Optical vs. Electrical Modules in Equivalent-Time Sampling Oscilloscopes
Filed under Modules, misc.
Equivalent-time sampling oscilloscopes require specialized plug-in modules to interface with high-speed digital systems. The choice between optical and electrical sampling modules depends directly on the nature of the signal source and the physical medium under test. Optical modules are designed to analyze optical waveforms directly from fiber connections, converting light signals to electrical currents for sampling. Electrical modules, on the other hand, measure electrical voltages directly over copper lines or coaxial cables.
Selecting the appropriate module involves managing significant trade-offs in bandwidth limits, noise floors, and signal conversion. While electrical modules typically offer higher raw bandwidths and lower voltage-based noise floors, optical modules introduce complex parameters such as optical-to-electrical conversion noise and integrated reference receiver filters. This guide examines these key technical differences to help engineers configure their testing mainframes effectively.
- Optical Wavelength Range
- 780 nm to 1650 nm
- Max Electrical Bandwidth
- 50 GHz
- Electrical Input Impedance
- 50 Ω nominal
- Fiber Types Supported
- 9/125 µm single-mode, 62.5 µm multi-mode
Bandwidth Capabilities and Transition Times
Bandwidth requirements differ sharply between optical and electrical domains. Purely electrical sampling modules, such as the Tektronix 80E01, can achieve high analog bandwidths up to 50 GHz with transition times (rise times) of 7.0 ps or less. This enables precise signal integrity analysis of ultra-fast electrical paths on backplanes and transmission lines.
Optical modules generally operate at lower bandwidths due to the physical limitations of photodetectors and internal transimpedance amplifiers. For instance, the Tektronix 80C08C has an unfiltered optical bandwidth of 12.5 GHz with an unfiltered transition time of 35 ps. Dual-purpose modules like the Keysight 86105A and Keysight 83485A provide selectable electrical and optical channel bandwidths, allowing users to toggle between 12.4 GHz and 20 GHz depending on the test requirements. Selecting 20 GHz on the electrical channel of the Keysight 83485A decreases the transition time to 17.5 ps compared to 28.2 ps at 12.4 GHz.
Noise Floors and Signal Conversion
A fundamental difference between optical and electrical modules is how noise is characterized. Because optical modules convert optical power to an electrical signal, their noise floors are specified in optical power (microwatts). For example, the Keysight 86105A exhibits a characteristic RMS noise of 8 µW at 12.4 GHz and 15 µW at 20 GHz on its optical channel. The Tektronix 80C08C achieves a lower noise floor of 1.5 µW at 12.5 GHz, which drops to 1.0 µW when using its integrated 9.953 Gb/s reference receiver filter.
Electrical channels measure noise in microvolts or millivolts. The electrical channel of both the Keysight 86105A and Keysight 83485A has a maximum RMS noise of 0.25 mV at 12.4 GHz, rising to 0.5 mV at 20 GHz. Ultra-high-bandwidth electrical modules like the Tektronix 80E01 maintain low noise levels relative to their bandwidth, with a typical RMS noise of 280 µV (450 µV maximum) at 50 GHz.
Optical Interfaces and Filtering
Optical modules must accommodate physical fiber-optic connections and standard-specific compliance testing. Modules like the Tektronix 80C08C are compatible with both single-mode and multi-mode fibers (using a 62.5 µm core diameter) and feature selectable hardware reference receiver filters for standardized rates such as 9.953 Gb/s, 10.3125 Gb/s, and up to 11.095 Gb/s. Dual-channel modules like the Keysight 86105A are optimized for 9/125 µm single-mode fibers across a wavelength range of 1000 nm to 1600 nm, with calibrated points at 1310 nm and 1550 nm.
For applications requiring synchronous signal analysis, clock recovery options (such as those on the Tektronix 80C08C) provide SMA outputs to lock the oscilloscope's timebase directly to the optical data stream.
Example instruments
Frequently asked questions
- What fiber core sizes are compatible with these optical modules?
- Compatibility depends on the module design. The Keysight 86105A is optimized for 9/125 µm single-mode fiber, while the Tektronix 80C08C features a 62.5 µm core diameter input that is compatible with both single-mode and multi-mode fiber.
- Why is optical noise specified in Watts instead of Volts?
- Optical sampling channels measure optical power using internal photodetectors. Therefore, the noise floor is characterized by RMS noise in microwatts (µW) rather than millivolts (mV).
- Can electrical modules handle the same voltage levels as optical modules?
- Electrical modules have strict voltage limits to prevent damage. For example, the Keysight 86105A has a maximum input signal of ±2 Vdc, and the Keysight 83485A has a maximum safe input of ±2 V plus peak AC (+16 dBm). The Tektronix 80E01 has a full-scale dynamic range of 1.0 V p-p and an offset range of ±1.6 V.