Calibration process of RF circulator and isolator

　　RF circulator and isolator are key components in RF system, and the accuracy of their performance parameters directly affects the quality of signal transmission. Calibration can ensure that the isolation, insertion loss, standing wave ratio and other parameters of the device within the working frequency band meet the design standards. The following is a detailed calibration process.

　　I. Preparation before calibration

　　(I) Equipment and environment preparation

　　Calibration requires a high-precision vector network analyzer (VNA, such as Keysight N5227A), whose frequency range must cover the working frequency band of the device to be calibrated (such as covering 790MHz-12GHz to adapt to multi-scenario devices), and the calibration parts must be metrologically certified (such as FLUKE calibration certificate within the validity period). At the same time, prepare auxiliary tools such as coaxial cable (low-loss RG-405 type), adapter (N type / SMA type conversion head), power meter (accuracy ±0.1dB), all connecting parts must be free of physical damage, and the interface must be clean and non-oxidized (the interface surface can be wiped with anhydrous alcohol).

　　The calibration environment must meet the following requirements: temperature 25℃±2℃, relative humidity 45%-55%, avoid strong electromagnetic interference (stay away from high-power transmitting equipment, motors, etc.), and lay anti-static mats on the work surface and ground it (grounding resistance ≤4Ω). For micro IoT devices (such as RFCI RFCR2895), special fixtures must be prepared to avoid measurement errors caused by hand-held (hand-held will introduce 0.5-1dB loss fluctuations).

　　(II) Device parameter confirmation

　　Before calibration, the key parameters of the device to be calibrated must be clarified: operating frequency range (such as S band 2700-2900MHz, Ku band 12-18GHz), rated power, interface type (SMA, N type, etc.). Check the device datasheet to obtain the nominal value, such as the typical isolation value of RFCI RFCR2895 is >21dB and the insertion loss is <0.5dB, as the basis for calibration qualification. At the same time, record the device serial number and establish a calibration file for subsequent traceability.

　　2. Vector Network Analyzer Calibration (Two-Port/Three-Port Calibration)

　　(I) Two-Port Calibration (Applicable to Isolators)

　　Connect the calibration parts: Connect port 1 of the VNA to the calibration parts (open, short, matched load) in sequence, and complete the "open-short-load" calibration according to the instrument prompts to ensure that the reflection parameter error in the test frequency band is less than 0.1dB.

　　Transmission calibration: Connect ports 1 and 2 of the VNA with low-loss cables, perform transmission/reflection calibration, eliminate cable loss and phase offset, and control the transmission loss measurement error within ±0.05dB.

　　(II) Three-Port Calibration (Applicable to Circulators)

　　Use the "TRL (Thru-Reflect-Line)" calibration method:

　　Thru: Connect any two of the three ports (such as ports 1-2, 2-3, 1-3) with a zero-length adapter and record the transmission parameters.

　　Reflection: Connect a high-precision short circuit (reflection coefficient>0.99) to each port and measure the reflection parameters.

　　Transmission line: Use a standard transmission line with known length (such as 100mm) and characteristic impedance (50Ω) to connect the port pair and calibrate the phase and delay errors.

　　After calibration, it is necessary to verify that the repeatability of the isolation measurement between ports is less than 0.3dB at the center frequency to ensure the effectiveness of the calibration.

　　III. Key parameter calibration steps

　　(I) Isolation calibration (core parameters)

　　Circulator isolation: Take the three-port circulator (port 1→2→3 is the forward direction) as an example. Port 1 of the VNA is connected to the signal source, port 2 is connected to the matching load (VSWR<1.1), and port 3 is connected to the VNA receiving end. Scan within the working frequency band and record the difference between the receiving power of port 3 and the transmitting power of port 1 (i.e., S31 parameter). This value is the isolation of port 1→3, which must meet the following requirements: ≥ nominal value in the entire frequency band (e.g., Sonoma Scientific Ku-band isolator requires ≥25dB).

　　Isolator isolation: The input end of the two-port isolator is connected to the signal source, the output end is connected to the load, and the reverse port (isolation end) is connected to the VNA receiving end. The reverse attenuation (S12 parameter) is measured. The typical value must be ≥20dB (IoT micro devices require ≥18dB).

