Clock Distribution

LimeSDR Mini board clock distribution block diagram is as shown in Figure 7.

Figure 7. LimeSDR Mini v2.4 board clock distribution block diagram

LimeSDR XTRX board features an on board 40.00 MHz VCTCXO as the reference clock for LMS7002M RF transceiver and FPGA PLLs.

Rakon E7355LF 40 MHz voltage controlled temperature compensated crystal oscillator (VCTCXO) is the clock source for the board. VCTCXO frequency can be tuned by using 10 bit (IC11) or 16 bit (IC16 unpopulated) DAC. Main VCTCXO parameters are listed in Table 5.

Table 5. Rakon E7355LF VCTCXO main parameters

Frequency parameter	Value
Calibration (25°C ± 1°C)	± 1 ppm max
Stability (-40 to 85 °C)	± 0.5 max
Long term stability (1 year, 10 years)	± 2 ppm max, ± 4 ppm max
Control voltage range	0.5 V .. 2.5 V
Frequency tuning	± 7 ppm min, ± 15 ppm max
Slope	+9 ppm/V

Analogue switch gives option to select clock source for RF transceiver and FPGA from buffered onboard VCTCXO clock (XO1) and external U.FL (J8). Buffered clock output (REF_CLK_OUT) is also available using U.FL (J7) connector.

The board clock lines and other related signals/information are listed in Table 6.

Table 6. LimeSDR XTRX main clock lines

Source	Schematic signal name	I/O standard	FPGA pin	Description
External	REF_CLK_IN	2.5V/3.3V		External reference clock input
Clock buffer (IC9)	REF_CLK_OUT	3.3V		Reference clock output
Clock buffer (IC9)	LMK_CLK	3.3V	A9	Reference clock connected to FPGA
RF transceiver (IC1)	RxPLL_CLK	1.8V		Reference clock input
	TxPLL_CLK	1.8V		Reference clock input
	LMS_MCLK1	2.5V/3.3V	H4
	LMS_FCLK1	2.5V/3.3V	H3
	LMS_MCLK2	2.5V/3.3V	D2
	LMS_FCLK2	2.5V/3.3V	D1
USB 3.0 controller (IC8)	FT_CLK	3.3V	D17	Clock output (100 MHz) from USB controller