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From: | Marcus D. Leech |
Subject: | Re: B200 clock and pulse timing |
Date: | Tue, 06 Apr 2021 21:34:43 -0400 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0 |
On 04/06/2021 07:14 PM, Mike wrote:
By "resolution of the clock", you presumably mean the Timestamp clock--there are many clocks used in a radio system. The timestamp clock "ticks" at the rate of the master-clock rate at which the AD9361 runs. Usually, the master clock rate is some multiple of the sample-rate. The clock architecture of the B2xx has an on-board TCXO that provides top-level clock-reference for a number of functions, including deriving the master-clock used by the AD9361 RFFE chip. I recall that TCXO runs at 40MHz, but the schematics can be consulted for a definitive answer. https://files.ettus.com/schematics/b200/b210.pdf When you use an external reference, it drives a PLL chip to provide the top-level clock for everything on the board. It is typically the case that you use an external reference because it is better than the on-board TCXO (or because you want synchronization across multiple units in some cases). The on-board TCXO is a +/- 2PPM part. External references such as an OCXO, GPSDO, Rubidium clock are very much better than this. There can be many many many samples "in flight" when a PPS arrives. What sample rates are you using? There is necessarily buffering within the systems USB drivers, and within UHD. You can tune this to a certain extent using the USB transport parameters: https://files.ettus.com/manual/page_transport.html#transport_usb But trimming back buffering to reduce average latency risks losing samples (overruns).
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