ALMA SPECIFICATION

Specifications for the

ALMA Front End Assembly

Prepared by: W. Wild, J. Payne

Issue: draft 1.4

Date: 31 August 2000

Approved:

Distribution:

(to team managers, division heads, WP managers and chairpersons of the following groups for further distribution as appropriate):

ALMA Executive Committee

ALMA Joint Receiver Design Group

ALMA Receiver Optics Group

ALMA Dewar Group

ALMA Cryocooler Group

ALMA Mixer Groups

ALMA Local Oscillator Group

ALMA Science Groups

ALMA System Groups

ALMA Scientific Advisory Committee

Revision control

1. Title: ALMA Receivers: Specifications adopted (by U.S. Division Heads)

Issue: N/A

By: L. D'Addario

Date: 20 April 2000

1. Title: Specifications for the ALMA Front End Assembly

Issue: draft 1.4

By: W. Wild, J. Payne

Date: 31 August 2000

Reason: Joint European/US receiver specs

Major changes:

• Receiver specific monitor/control system included in receiver subssystem

• LO components inside the cryostat included in the receiver subsystem, interface to LO subsystem at dewar feedthrough

• definition of terms

• band 3 starts at 84 GHz

Table of contents

1 Introduction 5

1.1 General 5

1.2 Definition of the front end assembly 5

1.3 Definition of terms 5

2 Document references 6

2.1 Applicable documents 6

2.2 Reference documents 6

3 Science driven specifications 6

3.1 Frequency coverage 6

3.2 Polarization 7

3.2.1 Polarization states 7

3.2.2 Maximum non-orthogonality 7

3.2.3 Maximum polarization mismatch 7

3.3 Optical coupling to the telescope 7

3.4 Receiver noise performance 7

3.5 Sidebands 8

3.6 IF bandwidth 8

3.7 Simultaneous operation of bands 8

3.8 Selection of a (pre-set) observing band 9

3.9 Selection of new observing band 9

3.10 Narrow-band frequency switching 9

3.11 Frequency changes within a band 9

3.12 Receiver stability 9

3.13 Receiver calibration 9

3.14 Water vapour radiometer 10

3.15 Solar observing and safety 10

4 Engineering specifications 10

4.1 IF interface 10

[4.2 IF passband ripple] 10

[4.3 LO range and power] 10

[4.4 LO interface] 10

[4.5 Optics] 10

4.6 Dewar size and mass 11

4.7 Cryocooling 11

4.8 Packaging 11

1 Introduction

1.1 General

This document describes the specifications for the ALMA front end subsystem. It is based on the document ALMA Receivers: Specifications adopted (by U.S. Division Heads), written by L. D'Addario. We distinguish between science driven specifications and engineering specifications as follows:

• Science driven specifications: these are the specifications which have immediate influence on the scientific capabilities of ALMA, such as frequency bands, IF b/w, noise temp, stability etc. These specifications are of concern to the future scientific users of ALMA, and a change would affect the scientific goals of ALMA.

• Engineering specifications: they describe details of the front end subsystem which have no direct influence on the scientific capabilities but are important engineering specs. Examples are IF interface specs (e.g. power level), breakdown of noise contributions, cool down time etc. A necessary change would not affect the scientific capabilities of ALMA. In any case, changes with an influence on interfaces will be discussed with and must be approved by the relevant groups.

1.2 Definition of the front end assembly

The front end (FE) assembly accepts the focused beam from the antenna's secondary reflector over a selected band of frequencies. It amplifies and converts this band to an intermediate frequency band in several channels (typically differing in polarization and/or sideband), and delivers the IF signals as outputs. It accepts as inputs local oscillator reference signals at the appropriate frequencies and levels to generate the local oscillator signal for conversion. The front end assembly includes:

• RF optics as required to couple the subreflector beam to its first electronic element.

• Mixers and amplifiers of the RF-IF signal path (separately for each band required to cover the overall frequency range), including any components needed to couple the LO signal to each mixer.

