A Survey of Selected Radio Telescope Receiver Types Dana Whitlow Microwave Receiver Specialist, Arecibo Observatory Denis Urbain Microwave Receiver Specialist, Arecibo Observatory In this talk we will consider several types of receivers: Single feed Focal plane arrays > Traditional (Arecibo ALFA, Parkes MB20) > Phased array (AO-40; upcoming at Arecibo) > Incoherent detector array (USRA SOFIA, GBT Mustang) DEFINITION 1: COHERENT RECEIVER Dual Polarization Feed, Typically a Circular WG Feed with OrthoMode Transducer (OMT) Polarization A BPF R I BPF L BPF R I BPF L Low-Noise Amplifiers (cooled) Polarization B Local Oscillator Power Detectors (diodes or ADCs + software) COHERENT RECEIVER: A receiver in which phase information is preserved through a gain process in the signal chain, regardless of whether or not the phase information is ever actually used. If your receiver has an amplifier prior to the detector, it is a coherent receiver. Coherent receivers are subject to a fundamental quantum sensitivity limit: Tnoise >= h /k DEFINITION 2: INCOHERENT RECEIVER Incoherent Detector (normally a cryo-cooled bolometer in radio or IR astronomy) Low-Noise DC Amplifier (cooled) Data Acquisition System INCOHERENT RECEIVER: A receiver in which the first element in the signal chain is a direct (power) detector, in which case phase information is destroyed. If the first active element in your receiver is a heat detector, photocell (not used as a mixer), etc, you have an incoherent receiver. The quantum sensitivity limit is said not to apply. Single-beam versus Multi-beam • Single beam (single pixel) operation seems like a waste of a perfectly good (well, almost) optical system. It’s especially inefficient for survey work. • Multiple beams permit considerably faster survey work, but having them is definitely an extra-cost (and extra-complication) option. G. Cortes-Medellin, K.F. Warnick, B. D. Jeffs, G. Rajagopalan, P. Perillat, M. Elmer, D. Carter, V. Asthana, T. Webb, A. Vishwas. “Field of View Characterization of Arecibo Radio Telescope with a Phased Array Feed”. IEEE Antennas and Prop Symposium, Spokane, WA, Jul 2011 Optical Axis "Ideal" reflector brings all rays from a given direction to a focus at a location which depends on that direction. Focal "Plane" Reflector Because of diffraction, each focus is a "blob", not a point. The blob's size depends on the wavelength and on the angle of convergence, and so does the depth of focus. In a coherent receiver, the feed- horn for the wavelength of interest must be sized to fit its blob. BASIC TELESCOPE OPTICS ALFA’s 7-ELEMENT CLOSE-PACKED FEED HORN ARRAY Optical Axis Unfortunately, the blobs arising from off-axis beams are oversize & distorted, placing them beyond the realm of good capture by practical feed horns. Array of Independent Conventional Receivers and Back Ends Array of Sized Feedhorns in the Focal "Plane" Reflector Also, the desire for close-packing of the feedhorns' mouths constrains against feed designs that produce a very favorable footprint illumination distribution on the main dish, leading to inferior aperture efficiency. The EM field distribution at the mouth of such a horn is overly concentrated so that its far field beam, which is basically an image of the focal plane's field distribution, is a fair bit narrower than the beam-to-beam spacing defined by the horns' non-overlapping physical openings. Thus, there are appreciable gaps between the beams. "TRADITIONAL" FOCAL PLANE ARRAY TRADITIONAL FOCAL PLANE ARRAY • Receivers are independent, with no phase connection. • Therefore each feed must take individual responsibility for matching its footprint to the main reflector, setting a minimum size requirement. • Feeds of this size (always too large) cannot adequately spatially sample the electromagnetic field configuration at the focal plane to correct for off-axis aberrations and permit creation of a pattern of contiguous beams. Example of Off-axis Aberration (this is primarily “coma”) FOCAL PLANE PHASED ARRAY • Here the array comprises a grid of small antenna elements spaced by slightly less than l/2, thereby meeting the Nyquist criterion for full spatial sampling of the electric field configuration over the focal plane. The elements are often implemented as shortened “halfwave” dipoles. • The outputs of the elements are vector summed with complex element- and beam-dependent weighting to produce the desired beam(s) on the sky. • Assuming that sufficient processing capability is available, simultaneous production of many beams is possible. • Beams can be well corrected for off-axis aberrations and (within reason) focus errors. • Within limits, pattern notches can be formed to mitigate RFI. • But there’s a catch: electrical interactions and noise coupling between the closely-packed elements seriously complicate the design process and tend to degrade overall noise performance. e + e j + e j + e j + j + Array of Small Antenna Elements in the Focal "Plane" Digital Beam Former Not shown: a ton of pesky RF hardware: amplifiers, filters. downconverters, etc, not to mention a big pile of ADCs and other digital hardware. Beam 3 Out Beam 4 Out Beam 5 Out + + Beam 0 Out Beam 1 Out Beam 2 Out + Each beam is formed as a unique complexweighted sum of the elements' individual outputs. Reflector FOCAL PLANE PHASED ARRAY CONCEPT BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY INCOHERENT DETECTOR ARRAYS • Incredibly, heat detectors (such as bolometers and arrays thereof) can be made sensitive enough to be very useful for astronomy. • Greatest usefulness (for “radio” astronomy) is in the mmwave and sub-mm-wave regimes where fundamental quantum behavior places severe limits on the noise performance of coherent receivers. • Incoherent detectors in general (including photon detectors as well as bolometers) extend astronomy upward in frequency all the way to the gamma ray regime. • A variety of useful detection mechanisms are known and used; all require cooling to sub-one-degree-Kelvin temperatures to work. In fact, usually well below one degree is required! SOME ADVANTAGES OF INCOHERENT DETECTION • Extends upper frequency limits of high-sensitivity radio astronomy beyond current practical (and even theoretical) limits of “conventional” (coherent) radio telescope receivers. • Uncouples the strict connection between beamwidth and effective aperture area that is characteristic of coherent receivers. This can sometimes be exploited to obtain a sensitivity advantage if diffraction-limited angular resolution is not required. • Very wide pre-detection bandwidth (tens of GHz) is available, which is really great for continuum work. SOME ISSUES WITH INCOHERENT DETECTION • No phase information is available from the detectors; thus neither off-axis aberration correction nor participation in interferometry is possible. • Sensors are inherently insensitive to polarization. • Spectroscopy is usually considered impractical since nothing can be done post-detection, and versatile or tight pre-detection filtering is extremely hard to implement. Some attempts have been made. • Extraordinary care is required in the design and implementation of the sensor (array) to keep out stray radiation everywhere in the electromagnetic spectrum, since the inherent bandwidth of a thermal sensor is essentially infinite. Accomplishing this adequately can be much more challenging than it looks at first glance. • Great attention is also required in the sensors’ output signal handling circuitry to avoid microphonics, 1/f noise, etc. • Cryogenic cooling is a challenge, especially in large arrays.