Jackson Labs Micro-JLT GNSS, Руководство пользователя

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© 2019 Jackson Labs Technologies, Inc. 41
Micro-JLT GNSS™ User Manual
The board may be configured lock to an external PPS source rather than the internal GNSS receiver.
An optional TTL or CMOS 3.3V/5V PPS signal may be applied to Pin 3 of connector J4, and this
signal may be used to feed an external CMOS rising-edge PPS signal with -0.5V < x < 5.0V signal
level, and >= 100us minimum pulse width into the unit. Also refer to
Table 2.1 for pin descriptions of
connector J4.
By default the unit is set to GNSS disciplining. The unit may be hard-coded to only use the external
PPS source by setting EXT, or it may be auto-switched by the firmware to the external PPS signal if
the internal GNSS receiver does not generate 1PPS signals for longer than 15 seconds due to a weak
signal, or GPS failure. When set to the AUTO setting, the unit will switch back to the internal GNSS
receiver once 1PPS signals are generated internally again. Setting the unit to the EXT setting will
cause the unit to disable the GNSS receiver, and this setting may also be used to initiate a manual
forced-holdover if the GNSS signal is suspect, or unreliable.
The command has the following format:
SYNChronization:SOURce:MODE <GPS | EXTernal | AUTO>
The following command returns the current mode setting stored in NV memory:
SYNChronization:SOURce:MODE?
3.6.5 SYNChronization:SOURce:MODE:EDGE
<NEGative|POSitive>
This command configures the triggering edge of the external PPS input source (Pin-3 of J4
connector) shown in
Figure 3.1 for both OCXO disciplining discussed in Section 3.6 and
time-stamping subsystem in Section 3.5.11 .
By default the external PPS input capturing is triggered at the rising (or “POSITIVE”) edge. The
triggering edge can be changed to falling (or “NEGATIVE”) edge with the
SYNC:SOUR:MODE:EDGE NEG command.
The NEGATIVE edge setting allows the Micro-JLT GNSS™ to synchronize the rising edge of 1PPS
output to the falling edge of external PPS input if SYNC:SOUR:MODE EXT command is configured
in Section
3.6.4 . Alternatively the NEGATIVE setting allows the Micro-JLT GNSS™ to compute
the time-stamp offset between the GPS 1PPS and the falling edge of external PPS input if
PTIMe:STAMp:MODE DIFFer command is configured in Section
3.5.12 .
To avoid undesired OCXO disciplining or time-stamping, JLT recommends confirming the
characteristics of the external PPS input signal, proper settings in
SYNChronization:SOURce:MODE? and PTIMe:STAMp:MODE? commands before changing to
NEGATIVE setting.
The following command returns the current configured edge for external PPS input:
SYNChronization:SOURce:MODE:EDGE?
3.6.6 SYNChronization:SOURce:STATE?
The SYNChronization:SOURce:STATE? command is unrelated to the Micro-JLT GNSS™
time-stamping subsystem. For more information on time-stamping, please refer to Section
3.5.11 .
This command returns the state of the SYNC:SOUR:MODE command, and may return GPS or EXT.