• Local oscillator components within the cryostat. The interface to the LO reference system is the feedthrough into the cryostat, at a frequency around 100 GHz. Each cartridge will have its own interface to the LO reference system. The interface specifications (for each cartridge) will be detailed in a corresponding ICD.

• Vacuum system and cryocoolers needed to achieve the appropriate operating temperatures for certain components, along with related thermal insulation and mechanical supports.

• Bias and control circuits to support the RF-IF amplifiers and mixers.

• Devices to select the desired frequency, including IF band switching and any required tuning or adjustment of amplifiers and mixers as a function of frequency.

• A monitor/control system allowing remote control of all functions and providing extensive remote diagnosis capability, with an appropriate interface to the general ALMA monitor and control bus. The implementation of such a system will be decided between the front-end, systems and software groups and corresponding ICDs developed.

• A water-vapor monitoring radiometer operating simultaneously with at least the observing bands 2 to 10.

• Devices that are placed directly in the input beam of the receiver and which include (but are not limited to) calibration systems, insertable components such as quarter wave plates for circular polarisation, and attenuators for solar observations.

It does not include the following elements, which belong to other subsystems:

• any local oscillator components outside the cryostat,

• general power supplies (which are part of the common infrastructure), and

• calibration devices located outside the receiver cabin (including any built into the subreflector).

1.3 Definition of terms

• Front end assembly: this is the front end subsystem as defined in 1.2 . It provides space for 10 cartridges (see below).

• Band: this is the range of RF frequencies which is received in dual linear polarization and defined in Table 1.

• Frequency channel: this is one receiving chain which receives one polarization within the specified band. A band has two channels.

• Cartridge: a device insertable into the main dewar, receiving RF frequencies within one specified band in dual polarization, containing optics, mixers, IF amplifiers, LO components. Receives the RF from external optics, accepts an LO signal at TBD frequency and power, and delivers IF signals.

• ICD: Interface Control Document. The document which specifies interfaces between subsystems and parts within a subsystem.

2 Document references

2.1 Applicable documents

The following documents form part of this specification document to the extent specified herein. In the case of conflict between documents referenced herein and the contents of this specification, the content of the specification shall be considered a superseding requirement.

ALMA Construction Project Book

Antenna/Receiver ICD No. 1

Antenna/Receiver Cabin Equipment Rack ICD No. 10

System PDR report

Front End/Local Oscillator ICD (to be written)

Front End/IF system ICD (to be written)

2.2 Reference documents

ALMA memos

Report of the ALMA Scientific Advisory Committee: March 2000 meeting

Review Reports: Antenna PDR, Antenna CDR, Multiplier LO PDR, Photonic System PDR (all available at http://www.alma.nrao.edu/administration/index.html)

3 Science driven specifications

3.1 Frequency coverage

The ALMA front end subsystem will cover frequencies between 30 GHz and 950 GHz as given in Table 1.

Table 1: Frequency bands for ALMA

^* lower sensitivity at lower end acceptable

3.2 Polarization

3.2.1 Polarization states

Simultaneous reception of two orthogonal polarizations is required, with each converted to (one or more) separate IF output(s). The nominal polarization states may be selected separately for each band so as to minimize the receiver noise temperature; that is, either linear or circular is acceptable as the nominal polarization.

Detailed specifications on polarization performance are under study.

3.2.2 Maximum non-orthogonality

At any frequency within the front end's tuning range, the polarization states of the two channels should conform to a maximum non-orthogonality of TBD (~ -20 to -25 dB), measured at the optical entrance to the front end subsystem.

(Remark: This specification affects the accuracy with which total intensity (not polarization) can be measured when cross-polarized correlations are not computed, thus enabling the frequency resolution of the correlator to be doubled.)

3.2.3 Maximum polarization mismatch

The front end contribution to the maximum polarization mismatch between any pair of antennas in the array shall not exceed -20 dB.

3.3 Optical coupling to the telescope

Under study, shall be maximized.

