This blog post details the design and construction of a high-performance, low-cost 10 MHz - 15 GHz vector network analyzer (VNA). The author discusses the limitations of existing cheap VNAs, motivates the need for a custom-built solution, and presents the architecture and component selection for the dual-source VNA design. Key aspects covered include directional coupler design, mixer selection (ADL5802), ADC choice (AD9238), FPGA usage for signal processing, PCB layout considerations for isolation, and the CNC-machined case design. Performance measurements, including dynamic range, stability, and calibration accuracy using various methods (SOLT, Unknown Thru, EightTerm, TRL), are presented. Finally, the VNA's performance is validated by measuring a 6 GHz bandpass filter and a varactor diode, comparing the results with commercial VNAs and SPICE models, respectively. The total component cost is around $300, making it a viable alternative to expensive commercial VNAs.
Designing a Cost-Effective, High-Performance VNA (10 MHz - 15 GHz)
Introduction (0:00): Vector Network Analyzers (VNAs) are essential for high-frequency circuit design, but commercial options are expensive. The author aims to create a better-performing, low-cost alternative.
Limitations of Existing VNAs (0:00): Existing low-cost VNAs like nanoVNA and LibreVNA have limitations in frequency range, accuracy, and measurement capabilities (e.g., nanoVNA's 1.5-receiver architecture). Professional VNAs from companies like Keysight are too expensive for the author's purposes.
VNA Architecture (0:00): A dual-source architecture is used to eliminate the need for a high-isolation port switch, using separate signal sources (LMX2594 PLL chips) for each port to improve isolation. Harmonics and the low output power are considered acceptable trade-offs.
Directional Couplers (0:00): Custom-designed resistive directional bridge couplers with coaxial cable baluns are used to sample incident and reflected waves. Performance measurements show about 20 dB directivity and acceptable isolation between coupled ports.
Receiver (0:00): The ADL5802 dual-channel mixer, though only rated to 6 GHz, is selected for its low cost. Higher-frequency performance relies on the mixer's out-of-spec operation.
ADC (0:00): The AD9238 ADC, with a 40 MHz sampling frequency and -143 dBFs/Hz NSD (Noise Spectral Density), is used, balancing cost and dynamic range.
FPGA (0:00): An FPGA handles data processing from the ADC, implementing digital signal processing (DSP) for I/Q signal extraction using discrete Fourier transform. Logic for I/O and communication with PC is also implemented in the FPGA.
PCB and Case (0:00): A 6-layer FR4 PCB is used with considerations for RF enclosure mounting. CNC-machined aluminum case provides shielding. Gaps around SMA connectors were addressed with aluminum foil and solder wick to minimize leakage.
Performance Testing (0:00): After addressing leakage issues, the VNA demonstrated excellent isolation and a dynamic range up to 120 dB at low frequencies, decreasing to 70 dB at 15 GHz.
Calibration and Stability (0:00): Calibration is done using self-made standards and the Unknown Thru algorithm. Thermal stability is examined.
Harmonic Mixing (0:00): Harmonic mixing is implemented beyond 6 GHz for higher frequency measurement using odd harmonics in the mixer.
Bandpass Filter Measurement (0:00): A 6 GHz bandpass filter is measured and compared to a commercial VNA, showing good agreement and validating the custom VNA's accuracy.
Calibration Algorithms (0:00): Explores different calibration algorithms (SOLT, Unknown Thru, EightTerm) and the advantages of Unknown Thru, which doesn't require a precisely known Thru standard, beneficial for low-cost VNAs.
TRL Calibration (0:00): Thru-Reflect-Line (TRL) calibration allows calibrating the reference plane on the PCB, useful for measuring SMD components. Transmission line parameters solved by the TRL calibration are validated.
Varactor Diode Measurement (0:00): A varactor diode is measured using TRL calibration, demonstrating the VNA's ability to characterize SMD components accurately. Bias-Ts were added to VNA ports for biasing the diode. Measurements are compared with a SPICE model and a datasheet, showing good agreement.
Conclusion (0:00): The custom-designed VNA achieves high measurement quality and performance surpassing existing low-cost VNAs. The total component cost is $300 + PCB + $75 CNC machined case. The schematic is provided for those interested in building their own.