
Report on VT LWA-1 Site Activity, Jan 7-12, 2010
S. Ellingson
Jan 16, 2009


Introduction
------------

I was on site Jan 7 through Jan 12.  My primary activities during that period were to install a temporary backend to support analog system checkout & integration, system commissioning, RFI monitoring, and to provide a rudimentary early science capability.  Joe Craig, Steve Tremblay, and Jake Hartman installed and checked out stands, cables, and ARX channels sufficient to make this temporary system operational (as well as a lot of other things I wasn't keeping track of).  


System Description
------------------

The system is currently as follows (also see associated pictures).  The east-west-oriented dipoles of the following stands of the "PS" subarray are used: 187, 189, 191, 210, 212, 214, 228, and 230.  The system components from dipoles to ARX output are "production" equipment.  The ARX outputs are converted from differential to single-ended using "balun boards" designed/constructed by Joe Craig.  The single-ended RF outputs are delayed using DB64 switch-programmable delay line units purchased from Stanford Research Systems.  The delayed outputs are summed using an 8-way RF combiner (Mini-Circuits ZFSC-8-4).  The beam output is then split 4 ways.  Two of these outputs go to S60 boards (more on this later), and the other two are terminated into matched loads.  The latter are provided to allow for easy hook-up of spectrum analyzers and other test equipment for monitoring, calibration, or other measurements.  Also, note that the beamformer is BNC-connectorized and is very nearly gain-neutral; thus, it is very simple to bypass it so as to do things such as observing one ARX output (i.e., dipole) at a time.

Each S60 board is essentially a 12-bit 120 MSPS A/D, connected to a big FPGA, connected to a 100baseT ethernet output.  A Mini-Circuits SLP-50+ 50 MHz lowpass filter is placed in front of the A/D for anti-aliasing.  The FPGA tunes a switch-programmable center frequency between 1.75 MHz and 54.25 MHz in 3.5 MHz steps.  (Frequencies above 60 MHz can also be accessed through undersampling; replacing the SLP-50 with a suitable bandpass filter.)  The FPGA output is baseband using 7 bits for "I" and 7 bits for "Q" (thus, 14 bits/sample) at 3.75 MSPS.  The output 1 dB bandwidth is 3.5 MHz; thus, the selectable channels meet at the 1 dB points.  Data streams continuously from the S60 board as UDP over 100baseT ethernet.  The FPGA firmware and associated PC capture software (described in the next paragraph) was developed by Qian Liu, one of my Ph.D. students; see her design report in the "refs" folder.

The S60 outputs are captured by custom-built mini-ITX-form-factor PCs (see figure) running Ubuntu Linux 9.04.  Data is captured coherently and contiguously using a Python script.  The maximum amount of data that can be captured coherently depends on the maximum acceptable file size, and how full the hard drive is.  The system has been tested for file sizes up to 980 MB, which is 140 seconds of data.  Bash scripts are used to iteratively run the Python capture program for nearly-continuous data capture, although with brief gaps between files.  The PCs have 500 GB hard drives.  My recommendation is not to exceed about 200 GB on each drive at a time, since the drives tend to slow down as they fill up, and at some point data may be lost.  For long nearly-continuous runs, I suggest 100 140-second files acquired at a time, which is 3.89 hours of observation, requiring 98 GB.          

I am not exactly sure what the beam looks like, but based on the dimensions of the array it must be roughly 35 degrees HPBW at zenith.  Currently the beam is set to point south along the meridian, at a zenith angle of 22 degrees.  The Crab Pulsar transits through the center of the beam, and is between the HP points for about 4 hours.  Currently the S60 center frequencies are set to 36.75 MHz and 50.75 MHz.  As mentioned above, both the pointing and frequency selection are set manually using switches.
   
It should be noted that there is currently a problem somewhere in the analog signal chain for Stand 187.  Thus, this input to the delay line unit has been terminated into a matched load and so does not currently contribute to the beam.  Once the problem is identified, it is very simple to reconnect this dipole to the beamformer. 

