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In the context of Software defined radios (SDRs), what are the differences between using an external 10MHz reference versus using a GPSDO? Why/when would you use one over the other? And how does the 1PPS relate to all this? Is the 1PPS only used for system time? What happens if you don't use a 1PPS?

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A GPSDO is a way of providing a 10MHz reference. A GPSDO can be an external device, but some SDR devices are also able to "host" a GPSDO so that you don't need an external unit in its own box with its own power and its own cable for the 10MHz; you just buy the GPSDO option, plug a GPS antenna into the port for it, and go. Other sources of 10MHz are available, of course, and if you have one of those you might prefer the simplicity of an external reference input.

PPS is another signal commonly available from GPSDOs and similar devices. Yes, it's usually synchronized to top-of-second, which means that it's useful for deriving time as well as frequency. In some radio applications that's not needed at all, but some systems (such as LTE) require participants to have good absolute time, and 10MHz won't give you that.

In theory 1PPS will give you frequency as well as time, but disciplining frequency from 1PPS is much slower (and much more demanding of oscillator stability) than using 10MHz, so if you have 10MHz available you will almost always use that, and if you need time and frequency you will use both.

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  • \$\begingroup\$ Not just "usually synchronized" -- it is by definition top-of-second. It is used both for timekeeping and precision navigation, as described here. \$\endgroup\$
    – Dave Tweed
    Jan 7 at 4:51
  • \$\begingroup\$ @DaveTweed Unless the device has been told to introduce an offset, subject to the precision of the device firmware, and subject to the accuracy of the constellation's idea of UTC. All else being equal, I've observed stable discrepancies of the order of 10 nSec between the American, European and Russian systems, of of the order of 100 nSec between those and the Chinese system. \$\endgroup\$ Jan 7 at 12:23
  • \$\begingroup\$ @MarkMorganLloyd: The offset is used to compensate for antenna cable length, and the rest reflects the current state of the art accuracy of the various systems. None of which negates what I said. \$\endgroup\$
    – Dave Tweed
    Jan 7 at 13:03
  • \$\begingroup\$ @DaveTweed PPS at top of second is conventional, and required for a bunch of industrial applications, but it may be configured differently. And consider a receiver where the timepulse and/or the nav rate can be configured for rates other than 1Hz. Modern uBlox has separate messages NAV-TIMEGPS (for exact specification of nav TOV) and TIM-TP (for exact specification of the next "PPS" pulse). \$\endgroup\$
    – hobbs
    Feb 8 at 18:53

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