[time-nuts] Comparing the performance of 17 different GPSDOs

[Mark Sims]

I recently completed some comparison tests of the PPS and 10 MHz outputs of 17 different GPSDOs.

The tests were done with a TAPR TICC time interval counter running in timestamp mode and the data was acquired and processed with Lady Heather.   The TICC was clocked by a HP-5071A cesium beam oscillator with the high-performance tube.   The TICC has a resolution of around 60 picoseconds.

The PPS signal from the GPSDO was connected to channel A of the TICC (for those GPSDOs that have an easily accessible PPS signal).  For receivers without a PPS signal available, Heather shows ADEV, HDEV, MDEV, and TDEV data for the 10 MHz signal.   For receivers with a PPS,  ADEV is shown for the 1PPS and 10 MHz signals.

The GPSDO output frequency (10 MHZ) was connected to channel B of the TICC through a TAPR TADD-2 Mini divider to get a 1 Hz output.

Note that PPS and 10 MHz plots have a display averaging filter applied.   Without this filter the plots mostly show noisy "grass" and not any interesting details.  A side effect of the display filter is to reduce the "span" (difference between the max and min values seen) of the plot data.  Typically the unfiltered spans are around twice the value shown in the plot header, but for a couple of receivers it is up to a 10x reduction of the raw span.

Each GPSDO was allowed to warm up for at least 24 hours,  or longer... until the DAC output voltage slope leveled off.

The GPSDO antenna was a L1/L2/GPS/GLONASS/BEIDOU antenna from China.  With a L1/L2 survey receiver this antenna produces location error ellipse in the 5-10mm range.  It is mounted on a 1 meter tripod and fed into an 8-way amplified L1/L2/Glonass/Beidou splitter.  The antenna is in a rather horrible location for multi-path and sky view.   Lots of tall trees and a 2 story stucco-over-wire mesh house.

http://www.eevblog.com/forum/metrology/comparing-the-performance-of-17-different-gpsdos/msg2025295/#msg2025295