The system should be robust to power outages.  The PCs are set to automatically reboot upon restoration of power.  The S60s are on a UPS; if the outage should outlast the UPS, then the S60 center frequencies will need to be reset since they initialize to Channel "0".


Data Collection
---------------

See the file "100116_Summary_of_S60_Data.txt" for a summary of data collected so far using the above system.  

The log files (see the "log" directory) describe other data collected including spectrum analyzer outputs, s12 measurements, cal data, etc.

The directory "QuickLook" shows a very quick and crude analysis I did to demonstrate the system using data collected on Jan 8.  This is from a single dipole.  We've grabbed 19 ms of data from each of the S60 tunings that fall within the ARX's 30-55 MHz filter bandpass.  "b3.png" shows the integrated spectrum put together to make a single plot.  I've made no attempt to calibrate the S60's bandpass (which has slight "horns" on the edges).  Don't panic at the apparent RFI -- things always look worse at this resolution, unless you zoom in real close.  Note the resolution bandwidth is << 1 kHz; much larger and the unresolved RFI makes the spectrum look cruddy, unless the resolution bandwidth is >> 1 kHz in which case the spectrum starts to look decent again because the RFI averages out.  (The latter is what you normally see on a spectrum analyzer.)  In other words, this shows you need spectral resolution < 1 kHz to resolve most of the RFI.  Just to underscore the point that the spectrum is mostly empty, I have included a second plot (b3zoom.png) which zooms in to the 38 MHz Radio Astronomy band, and a third plot (b3zoom2.png) which shows "good" and "bad" sections close to each other.


What's it Good For?
-------------------

Recommended uses for this new capability and the data already collected include the following:

-- Analog system characterization and check out.  Especially, looking for self-RFI, gain modulation, and various other "jabbering" behaviors that are difficult to catch using a spectrum analyzer.

-- RFI survey.  Nothing beats coherent time-domain data for RFI characterization.  
-- -- Spectrum can be assessed at any frequency resolution.  
-- -- Data should be searched for impulsive components.

-- Deep integration to assess stability of production analog electronics and low-level RFI

-- Early science:  Might be able to detect Crab giant pulses

-- Early science:  Might be able to detect very strong pulsars

-- Early science:  Blind single pulse searches (e.g., PBH explosions) 

-- Early science:  Solar spectrum in high time-frequency resolution

Data taking can be controlled over the internet.  However (as noted above) beam pointing and S60 frequency selection requires manual setting of switches on site.  Also, useful data volumes are typically too large to send back over the internet; better to write these out to attached hard drives for later pick-up.


Inventory
---------

Much of the additional equipment installed during this trip belongs to VT (in fact, was cannibalized from ETA) and it's use is intended to be temporary.  To aid in recovery later, here is an inventory of VT-owned equipment currently in-system:

Bundle of 8 long RG-58 cables, SMA right angle both ends, labeled 1-8
7 BNC male to SMA female adapters (the 8th one was borrowed from Joe Craig)
4 Stanford Research Systems DB64 delay line units, labeled "VT" (the other 4 belong to UNM)
Bundle of 8 long RG-58 cables, BNC straight on one end, BNC right angle on the other end, yellow band on one end
MCL ZFSC-8-1 8-way combiner
BNC to right angle SMA jumper cable, RG58
5 short beige SMA-SMA jumper cables
MCL 4-way combiner
4 ea. MCL SAT-3 coaxial attenuators
1 ea. MCS SAT-6 coaxial attenuator
2 MCL SLP-50+ coaxial filters
2 Altera S60 FPGA development boards, each with power cords and adapters.
19" rack mount tray to which the above are fastened.
2 ea. 7' CAT5 ethernet cables, running from S60s to PCs.
2 ea. PCs, mini-ITX form factor (labeled "VT k1" and "VT k2"), with power cords
2 ea. long CAT5 ethernet cables, running from PCs to shelter LAN.
Power strip used for PCs, black in color


 