3.4 Receiver noise performance

The noise temperature measured at the dewar input window to the cartridge shall not exceed the values of T(SSB) for SSB response and 0.5 x T(SSB) for DSB response as given in Table 2. Specifications and goals are given. The first number of the specifications and goals, respectively, refers to T(SSB) that must not be exceeded over the 80% range of the nominal bandwidth that has the best performance, whereas the second value may not be exceeded at any frequency within the nominal bandwidth.

Table 2: Specifications and goals for receiver noise temperatures

Following an ASAC recommendation the values in Table 2 were calculated with the following formula:

T(SSB)= A * (h*freq/k) + 4 K

where h and k are the usual physical constants, and freq was taken as the center frequency of a particular band. The frequency dependent quantity A has the following specification and values (over 80% / at any freq):

Bands 1-6 (below 275 GHz) Spec: A = 6 / 10 Goal: A = 3 / 5

Bands 7-8 (275-500 GHz) Spec: A = 8 / 12 Goal: A = 4 / 8

Band 9 (602-720 GHz) Spec: A = 10 / 15 Goal: A = 6 / 9

Band 10 (787-950 GHz) Spec: A = 10 / 15 Goal: A = 8 / 12

[detailed break down of noise contributions within cartridge (optics, mixer, IF amps, LO): TBD]

[how to measure the noise temperature: TBD]

3.5 Sidebands

Each frequency channel may be double sideband (DSB), single sideband (SSB) or dual sideband (2SB). An attempt shall be made to optimize the overall sensitivity for both spectral line and continuum observations. SSB and 2SB channels shall provide at least 10 dB image band suppression.

3.6 IF bandwidth

The ALMA IF system and IF transport from each antenna to the correlator are designed for an IF bandwidth of 8 GHz per polarization and two polarizations (i.e. a maximum of 16 GHz IF per antenna). Each frequency channel shall provide 8 GHz total IF bandwidth (per polarization) using one of the following alternatives:

• 8 GHz single-sideband (SSB), upper or lower sideband, or

• 8 GHz double-sideband (DSB), or

• 4 GHz dual-sideband (2SB), upper and lower sideband.

Sideband separation in the correlator for DSB mode will be possible for integration times in multiples of 1 sec. Detailed IF interface specifications are contained in the corresponding ICD.

3.7 Simultaneous operation of bands

Astronomical observations will be done in only one frequency band at any one time (no dual frequency observations). The water-vapor monitoring radiometer shall operate simultaneously with any of the observing bands, but at least with bands 2 to 10.

In order to allow fast phase switching, band 3 shall be ready for operation at all times.

3.8 Selection of a (pre-set) observing band

Selection and operation of a pre-set (i.e. switched on and tuned) observing band shall be possible in less than 1.5 sec. This provides support for fast phase-switching and switching to another (pre-set) observing band. The reselection of a particular frequency band at a particular antenna shall result in pointing errors not to exceed 0.2" (repeatability of selection mechanisms), and phase changes not to exceed (TBD).

3.9 Selection of new observing band

Switching to a new observing band shall be possible in less than 1.5 sec if the electronics of this particular observing band has been switched on the time t_warmup before using the band. t_warmup shall not exceed 15 min, and a much shorter time is desirable.This is to reach thermal equilibrium and maximum stability. In practice, t_warmup may be much shorter, but is not known at this time. Since due to cryogenic limitations not all bands can be switched on at all times, this spec was introduced to minimize the impact on scheduling. It is intended to have at least three bands (two observing bands and band 3) plus the WVR switched on at any one time.

3.10 Narrow-band frequency switching

Changing between two frequencies within .03% of each other (30 MHz at 100 GHz, 285 MHz at 950 GHz) and in the same band shall require no more than 10 msec (goal of 1 msec). This provides support for narrow-band frequency switching.

3.11 Frequency changes within a band

Changing between frequencies more than .03% apart and in the same band shall not require more than 5 sec.

