The **Keysight N4373C** is a high-performance **Lightwave Component Analyzer (LCA)**. It is designed to measure the electro-optical transfer function of components such as laser diodes, modulators, and photodiodes. Below is a breakdown of its primary electronic parts and system components. --- ## 1. Core System Components The N4373C is not a standalone box but an integrated system consisting of several high-end instruments working in tandem. | Component | Function | | :--- | :--- | | **Vector Network Analyzer (VNA)** | The "brain" of the system (typically a PNA or PNA-X series). It generates the RF modulation signal and measures the resulting electrical response. | | **Optical Test Set** | Contains the optical source (laser), modulators, and internal receivers necessary to convert electrical signals to optical and vice versa. | | **Calibration Kit** | Electronic and optical standards used to remove systematic errors from the measurement path. | --- ## 2. Key Internal Electronic/Optical Modules The internal circuitry of the N4373C focuses on high-bandwidth signal conversion. ### A. The Optical Transmitter (E/O Converter) This section converts the electrical RF signal from the VNA into a modulated optical signal. * **Mach-Zehnder Modulator (MZM):** A wideband lithium niobate ($LiNbO_3$) modulator that imprints the RF signal onto a laser carrier. * **Bias Controller:** Electronic circuitry that maintains the modulator at the "Quadrature" point to ensure linear modulation and prevent distortion. ### B. The Optical Receiver (O/E Converter) This section converts the optical signal back into an electrical RF signal for measurement. * **Photodetector (PIN Diode):** A high-speed InGaAs photodiode designed for flat frequency response up to 67 GHz (depending on the specific model option). * **Transimpedance Amplifier (TIA):** Low-noise electronics that amplify the tiny current from the photodiode into a measurable voltage. ### C. Signal Processing Board * **DSP (Digital Signal Processing):** Handles the de-embedding of the test fixture. It subtracts the known frequency response of the N4373C’s internal parts so that only the "Device Under Test" (DUT) performance is shown. --- ## 3. Technical Specifications Table | Feature | Specification Details | | :--- | :--- | | **Frequency Range** | Typically 10 MHz to 40 GHz / 50 GHz / 67 GHz | | **Wavelength Range** | 1260 nm to 1640 nm (Single-mode) | | **RF Interface** | 3.5 mm, 2.4 mm, or 1.85 mm male/female connectors | | **Optical Interface** | FC/APC (standard for high return loss) | --- ## 4. Operational Block Diagram Logic 1. **RF Out:** The VNA sends a swept RF signal. 2. **Modulation:** The internal laser is modulated by this RF signal. 3. **DUT Interaction:** The modulated light passes through your component. 4. **Detection:** The internal receiver converts the light back to RF. 5. **Analysis:** The VNA compares the **Phase** and **Magnitude** of the sent signal vs. the received signal.