ALMA Band 2 LNA

145 LNAs for ALMA Band 2.

As part of the Wideband Sensitivity Upgrade (WSU) of the ALMA (Atacama Large Millimeter/submillimeter Array) observatory, the division was awarded a production contract by the European Southern Observatory (ESO) for the first-stage cryogenic low-noise amplifier (LNA) used in the new Band 2 receivers. These receivers cover the 67–116 GHz frequency range. The contract award followed a competitive phase, where the stringent performance specifications set by ALMA could be reproducibly met for a small series production.

The production and qualification process reached its successful conclusion at the end of 2025 with all 145 LNAs delivered and accepted by ESO. Receiver implementation for Band 2 at the ALMA site is currently underway, with initial results confirming the state-of-the-art performance.

The results delivered for the whole production run, in particular the low noise, are unprecedented and cutting edge. This achievement is the result of several years of joint development between the Max Planck Institute for Radio Astronomy (MPIfR) and the Fraunhofer Institute for Applied Solid-State Physics (IAF) and marks the pinnacle of this nearly 20-year-long collaboration between the institutes.

On the left: the LNA split-block metal housing, a gold-plated block with a great number of holes for screws. It is 18.92 mm wide. There is a board in the upper half. On the right is the zoomed in RF section. The W-Band MMIC in the center has a length of 1.5 mm. To the left and right, channels are visible that connect to larger waveguides on either side, one labelled input and one output. Above are the DC board and the DC bias filter network. — **Fig. 1:** Left: populated bottom half of the LNA split-block housing. Right: The components of the RF section. All chip components are attached with silver epoxy. The thermocompression bonds (RF and DC) at the MMIC are realized in 17.5 µm wire and the other DC bonds 50 × 8 µm² ribbon. For scale, the MMIC size is 1.5 × 1 mm².

© MPIfR

**Fig. 1:** Left: populated bottom half of the LNA split-block housing. Right: The components of the RF section. All chip components are attached with silver epoxy. The thermocompression bonds (RF and DC) at the MMIC are realized in 17.5 µm wire and the other DC bonds 50 × 8 µm² ribbon. For scale, the MMIC size is 1.5 × 1 mm².

© MPIfR

Part of the development was carried out within the framework of European Union research programs such as AETHRA (Advanced European Technologies for Heterodyne Receivers for Astronomy), which supported critical aspects of amplifier optimization and fabrication process refinement. To support production-level testing and qualification, a dedicated cryogenic 67–116 GHz test and qualification setup was developed and commissioned by the division and put into near continuous operation during production. This setup features test channels for noise, s-parameters and spectral stability criteria. This system enables precise and repeatable performance verification of each LNA unit under cryogenic conditions for evaluation against the demanding specifications defined by ESO.

Fig. 1 depicts the lower waveguide split block housing part with internal components. The image on the right shows a close-up view of the central area with MMIC amplifier chip. The housings are milled in MPIfR’s own precision machining workshop in which critical components such as waveguide features can be milled with consistent accuracy in the range of a few micrometers. It was possible to utilize the newly acquired 5-axis CNC that simplifies the complicated milling process. Similarly, the housings are gold plated at MPIfR’s electroplating workshop with cryogenically proven processes. Trials with industry partners led to inferior results when compared to those of our inhouse workshop. This underlines the necessity of inhouse machining facilities for MPIfR’s state-of-the-art instrumentation development.

In the top half of the picture, a schematic representation of two RF measurement channels is presented. The individual electronic components are depicted as chain of boxes for each channel. The inside of the cryostat is connected via a stainless steel WR10 waveguides and vacuum feedthroughs to the VNA extension modules. that connect to a VNA (not depicted). In the bottom half of the picture a top-down photo of the complete test setup is shown. The cryostat is the large metallic box is at the center. Inside the box, with lid removed, a great number of wires are visible. On the left and right, electronic components are visible that process the RF signals. — **Fig. 2:** Cryogenic measurement test setup used for qualification of the W-band LNA. Top panel: Schematic of the two RF measurement channels. Bottom panel: Top view of the test setup with vacuum lid and radiation shield removed; there are two LNA as DUT (Devices Under Test) in the center of the cryostat. Outside of the cryostat on the right is the noise measurement channel with downconverter. On the left is the VNA based measurement channel with movable waveguides connected on either side and the linear translation stages for the frequency extenders used with the vector network analyzer.

© MPIfR

**Fig. 2:** Cryogenic measurement test setup used for qualification of the W-band LNA. Top panel: Schematic of the two RF measurement channels. Bottom panel: Top view of the test setup with vacuum lid and radiation shield removed; there are two LNA as DUT (Devices Under Test) in the center of the cryostat. Outside of the cryostat on the right is the noise measurement channel with downconverter. On the left is the VNA based measurement channel with movable waveguides connected on either side and the linear translation stages for the frequency extenders used with the vector network analyzer.

© MPIfR

A graph with the statistically averaged data curves for effective noise temperature in Kelvin (left y-axis) and gain in dB (right y-axis) vs. frequency 67 to 116 GHz of the whole ESO production run. For averaged noise, a rising sloped curve from 15 to 28 K is shown in the bottom part of the figure. The averaged gain in the top part of the figure is a falling slope from 40 to 32 dB. The 1-sigma uncertainty of the averages is depicted as shaded area in orange. — **Fig. 3:** Summary of the main test results for the full ESO production run. Statistical analysis of the main RF performance parameters of the qualified LNA measured at T_amb of 15 K. The traces show the average data for all 145 LNA qualified and delivered to ESO with 1-sigma errors plotted in orange. Depicted are the corrected Teff in (K) in the lower half of the plot and gain as mag(S21) in (dB).

© MPIfR

**Fig. 3:** Summary of the main test results for the full ESO production run. Statistical analysis of the main RF performance parameters of the qualified LNA measured at T_amb of 15 K. The traces show the average data for all 145 LNA qualified and delivered to ESO with 1-sigma errors plotted in orange. Depicted are the corrected Teff in (K) in the lower half of the plot and gain as mag(S21) in (dB).

© MPIfR

The LNAs were evaluated and qualified against an extensive specification sheet from ESO, with all performance critical testing performed at T_amb = 15 K. Cryogenic testing is very resource intensive due to the duration required for cool-down and warm-up and needs to be carefully planned for in a production process. A dedicated measurement test setup was developed that incorporates two test channels under software automation (Fig. 2). Data analysis is performed using a pipeline that qualifies all required test parameters for their compliance and generates the test reports. These production and qualification tools can also be instrumental in tracking hundreds of components for even larger scale projects in the future. Furthermore, the test setup features a novel approach for effective measurement of the phase and amplitude stabilities of the LNA utilizing only single additional VNA-based (Vector Network Analyzer) measurement (to be published).

Fig. 3 summarizes the main test results for effective noise temperature Teff (K) and gain as mag(S21) (dB), two crucial figures of merit for LNA performance evaluation, for the whole production run. Based on their superior demonstrated noise performance, ESO selected our LNAs as the first und crucial amplification stage of the ALMA Band 2 receivers. This project has generated an extensive homogeneous data set for these high-performance LNAs that will improve our general understanding of cryogenic transistor models developed at Fraunhofer IAF. The performance of the W-band channels in the upcoming 3-band receivers will greatly profit from the performance of these LNAs.