Feedback-Controlled Beam Pattern Measurement Method Using a Power-Variable Calibration Source for Cosmic Microwave Background Telescopes

TL;DR

The paper presents a feedback-controlled beam pattern measurement method that uses a power-variable calibration source to extend the dynamic range of telescope beam measurements while avoiding detector nonlinearity. By employing closed-loop control from the detector under test to the source monitor, the method maintains constant received power within the DUT’s linear range, shifting nonlinearity constraints to a more linear source-monitor detector. In a laboratory proof-of-concept at 81 GHz, the authors achieve an additional dynamic range of 60.3 dB, yielding a total practical dynamic range of 77.7 dB when combined with the DUT’s range, and demonstrate consistency with conventional reference measurements. The approach offers a practical path to high-dynamic-range side-lobe characterization for current and future CMB telescopes and can be generalized to other optical measurements requiring precise, high-dynamic-range beam or pattern assessments.

Abstract

We demonstrate a novel beam pattern measurement method for the side lobe characterization of cosmic microwave background telescopes. The method employs a power-variable artificial microwave source under feedback control from the detector under test on the telescope. It enables us to extend the dynamic range of the beam pattern measurement without introducing nonlinearity effects from the detector. We conducted a laboratory-based proof-of-concept experiment, measuring the H-plane beam pattern of a horn antenna coupled to a diode detector at 81 GHz. We gained an additional dynamic range of 60.3 dB attributed to the feedback control. In addition, we verified the measurement by comparing it with other reference measurements obtained using conventional methods. The method is also applicable to general optical measurements requiring a high dynamic range to detect subtle nonidealities in the characteristics of optical devices.

Figures (9)

• Figure 1: Concept of the feedback-controlled beam pattern measurement method. The beam pattern measurement is conducted using the power-variable calibration source under feedback control from the detector under test on the telescope. The feedback control keeps the received power by the detector constant to avoid detector nonlinearity during the measurement.
• Figure 2: General model of the feedback-controlled beam pattern measurement system. The microwave generated by the power-variable source propagates from the feed horn to free space, passes the optics (e.g., a compact range or a far-field distance), and illuminates the antenna under test coupled to the detector under test. The source monitor tracks the source power through the directional coupler. The closed-loop feedback control from the detector under test adjusts the source power during the measurement.
• Figure 3: Block diagram of the feedback-controlled beam pattern measurement system used for the proof-of-concept demonstration. The power-variable continuous wave (CW) generator and frequency multiplier produce a W-band (75110) microwave, which is pulse-modulated at 10 by the RF switch. The 20 directional coupler (Coupler A) transmits the microwave to the feed horn attached to the through port, and two source monitor detectors (diode amplitude detectors) via the 10 directional coupler (Coupler B) attached to the coupling port. One of the source monitor detectors is given a 30 attenuation by Coupler B and the 20 attenuator. The feed horn feeds the compact range optics, which consists of the off-axis parabolic mirror, and illuminates the antenna under test connected to the detector under test. All detector outputs are amplified, low-pass-filtered, digitized, and processed in the system-control computer, which also runs the feedback control.
• Figure 4: Photographs of the feedback-controlled beam pattern measurement system. Left: Compact range. It was fully shielded by microwave absorbers during the measurement to suppress unexpected stray lights. Some absorbers are temporarily removed for the photograph. The antenna under test (AUT) and detector under test (DUT) are mounted on an automatic rotation stage to change the AUT angle. Right: Close-up of the source and source monitor (the part framed with red dashed lines in the left panel). The continuous wave generator and RF switch are not shown in this photograph.
• Figure 5: Flowchart of the routine in the feedback-controlled beam pattern measurement. The routine was repeated until all the angular points were covered. The subroutine boxed in the dashed lines is the closed-loop feedback iteration adjusting the source power.