3.12 Receiver stability

Specifications on receiver phase stability and total power gain stability (including 1/f noise) are under study. These are in addition to, and independent of, stability specifications imposed on the LO and on the antenna structure. Preliminary suggestions for gain fluctuation limits are: 1e-4 rms over a 1 sec interval (ASAC report, March 2000), and 1e-4 rms over a 0.1 sec interval (Wright, ALMA memo # 289). A complete specification should give the limit as a function of time interval over a wide range.

3.13 Receiver calibration

Specifications on overall calibration concepts are under study by several groups (U.S. Calibration & Imaging group, European Observational Concept & Calibration group, receiver WBS 4.8.1: Receiver calibration system). The specifications for the front end calibration accuracy (as part of the overall calibration requirements) need to be seen in the context of a system wide calibration concept and strategy. In any case, front end calibration will be supplied in terms of Volts output per degree input and is for the front end assembly only (not including atmospheric calibration).

In any case, an effort shall be made to reach the best receiver calibration accuracy possible.

3.14 Water vapour radiometer

The receiver shall include a radiometer for measurement of water vapor along the signal path, using the 183 GHz line. This instrument shall operate simultaneously with the selected astronomy band (all bands, but at least bands 2 to 10), illuminating the subreflector so as to produce a beam on the sky that is offset from that of an astronomy channel by no more than 10 arcmin. It must include all necessary LO sources and signal processing. Its outputs shall be digital total power measurements in each of [TBD] frequency channels at [TBD list of frequencies], along with calibration measurements that allow each to be reduced to an antenna temperature.

Detailed specifications for the water vapour radiometer are under study and will be included when available.

3.15 Solar observing and safety

No components shall be damaged if the receiver input is illuminated by 0.3 W/cm² of solar optical and infrared radiation. Provisions shall be taken to allow observations of the sun.

4 Engineering specifications

4.1 IF interface

The IF shall be delivered in the range from 4 to 12 GHz, at a nominal power spectral density of [TBD, tentatively -30] dBm/GHz when the antenna temperature at the receiver input is 290 K. There will be four such signals to support 2SB, dual-polarization receivers; SSB or DSB receivers will use only two of them. If the instantaneous bandwidth B of one IF channel is 4GHz<B<=8 GHz, the center frequency should be 8.0 GHz; if B=4.0 GHz, then the center frequency should be 6.0 or 10.0 GHz. More detailed specs will be part of an ICD.

[4.2 IF passband ripple]

TBD, FE-IF ICD

[4.3 LO range and power]

LO tuning range, min. output power vs frequency

[4.4 LO interface]

At the feed through to dewar at a frequency of (TBD) and a power level of (TBD).

[4.5 Optics]

Beam quality, alignment tolerances etc under study

4.6 Dewar size and mass

The maximum size and mass of the dewar as well as mechanical interfaces with the antenna are specified in ICD #1 Antenna/Receiver interface.

4.7 Cryocooling

- temp stages: 4 K, 12-15 K, and 70 K

- stability for each stage: 4K <2 mK in 1 min, 12-15 K under study,

70 K <0.5 K in 1 min

- max thermal load for each stage: under study

- max thermal load for each cartridge: under study

- operational requirements: - # of bands switched on: min 3 + WVR

- what mounted where:

4K: cartridge optics, SIS mixers, first IF amps

12-15K: second IF amps, HFETs (bands 1 and 2, possibly 3)

70K: LO multipliers or photomixer

- dewar cooldown time: max 24 h. goal of 6 to 12 h]

4.8 Packaging

All components specific to one band shall be packaged into an assembly that can be removed and tested separately from the others, except that it may be broken into two assemblies, one for the cryogenically cooled components and one for the room temperature components. Components common to several or all bands, or similar among bands (e.g., bias circuits), shall be packaged in their own removable assemblies.

Exception: Large optical components for one band may be mounted separately from that band's removable assembly if necessary to allow easy removal.

The complete front end package, including components for all bands but not necessarily including some mechanical components (crycooler compressors, vacuum pumps, etc.), shall be removable from the antenna without major disassembly. Its mass, size, center of gravity etc. shall conform to the Antenna/Receiver ICD.