diff options
author | David Raeman <david@synopticengineering.com> | 2022-02-21 15:11:32 +0000 |
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committer | Aki Tomita <121511582+atomita-ni@users.noreply.github.com> | 2024-02-21 06:40:57 -0600 |
commit | 76e004db07a3714139e64ff396a81b90c372b2bb (patch) | |
tree | 3da50b61106f7905ee0d56c0d495e4f2530c2368 /host | |
parent | multi_usrp: Fix setters for ALL_CHANS (diff) | |
download | uhd-76e004db07a3714139e64ff396a81b90c372b2bb.tar.xz uhd-76e004db07a3714139e64ff396a81b90c372b2bb.zip |
n3xx: Add comments on clock_source=external,time_source=gpsdo
It is, in principle, possible to use an external clock to synchronize
N3x0 devices in frequency, and then use a GPSDO for a coarse time
synchronization. This use case is deliberately not supported, as the
GPSDO PPS signal and the external clock signal are by definition not
matched, which will remove any guarantees on time/phase alignment.
Because there are certain, niche use cases where the lack of phase
alignment is acceptable, but only an external clock is available (no
shared external PPS), usage of GPS for generating a PPS signal may be
fine. This patch does not enable the usage of this combination out of
the box, but adds comments and an update to the manual to explain the
risks of this combination, and how to enable it (by patching MPM).
Diffstat (limited to 'host')
-rw-r--r-- | host/docs/sync.dox | 3 | ||||
-rw-r--r-- | host/docs/usrp_n3xx.dox | 22 |
2 files changed, 24 insertions, 1 deletions
diff --git a/host/docs/sync.dox b/host/docs/sync.dox index 9ce618010..9174bf5de 100644 --- a/host/docs/sync.dox +++ b/host/docs/sync.dox @@ -17,7 +17,8 @@ USRP devices take two reference signals in order to synchronize clocks and time: - A 10 MHz reference to provide a single frequency reference for all - devices. + devices (in some devices, other reference frequencies than 10 MHz are + also supported). - A pulse-per-second (PPS) to synchronize the sample time across devices. diff --git a/host/docs/usrp_n3xx.dox b/host/docs/usrp_n3xx.dox index 21b156296..4524a2e03 100644 --- a/host/docs/usrp_n3xx.dox +++ b/host/docs/usrp_n3xx.dox @@ -671,6 +671,28 @@ will look like this: \section n3xx_synchronization Clock/Time Synchronization +\subsection n3xx_synchronization_overview Overview + +To choose a source for clock (frequency) and PPS (time), use the `clock_source` +and `time_source` device arguments just like with any other USRP (see also +\ref page_sync). The following combinations are supported: + +`clock_source` | `time_source` | Notes +----------------|----------------|--------------------------------------------- +`internal` | `internal` | Default value +`external` | `internal` | The PPS will be generated from the reference signal, but this does not allow time synchronization between devices. +`external` | `external` | The device assumes that the external reference clock and PPS are synchronized. +`gpsdo` | `gpsdo` | This will work even without GPS reception (see below). + +The combination of `clock_source=external,time_source=gpsdo` is not available +out of the box, but can be enabled by patching the MPM source code (n3xx.py). +This is considered an advanced and unsupported use case. Read the appropriate +source code for a description of drawbacks. Using this mode will cause unreliable +phase alignment. + +See the following sections for more details on the individual synchronization +options. + \subsection n3xx_synchronization_internal Internal references The N3xx series has an onboard GPSDO as well as a 25 MHz reference oscillator, |