graph showing the phase difference between an NEC/Epson and HP Z3801A GPSDOs.  You can also read about some similar measurements here:

https://www.febo.com/pages/gpsdo_comparison/

Note that with two different types of GPSDO, the actual amount of phase difference might be impossible to determine beforehand.

[AG6QR]

Of course there will be a phase difference.

For a hypothetical, let's start with two absolutely identical GPSDOs with antennas at the same place, near as possible, and identical length cables.  To the extent that we manage to get everything exactly identical, the 10 MHz oscillators ought to be exactly in phase.

If we lengthen the cable of one of the oscillators, its phase will be delayed by the amount of time it takes the signal to travel the extra coax length.

10MHz is a wavelength of about 30 meters.  If we add a half wavelength of coax to the cable between one of the antennas and its oscillator, that will delay the signal by a half-wavelength, and put the oscillator 180 degrees out of phase from its twin.  To be more precise, we'd have to divide by the velocity factor, so 18.75 meters of coax would be about a half of a 10MHz wavelength after accounting for a velocity factor of 0.8.

You say you've got about 45 feet of difference in the coax lengths.  That's approximately 15 meters.  That's on the same order of magnitude as a 180 degree phase shift.

There may be other sources of phase shift besides the cable length, as edpalmer42 discusses.  But the cable length alone should cause an easily observable phase difference.

[tomato]

Hi,

I have two identical GPDSO (Nortel GPSTM) that are connected to their own antennas.  Antennas and cable length are not the same.  One has less than 5 feet of RG58 and the other has about 50 feet of it.  Antennas come from different manufacturers and located at different parts of my house.

Feeding 10Mhz outputs to each channel of 2 channel scope, they stabilized with some phase difference.  They are not changing relative to each other.

Is this an expected result?  If I make everything the same, say feed off of one antenna, would phase sync exactly as well?  (power injection would be little tricky….)

Identical oscillators, even when connected to the same antenna, will generally have a phase difference that wanders about.

[Zucca]

was already done:

https://www.eevblog.com/forum/testgear/comparing-two-hp-z3805a-gpsdo-10-mhz-frequencies-for-lab-use/