Inside Homer

A block diagram of a generic Homer Autotuner is shown in Fig. 1. The Homer Analyzer part of the system is based on the six-port reflectometer (SPR) principle. SPR is suitable for high-power industrial applications due to its capability of precision on-line measurement of

  • complex (vector) reflection coefficient of microwave applicators,

  • incident, reflected and absorbed powers.

at full working-power conditions of industrial installations. Moreover, the complex reflection coefficient can be used in predictive impedance matching algorithms actuating the Mototuner, enabling thus the design of fast, compact Autotuners.

From voltages obtained by its four amplitude detectors, the SPR computes :

  • Complex reflection coefficient G of a device under test (DUT), possibly transformed by the Tuner.

  • Incident power Pi

The detector signals are appropriate combinations (determined by the SPR hardware) of the wave incident on and reflected from the measured load G, one of the signals (the reference signal) being a sample of predominantly the incident wave. The SPR parameters required for the computations in addition to the detector voltages are obtained in the process of calibration where a set of impedance standards are connected in place of DUT.

Fig. 1.  Homer Autotuner block diagram.

Principal microwave components of the SPR are three combination voltage probes (P1, P2, P3) and a reference coupler. The voltage probes effectively sample the standing wave pattern, i.e., the magnitude of the vector sum of the incident and reflected waves at three points along the line. An optimum performance is achieved at mutual probe distances d = lg/6 (electrically 60°), with lg being the guide wavelength. Insertion depth h of the probes governs the probe coupling factor C hence the nominal working power of the system. For high working powers the probes are in fact recessed.

The reference coupler should ideally, although not necessarily, probe solely the wave incident on the load.

Microwave signals from the combination probes and the reference coupler are converted to DC in amplitude detectors, then amplified and converted to digital in a multifunction PC-card of in-house design (SD-2000), containing 4 separate channels of precision programmable amplifiers (PGA) followed by A/D converters (ADC). Solid-state switches preceding the detectors serve for capturing offsets of the DC circuitry (apparent signals in absence of microwave power), which must be subtracted from the measured voltages V1 to V4. The switches are controlled by a digital input/output (DIO) portion of SD-2000. In a single-board computer (SBC), connected with SD-2000 via PC/104 interface, the digitized voltages undergo a correction routine which eliminates nonlinearity and temperature dependence of the detector transfer characteristics. The inner temperature is measured by a sensor with serial output, connected to SD-2000 by means of I2C bus. The corrected voltages along with signal frequency and stored SPR calibration data are used to compute the reflection coefficient G and incident power Pi.

Accurate knowledge of signal frequency is a precondition for accurate measurement. Since magnetrons used in industrial applications are free-running sources with frequency depending on working conditions, frequency is not known in advance but must be measured. A frequency counter is therefore integrated with the system. The counter uses a separate directional coupler (Frequency Coupler) of basically the same design as the reference coupler. The coupled signal frequency is divided in a microwave prescaler down to about 50 – 100 MHz, which is then measured by a 150-MHz counter (CNT) implemented in SD-2000. Gating intervals down to 16 ms plus the triggering built in the SD-2000 software enable reasonable measurement even of pulsed signals with pulse duration down to about 100 ms.

The system communicates with an external controller (e.g., a personal computer) via RS232 or CAN Bus interface, the latter being implemented in SD-2000. Another function implemented in SD-2000 is a full control (MC in Fig. 1) of three stepper motors for the purpose of automatic impedance matching.

Residing inside SBC is Homer firmware, which is a system of files that control its operation and behavior. The firmware consists of an assembly of executable files (Servers), memory files (containing individual information, like system identity and calibration data) and auxiliary configuration files, defining the Homer startup behavior. The Server programs run in SBC one at a time, performing all tasks associated with acquiring data, computations of results, receiving commands from PC and sending responses and measurement results. The results include real and imaginary part of measured and load (deembedded) reflection coefficients, power incident on and reflected from DUT, frequency, internal temperature and error byte. In the case of Autotuner, the data include tuning stub positions and motors status. The configuration files, read by Server at its start, contain various Homer power-up settings.

While configuration files can be transferred from one Homer to another to ensure the same startup behavior, memory files are strictly individual. Any attempt to transfer them to a non-native Homer results in a Homer Identity Error and causes the system to refuse to perform, except for some basic operations, such as file transfers.

See more about the firmware in Homer Internal Files.

The Windows HomSoft application described in this document enables full Homer control, receiving of results, converting them to a variety of formats, displaying and storing them, as well as long-term monitoring and data logging.

Learn about Homer Analyzer measurement accuracy.