THE ARO 1.3mm IMAGE-SEPARATING MIXER RECEIVER SYSTEM. Revision 1.0

Transcription

1 THE ARO 1.3mm IMAGE-SEPARATING MIXER RECEIVER SYSTEM Revision 1.0 September, 2006

2 Table of Contents 1 System Overview Front-End Block Diagram IF System OPERATING PROCEDURES SETTING THE MIXER BIAS SYSTEM DIAGRAMS Phase 1 System Diagram Phase 2 System Diagram Phase 3 System Diagram SPECIFICATIONS SCHEMATICS MECHANICAL DRAWINGS... 16

3 1 System Overview The Arizona Radio Observatory s 1.3mm JT receiver system is the first of its kind to incorporate the latest SIS mixer technology; image separating mixers with internal IF amplifiers (mixer preamp module). These mixers were developed at the Central Development Laboratory of the National Radio Astronomy Observatory as part of the ALMA project, and the ARO has had the privilege to be the first observatory to integrate these mixers into a radiometer system for astronomical observations. These mixer preamps have demonstrated state of the art performance for sensitivity, and have no moving parts which makes them far easier to use than the conventional receiver systems using quasioptical techniques for image separation and are far more sensitive. Image separating mixers do not operate in the conventional way quasioptical single sideband receivers do. In the traditional quasioptical SSB system, the mixer is still being used as a DSB mixer where both sidebands are downconverted. The difference being that the image sideband is terminated in the cold dump of the optical system. This image noise is still downconverted thus adding a considerable amount of noise in the signal port. In an image separating mixer, the image noise and signal is truly separated from the signal noise thus isolating it from the desired sideband. This means that for an ideal image separating mixer, the noise performance would approach that of a DSB mixer except from the additional 4 K of added noise from the cold termination of the image port of the RF hybrid This radiometer system is unique over its predecessor because both sidebands are available simultaneously (thus, the term image separating mixer). This means that for a dual polarization system, four independent IF s are available. This can be extremely advantageous because it can allow the observation of two separate spectral lines simultaneously, thus reducing the observation time in half. The instantaneous IF bandwidth of each channel is 4 GHz (from 4 to 8 GHz), but capable of providing 8 GHz of IF bandwidth (from 4 to 12 GHz). This amount of IF bandwidth has made the system back end limited because of the limited amount of bandwidth available from the current spectrometer systems at the SMT. The current realization of the front end is an interim system. It uses the old 1.3mm quasioptical JT Dewar and cross grid (at room temperature) to separate the two orthogonal linear polarizations. Two inserts are used for each polarization. Improved performance will be achieved if the room temperature cross grid which is known to add a non-negligible amount of noise. Later, a waveguide OMT will be used which will enhance the sensitivity, and dramatically reduce the size of the front end because both polarizations will be attached to the OMT. This improved package will later be integrated into a new much smaller Dewar. This system is intended as a facility instrument primarily for spectroscopic observations. Due to the immense amount of work that needs to be done in such a short period of time, implementation of the receiver system will occur in 3 phases. Phase 1 will only have two IF channels available at one time. This means that the user will be restricted to two observing modes: either USB or LSB for each polarization, or both sidebands but with only one polarization. Full IF channel steering and sub-band selection will be available. Phase 2 will implement full four channel capability. This system will replace the existing two-channel IF processor with a four-channel system. This will allow independent accessibility of each of the four IF outputs from the front-end. A four channel total power facility continuum system with zero check capability and external four channel test ports will be used for all of the front ends in both receiver rooms. This will replace the existing two channel continuum system on the left side and the 2 channel IF processor unit in the receiver frame currently being used in phase 1. Full spectrometer configuration will also be available. This will allow the spectrometers to be arranged in a series or parallel mode with respect to each other. Phase 3 will have the same capability as Phase 2, but will incorporate the left side receiver room into the upgraded IF system. This includes the 345 and 490 GHz receivers. The 345 GHz receiver will change its

4 IF from 1.5 GHz to 4-8 GHz, and the 490 GHz receiver will still use its 1.5 GHz IF. An upconverter will be installed to convert the 1.5 GHz IF center frequency to 6 GHz.

5 1.1 Front-End Block Diagram

6 1.2 IF System General Comments: Phase 1 of the IF system uses the existing IF processor which was used with the legacy 1.3mm receiver. It is somewhat limited because it can only handle two of the four channels now available from the front-end. It allows the user to observe in either sideband for both polarizations, or both upper and lower sideband for one polarization. The spectrometers are configured in the following matter for each IF routing from each of the four channels: The following table describes which spectrometers are assigned for any particular IF routing configuration. The four IF routing options are selected on the JTtune screen. PHASE 1 IF ROUTING/SPECTROMETER CONFIGURATION IF Routing FFB-A FFB-B AOS-A AOS-B 1.3mm Pol. H (single pol.) LSB USB LSB USB 1.3mm Pol. V (single pol.) USB LSB USB LSB 1.3mm USB (dual pol.) Pol. V: USB Pol. H: USB Pol. V: USB Pol. H: USB 1.3mm LSB (dual pol.) Pol. H: LSB Pol. V: LSB Pol. H: LSB Pol. V: LSB