　　(II) Insertion loss calibration

　　Forward insertion loss: The forward path of the circulator (e.g., 1→2) is connected to the signal source and power meter, and the input power Pin and output power Pout are measured. The insertion loss IL=10lg (Pout/Pin) must be ≤ nominal value + 0.2dB (e.g., China Electronics Technology's ultra-wideband circulator requires IL<0.6dB).

　　Frequency response flatness: Measure the insertion loss every 10MHz in the working frequency band, calculate the difference between the maximum and minimum values, which must be ≤0.3dB (≤0.5dB for wideband devices) to ensure the consistency of signal transmission.

　　(III) VSWR calibration

　　Connect the test port of the VNA to any port of the circulator/isolator through a cable, connect the other ports to matching loads, measure the reflection coefficient Γ of the port, and calculate VSWR=(1+|Γ|)/(1-|Γ|). The VSWR of each port must be ≤1.2 (standard value), which can be relaxed to ≤1.3 for micro devices (such as those used in the Internet of Things), but it must be ensured that there is no mutation in the entire frequency band (mutation>0.2dB requires troubleshooting of interface problems).

　　4. Power Withstand Calibration (optional, high-power devices)

　　For devices with rated power > 10W (such as industrial circulators), a power withstand test is required:

　　Use a signal generator (such as Agilent E8257D) to output rated power (such as 250W) at the center frequency, connect the device input through a directional coupler, and connect the output to a water load (power capacity ≥ 1.5 times the rated value).

　　Continuously load the power for 30 minutes (or according to the device standard duration), during which the device surface temperature is monitored with an infrared thermometer (≤ 65°C). After the end, re-measure the isolation and insertion loss. The change must be ≤ 0.5dB, otherwise it is judged as unqualified power tolerance.

　　5. Data processing and verification after calibration

　　(I) Data recording and analysis

　　Enter the calibration data (frequency point, isolation, insertion loss, VSWR) into a table, draw a parameter curve (such as isolation versus frequency curve), and compare it with the nominal curve. The frequency points with excessive deviations must be marked and the reasons analyzed (such as loose interfaces and device aging). For example, the isolation of an S-band circulator at 2800MHz is only 17dB (nominal 21dB), and it is necessary to check whether the port is oxidized or the internal ferrite is demagnetized.

　　(II) Qualification judgment and issuance of certificate

　　According to the calibration results, the following conditions are met and it is judged to be qualified:

　　Isolation in the entire frequency band ≥ nominal value - 1dB;

　　Insertion loss ≤ nominal value + 0.3dB;

　　VSWR ≤ 1.3 (micro device ≤ 1.4);

　　Parameter change after power test ≤ 0.5dB.

　　Qualified devices need to be affixed with a calibration label (including calibration date, validity period, operator), and a calibration certificate (including test data, uncertainty analysis, and expanded uncertainty k = 2 ≤ 0.3dB). The calibration cycle is usually 1 year (6 months is recommended for high-frequency band devices).

　　VI. Calibration considerations for special scenarios

　　(I) Calibration of micro IoT devices

　　For devices with a volume of less than 15mm (such as RFCI RFCR2895), a probe station or micro calibration parts (such as Anritsu 3670 series) are required to avoid mechanical stress caused by conventional fixtures (stress will cause parameter drift of 0.2-0.5dB). Low-power signals (≤10mW) are used during calibration to prevent excessive power from damaging the micro ferrite structure.

　　(II) Calibration of wideband devices

　　Ultra-wideband circulators (such as those covering 1-18GHz) require an increase in the number of calibration points, with one test point per 100MHz, to ensure parameter accuracy at the edge of the frequency band (such as 1GHz and 18GHz). Use multiple sets of calibration parts (N type for low frequency and SMA type for high frequency) at the same time to avoid error accumulation caused by cross-band calibration.

　　By strictly following the above calibration process, the performance of RF circulators and isolators can be ensured to be stable, providing guarantee for the reliable operation of RF systems (such as IoT communications, radar, and satellite communications). The entire calibration process must be recorded to achieve “data traceability and process reproducibility” and meet the requirements of the ISO 9001 quality management system.

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