Reference Clock

The LimeSDR XTRX clock system is based on a high stability 26 MHz VCTCXO (Voltage Controlled Temperature Compensated Crystal Oscillator) which can be tuned via an external 1PPS reference signal or GPSDO function.

The board provides reference clock and 1PPS input and output via Mini PCIe connector, as well as dedicated U.FL connectors for reference clock input/output (X7) and 1PPS input (X6).

Table 2. Clock Functions

Function	Specification	Notes
On-board Oscillator	26 MHz VCTCXO	Rakon E6982LF, ±0.2 ppm stability
External Clock Input	mPCIe pin 19 or U.FL (X7)	10-52 MHz, 1.8V - 3.3V
Clock Output	mPCIe pin 30 or U.FL (X7)	3.3V CMOS
1PPS Input	mPCIe pin 3 or U.FL (X6)	3.3V CMOS
1PPS Output	mPCIe pin 5	3.3V CMOS

Note

By default clock connectors (X7) function is an external clock input. It can be changed to a clock output by removing resistor R155 and populating resistor R156 (0-ohm).

Warning

When using external clock references, ensure signal levels and frequencies match specifications.

Improper clock signals may cause unstable operation and potential damage to the device.