STEMlab 125-14 External Clock (Discontinued)

Note

STEMlab 125-14 External Clock is discontinued (no longer produced). The documentation here is provided as a reference for existing users.

Overview

The STEMlab 125-14 External Clock is a hardware-modified version of the standard STEMlab 125-14 that accepts an external clock source. SMD resistors have been relocated at the factory to connect the Ext. ADC Clk± pins on the E2 connector directly to the ADC clock input, bypassing the on-board oscillator.

This variant is intended for applications requiring clock synchronisation with external equipment or operation at non-standard sampling rates.

Features

• 14-bit, 125 MS/s ADC and DAC (at 125 MHz clock)

• External clock input support (pre-modified hardware)

• Dual-core ARM Cortex-A9 processor

• FPGA Xilinx Zynq 7010 SoC

• 512 MB RAM

• 16 digital I/Os, 4 analog inputs, 4 analog outputs

• Multiple communication interfaces: I2C, SPI, UART, CAN

• Micro USB connectivity for power and console

Quick Reference

Category	Key Specifications
ADC	2 channels, 14-bit, 125 MS/s, DC-60 MHz
DAC	2 channels, 14-bit, 125 MS/s, DC-60 MHz
Processor	Dual-core ARM Cortex-A9
FPGA	Xilinx Zynq 7010 SoC
RAM	512 MB
Digital I/O	16 GPIOs @ 3.3V
Analog I/O	4 inputs (12-bit), 4 outputs (8-bit)
Connectivity	Ethernet, USB, Extension connectors
Special Features	External clock input (LVDS)

Differences from Standard STEMlab 125-14

This board is electrically identical to the STEMlab 125-14 with the following hardware changes already applied at the factory:

Parameter	STEMlab 125-14	STEMlab 125-14 External Clock
On-board 125 MHz oscillator	Active (drives ADC, FPGA, DAC)	Bypassed (disconnected from clock path)
E2 pins 23-24 (Ext. ADC Clk±)	Not connected	Active — must receive a valid LVDS clock
External ADC clock	No (requires hardware modification)	Yes (pre-modified, plug-and-use)
Resistors R25, R26	In default position	Relocated to R23, R24
Operation without external clock	Normal (uses onboard oscillator)	FPGA non-functional; PS boots from 33 MHz

Warning

The on-board oscillator is bypassed. The board will not perform signal acquisition or generation without a valid external LVDS clock on E2 pins 23-24 (Ext. ADC Clk±). Leaving these pins floating or supplying an incorrect signal will prevent FPGA operation.

• OS 2.07-48 or higher: The Linux OS will still boot using an internal 33 MHz oscillator, but FPGA functionality will not be available.

• OS versions prior to 2.07-48: The board will fail to boot without a valid external clock signal.

Note

When using this board with synchronisation features:

• X-channel 2.0 (Click Shield) synchronisation is compatible with this board.

• X-channel synchronisation requires the X-channel system (primary and secondary boards), which differ from external clock models.

Technical Specifications

The STEMlab 125-14 External Clock has the same specifications as the standard STEMlab 125-14, with the following addition:

• External ADC clock: Yes (hardware pre-modified for external clock input)

• Hardware modification: resistors R25, R26 relocated to R23, R24 to enable external clock input

For full technical specifications, please refer to the STEMlab 125-14 specifications.

See also

For more detailed information, please refer to the Original Gen board comparison table.

Performance & Measurements

You can find the measurements of the fast analog frontend here:

• Original Gen - STEMlab 125-14.

Schematics & 3D Models

Schematics

• Schematics_STEM_125-14_v1.1.pdf.

Note

The external clock variant uses the same PCB as the standard STEMlab 125-14. The only physical difference is the relocation of resistors R25, R26 to R23, R24.

Mechanical Specifications & 3D Models

• STEP 3D_STEM_125-14_v1.0.zip.

Advanced Features

External ADC Clock

The main ADC, DAC, and FPGA CLK signal can be supplied from an external source through the Ext. ADC Clk± ports on the E2 connector connector.

Signal Path

The clock signal travels directly through the ADC, then to the FPGA, and DAC, ensuring consistent timing across the signal acquisition path.

External Clock Specifications

The external clock should be a differential LVDS signal:

Parameter		Description	Min	Typ	Max	Unit
	Clock input pins	23 (Clk+) and 24 (Clk-) on E2 connector
	Input standards	LVDS
	Input clock coupling	AC (on-board capacitors)
\(f_{CLK}\)	Input frequency range	ADC and DAC dependant	1		125	MHz
\(V_{ID,DIFF,PP}\)	Input voltage swing	Differential peak-to-peak		0.35	0.8	V
IDC	Input clock duty cycle		45%		55%

Note

The typical operating frequency depends on the board variant:

• STEMlab 125-14: 125 MHz

• SDRlab 122-16: 122.88 MHz

Operating outside the board’s specified frequency is possible but voids all performance guarantees. See the warning below.

For exact voltage levels and timing requirements, please refer to the oscillator datasheet.

Warning

Not a reference clock input: The Ext. ADC Clk± ports are not a reference clock input for a PLL or any other timing circuit. They directly drive the main sampling clock for the ADC, DAC, and FPGA. Connecting a reference clock signal intended for a PLL will not function as expected.

Warning

Changing the external clock frequency during operation is not supported.

The Zynq 7010/7020 PL uses Mixed-Mode Clock Managers (MMCMs) and PLLs to derive internal fabric clocks from the input clock. Changing the external clock frequency during operation will cause the MMCM/PLL to lose lock, resulting in undefined output clocks and acquisition/generation errors. A full FPGA reset or reconfiguration is required after any clock frequency change.

Note

Operating at non-standard clock frequencies:

The Red Pitaya FPGA is designed, tested, and guaranteed to operate correctly at the board’s specified core clock frequency (125 MHz for STEMlab 125-14, 122.88 MHz for SDRlab 122-16, etc).

While it is possible to run the board at different clock frequencies, please be aware that:

1. FPGA functionality: The official FPGA configuration may not function as intended at non-standard frequencies and requires thorough testing

2. Sampling rates: The ADC and DAC sampling rates will change proportionally with the clock frequency

3. Analog bandwidth: The analog bandwidth of the board will change with the clock frequency

4. Absence of external clock:

   • OS 2.07-48 or higher: If no valid external clock is present, the PS side will boot using an internal 33 MHz oscillator, but the FPGA will not operate (no signal acquisition/generation functionality)

   • OS versions prior to 2.07-48: The board will fail to boot (stuck in reboot cycle) if no valid external clock is detected

Note

Advanced: Dynamic clock reconfiguration

The standard Red Pitaya FPGA bitstream does not implement the Dynamic Reconfiguration Port (DRP) interface that AMD (Xilinx) provides for runtime MMCM/PLL reconfiguration. A custom FPGA design using DRP-based MMCM reconfiguration with active lock monitoring could theoretically support runtime clock frequency changes, but this requires extensive FPGA development and verification. The PS side (ARM/Linux) is not affected by clock changes, as it runs from its own independent 33 MHz crystal-derived PLL.

Additional Resources

For additional specifications and measurements, please refer to:

• STEMlab 125-14 - Standard STEMlab 125-14 specifications

• Original Gen hardware specifications - Common Original Gen specifications

• Original Gen board comparison table - Comparison across all Red Pitaya Original Gen models

Legal & Disclaimers

Note

The information provided by Red Pitaya d.o.o. is believed to be accurate and reliable. However, no liability is accepted for its use. Please note that the contents may be subject to change without prior notice.