Hello, I am also using multiple distributed E320s for an application that uses the integrated GPSDO to have a common sense of time for precise TOA estimation. I don't have the measurements at my fingertips at the moment, but I found that synchronization was substantially better using a high-quality external clock/time source (e.g. Octoclock or a good OCXO), compared to using the integrated GPSDO.
The synchronization performance of the integrated GPSDO improved if I let the radios sit for quite a while. For example, if two radios had GPS lock for 5 minutes before running an experiment, relative carrier drift would be measurably worse than if they were allowed to sit with GPS lock for an hour. I believe that once the GPS receiver acquires time, the 10 MHz reference convergence occurs very slowly. In the case of a using an external clock, to get good phase performance I also had to power down the internal GPSDO. The RF switch used to select between the two sources seems to introduce some coupling/interference when both sources are present at the switch inputs. My old mailing list inquiry about the problem is here<https://lists.ettus.com/empathy/thread/XK7DJMNXTZVTKHGVAPZFJL3NZEF73XME>. I pushed a patch to Ettus, incorporated into UHD 4.4, which allows you to power off the integrated GPSDO by passing "enable_gps=0" in your UHD session args. Interestingly, in that thread somebody mentions having a similar issue on a B210 device if the GPSDO add-on module is installed, which doesn't seem to have been an issue with your prior results. I can try to follow-up with some measurements from our experiments, and I'd appreciate any additional insights you might find as well since this is still an active project for me. -David From: edenmclaughlin...@gmail.com <edenmclaughlin...@gmail.com> Sent: Monday, April 22, 2024 4:24 AM To: usrp-users@lists.ettus.com Subject: [USRP-users] About Two E320's Synchronization using GPSDO's We have a setup with two USRP devices, attempting to acquire signals' time of arrival (TOA) with matched filters. We can observe stable and accurate TOA using USB-driven devices (i.e., B210's) with a single external PPS and 10MHz feed. However, when testing the design with two E320's using their integrated GPSDO as the clock and timing source, we could not achieve a steady TOA anywhere close to the clock performance specified here<https://www.ettus.com/wp-content/uploads/2019/01/USRP_E320_Datasheet.pdf>. A constant drift over time (i.e., +50 ns over ~120 seconds) was observed on one E320 device with respect to the other for each run. We are almost certain that neither of the devices is damaged (they are fresh out of the box), also we are aware of this<https://lists.ettus.com/empathy/thread/EDXCIKFY5LW7IEQN2JZOGXGWNCBELU3F?hash=4KGAPCNGJL6WXKC4IBZBITVNSFTDXXRK#4KGAPCNGJL6WXKC4IBZBITVNSFTDXXRK> issue. The setup includes one source signal symmetrically fed to the devices from the same RF channels. The E320 devices share the same GPS antenna, which is an externally fed active GPS antenna while the E320 GPSDO's ports are DC blocked. We are yet to try: 1. Observing the behavior with another pair of E320 devices. 2. Feeding the source signal from the other RF ends. 3. Synchronizing the E320's with an external clock and timer, as we have done with the B210 devices. We are wondering if there are any software tricks applicable specific to the E320 devices that we should be employing. Any help will be appreciated!
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