7 2 OPERATING PROCEDURES 2.1 SETTING THE MIXER BIAS This receiver has four independent mixers whose bias need to be set. The mixer voltage is independently adjusted for each of the four mixers, but the LO level is set for each polarization. This is because each polarization has two mixers, and they share the same LO port of the mixer block. The voltage level and LO drive are controlled by the JT tune window Protecting the mixers An added protection feature has been added to the mixer bias modules for each of the inserts. The mixer bias modules are located on the base plate of each insert, and there are two modules on each insert (see Fig. 1). This procedure is to be performed any time the operator intends to put the receiver into safe mode for any condition where they feel necessary to protect the mixers, including lightning. mixer bias modules mixer bias modules a. b. Figure 1. Location of the mixer bias modules for each polarization: Horizontal (a.) and Vertical (b.). Each mixer module has a 3 toggle switches arranged vertically labeled short 2, short 1, and zero. These switches short the input to the bias-t of the mixer and zeroes the input of the bias supply to prevent putting the receiver in operation or safe mode for the junctions. The basic thing that needs to remembered when throwing the switches is that the zero switch should be closed first when putting the receiver in safe mode and opened last when putting the receiver into operation. The procedure is as follows: To turn the receiver off (i.e., to put it into safe mode), 1. Set the mixer current (LO drive) and bias voltage to zero. 2. Throw the zero switch inwards (again, towards the middle of the insert). 3. Throw the short 2 & and short 1 switches (the order of these is not important). Note that to throw the switches, they will in the in position as labeled on the bias modules. This means that both gangs of switches will be thrown in towards the center part of the insert. 4. Repeat the procedure for the other mixer bias module of the insert.

8 5. Repeat steps 1 3 for the other polarization. The vertical polarization may be a little more tricky to do because the bias modules are oriented so that the switches are facing towards the receiver flange as shown in Figure 1b. In this case, the switches of the module closet to the operator (channel A of the vertical polarization) will be pointing away from the operator, and the switches for channel B of the vertical polarization should be pointing towards the operator. If one were to be able to position one s self in front of the receiver flange, they would notice that both gangs of switches for each channel would be pointed towards the middle of the insert as in the horizontal polarization. To put the receiver in operation mode, 1. Throw the short 2 & short 1 switches outwards (i.e. pointing away from the middle of the insert.) 2. Throw the zero switch outwards (again, pointing away from the middle of the insert.) 3. Repeat the procedure for the other mixer module of the insert. 4. Repeat steps 1 3 for the other polarization. If there are any questions about this procedure, don t hesitate to call one of the staff engineers Setting the bias 2.2 Warming up the receiver The receiver now has two 65 Watt cartridge heaters on the 1 st stage of the refrigerator and the JT stage. These heaters allow the dewar to be opened about 3.5 hrs. after they are turned on. 1. Locate the brown molded AC cord underneath the receiver frame. It has a tag attached with red letters stating not to plug it into a socket unless one desires to warm the receiver up. 2. Locate the isolated variable transformer underneath the receiver frame. Receiver heater cord

9 3. Plug the receiver heater cord into the transformer receptacle. 4. Plug the isolation transformer into a socket. (*Note*: Make sure that it is not plugged into a receptacle that is running off of the receiver UPS! Typically, the power strip on the receiver frame is plugged into the wall and can be used, but make sure that it is not plugged into the receiver UPS located on the bottom left hand side of the receiver frame.)

10 5. Turn the transformer on and set the rheostat to 83% (the dot just above 80). Set the dial here. A few precautions: The heaters are controlled by bimetallic thermal switches (also known as klixons ), should turn off at about 300 K. Of course, they are not rated to operate at 4K, so one needs to pay attention to make sure that they are operating properly. It is strongly recommended to keep an eye on the temperature of the receiver while it is warming up. For example, it is not recommended to turn the heater on and leave it unattended (e.g. the operator goes to bed). Keep an eye on the temperature of the receiver, especially after about 3 hours after the warm up where the thermal switches should start to open. If they remain closed, the heaters will keep running and result in damage to the electronics inside receiver! 6. To turn the heaters off, set the rheostat to 0 and turn off the power switch to the transformer.

11 3 SYSTEM DIAGRAMS 3.1 Phase 1 System Diagram

12 3.2 Phase 2 System Diagram

13 3.3 Phase 3 System Diagram

14 4 SPECIFICATIONS 1. Receiver configuration: Dual-linear polarization employing two image-separating (2SB) mixers, both upper and lower sidebands available simultaneously. Polarization separation done with a room temperature wire cross-grid. 2. Operating frequency: GHz; 210 GHz LSB only, 279 GHz USB only. 3. Image rejection: 12 to better than 20 db, depending on the LO and IF used for a particular sideband. Better IR is obtained in the lower sideband. 4. Instantaneous IF bandwidth: 4 8 GHz 5. IF outputs: Four available channels (i.e., a USB channel and LSB channel from each polarization.)* *However, only two of the four IF outputs will be available in Phase 1 of the receiver upgrade. 6. Spectrometers: Two 1 MHz filterbanks (FFBA & FFBB), 1024 channels each. Two 250 khz filterbanks, 1024 channels each. Two AOS spectrometers, each 970 MHz wide. Note: Each spectrometer s 1 GHz IF bandwidth is steerable within the 4 8 GHz IF range. In Phase 1 of the receiver upgrade, the spectrometers can only put in parallel meaning that only 1 GHz of instantaneous IF bandwidth is available.

15 5 SCHEMATICS

16 6 MECHANICAL DRAWINGS