Advanced Low Noise Ambient-Temperature Amplifiers S. Weinreb and Jun Shi Caltech, Pasadena, CA

1. Advanced Low Noise Ambient-Temperature Amplifiers S. Weinreb and Jun Shi Caltech, Pasadena, CA Sep 3, 2019 • Rationale • Description • LNA Performance • Noise, Gain, and S-Parameter as Functions of Temperature and Frequency • System Performance on 4.65m Telescope • Calibration of Noise Measurement • Noise Model – Theoretical Studies • Future Perspective

2. Advanced Low Noise Ambient-Temperature Amplifiers LNAs operating at 25C have been developed with measured noise temperatures a factor of four lower than commercially available amplifiers in the 1.4 GHz frequency range. These new LNAs can have a transformational effect on radio astronomy arrays operating below 2 GHz by reducing the total system noise from 60K to <30K. In the past this noise level was only available with cryogenic cooling which forced the optimum size reflector for a given array sensitivity to be > 15m. Thus large arrays of small antennas have become feasible and provide large total collecting area with order of magnitude lower cost and unprecedented survey speed. The new LNAs have been in development for nearly 1 year and 6 revisions with 17 prototypes have been tested. The developments have focused on manufacturability, reliability, and ease of use in a large system as well as low noise. • . Cryogenic receiver requires external helium compressor, kWatts of AC power, and typical cost of >$100K. Ambient receiver, feed and two LNAs, costing <$5K and operating on 0.5W of power supplied on output coax..

3. Advanced Low Noise Ambient-Temperature Amplifiers Size: 8.3cm x 4cm x 1.8cm • Record L-Band Noise, < 9K (0.13 dB NF) at 50 ohm input connector at 25C. • Powered and controlled through output coaxial cable • Internal noise source to easily measure Tsys controlled by tone signal on output coax. LED indicator indicates cal source on • Weathertight enclosure • Optional Internal 1900 MHz band reject filter; <1100 MHz will be rejected by feed cutoff. • Similar noise performance in 17 prototypes that have been constructed over a 9 month period for improving the manufacturing repeatability and reliability. • Amplifier has been tested from +40C to -40C and the gain variation is 0.50 dB, a coefficient of .008 dB/C • The measured noise at -40C is <4K which can be achieved with thermoelectric cooling in future versions.

4. Measured Noise, Gain, and Return Loss for Early Prototype LNA, SN7 Noise is < 11.6K in DSA band. Spikes around 2 GHz are due to WiFi and cell bands near 2GHz Measured S parameters of SN7 LNA with applied 5V at 53.8mA Noise with internal calibration signal turned on to add ~20K to the system noise.

6. LNA Noise and Internal Calibration Noise As Function of Frequency and Physical Temperature for LNA SN22

7. S-Parameters of Two LNAs, 1 to 2 GHz, at 30C • Gain is typically 38 +/- 1 dB in the DSA110 range of 1.28 to 1.53 GHz • Gain at 1.4 GHz varies by < 0.5dB from -40C to +40C; <.006 dB/C • S11 is typically < -10 dB in the DSA110 range • S22 is typically <-15 dB in this range • Both S11 and S22 can be tuned to be lower if required.

8. System Noise Measurements at 1.4 GHz with Ambient Temperature LNAs • LNAs have internal calibration signal and are directly mounted on feed • 4.65m prime focus reflector with F/D = 0.315 elevation only drive • Simple, low-cost feed with 14.6cm diameter pipe surrounded with cake pans • Fiber-glass feed supports, transparent to RF at these wavelengths • Approximate breakdown of 26K Tsys: LNA 9K, Sky 7K (CMB 2.7K, ~3K atmos,~ 1K galaxy), Spillover 5K, Feed Loss 1K, Post LNA Noise 1K. and Feed Blockage, 3K.

9. Calibration of Noise Temperature Measurements Most NF measurements are made with noise sources with calibration uncertainty of +/- 0.15 dB which results in a noise temperature measurement of +/- 11K which is inadequate for an LNA with <10K noise. • Our noise temperature measurements have been calibrated to within +/- 1K with thermal noise standards such as the LN2 dewar shown at left or a SS line dipped in LN2 as shown at right • Both methods agree within +/- 0.5K in the 1.2 to 1.6 GHz range as shown below.

10. Tests of OMMIC Transistor vs Temperature

11. Modeled Composition of the Noise in the Measured DSA 1.4 GHz LNA • The reduction of noise by approximately a factor of two from +40C to -40C cannot be explained by the (233/313 K = 0.74) reduction of thermal noise in the circuit and transistor. There must be also a large reduction in the hot-electron channel noise in the HEMT transistor. This is the topic of theoretical studies by the Minnich group at Caltech. • The sources of noise in the LNAs can be modeled in MWO as below

12. Future Perspective • SKA1 Dish Band 2 – Could reduce capital cost by of the order of $10M by eliminating cryogenics as well as a substantial decrease of power cost. However it would increase Tsys by 10%to 20% and take two years for design • SKA2 – Development effort recommended for lower microwave bands with focus on wider bandwidth, feed integration and packaging for Peltier coolers. “Smart” LNAs with built-in microprocessors for bias optimization, adaptive filtering, and monitoring should be considered. • ngVLA – Same as above. Could save much in capital and operating cost but the Tsys impact needs to be considered • PAFs – Promotes larger (>100) arrays with low loss simple feeds and no cryogenics • Large N, Small D Arrays Such as DSA2000 – Large impact on the optimum size antennas and feasibility of arrays with very large survey speed in the lower microwave range. Needs further development of low noise feeds and optimum optics (offset or symmetric), wider bandwidth, packaging for solid-state coolers to -40C, and newer transistors.