# SETI Spectrometer

The SERENDIP V SETI Spectrometer is a 128M channel, 200MHz, 1 pol spectrometer on the iBOB/BEE2 and was designed for applications in SETI, FX correlators, pulsar, and general radio astronomy.

The signal processing is implemented in a Xilinx Virtex-II Field Programmable Gate Array (FPGA) chip, which can be dynamically programmed for various applications.

The spectrometer uses a polyphase filter bank (PFB) to implement a bank of steep cut-off bandpass filters, providing a significant improvement in signal-to-noise and out-of-band-rejection over the common FFT algorithm. The PFB uses a biplex pipelined architecture which allows data to be input and output at the full sample rate, and can do two polarizations simultaneously with no extra cost in hardware or data flow rate.

For information on SERENDIP science, visit UC Berkeley SETI

Dan Werthimer

Jim Cordes

## Assembly

Connect power to IBOB, BEE2, and BEE2 fans

Connect ethernet to IBOB (IBOB has IP address 192.168.2.6 where the last two octets are set on the headers) for more info on communication with IBOB, see: IBOB UDP server documentation

Connect ethernet to BEE2 (Check board for IP address)

Connect XAUI1 to F2-0 on BEE2

Connect Passive LVPECL termination to J117 on BEE2

Connect LVCMOS to LVPECL converter to SMA1 (converter should be connected to 5V)

Connect SMA to one of outputs of LVPECL converter to 3db attenuator to LVPECL termination

Connect F1-3 to 10GbE NIC on your computer

## SETI Spectrometer Software

### Initialization Scripts

All of the bee2 bof files and initialization scripts are available in svn http://casper.berkeley.edu/svn/trunk/projects/seti_spec/bee2_software/. After copying these files to the bee2, either add the checked out bin directory to the path or move the read_reg and write_reg binaries into a directory already in the path.

The top level script start_seti.sh will kill any previous setispec run, reload all the bof files, and configure the chips. The bofs/ directory contains the current versions of the bof files and the bin/ directory contains all the subscripts for initializing and restarting the bee2.

The 10GbE block is configured by the 10gbe.conf file. There is documentation on editing this file here: 10GbE configuration.

Most initialization values are hard coded into the scripts but a few parameters can be modified via a configuration file in the bin/ directory named bee2.config. The file in svn is currently empty so the scripts will use default values. If a line in the config file is found with a parameter name followed by the value then the script will use that value instead of the defaults.

Here is an example of a config file that sets max_hits_per_bin to 25 and scale_threshold to 96:

max_hits_per_bin 25
scale_threshold 96


Currently the scripts will only look for max_hits_per_bin and scale_threshold. Additional parameters can be added to the script by defining a variable using the following code and replacing max_hits_per_bin with an appropriate variable name.

config_file="bee2.config"

#read in parameters from config file
max_hits_per_bin=$(awk '/max_hits_per_bin/ {print$2}' $config_file) #set default values if parameters are not defined in the config file max_hits_per_bin=${max_hits_per_bin:-25}

#write max_hits_per_bin into the register
write_reg $thrproc thr_comp1_thr_lim$max_hits_per_bin


### Data Recorder

To run the test software and the data recorder the following packages are required: g++, tk, tk-dev, and grace

To set up the data recorder svn checkout the http://casper.berkeley.edu/svn/trunk/projects/seti_spec/server_software/datarecorder2/ directory from the casper svn and go through the following steps. (Note all paths are relative to this datarecorder2 directory).

• Verify that the top level Makefile has the correct include path for tk. This is set in the third line of the makefile and is set by default to /usr/include/tk.
• Next, you need to rebuild some of the include libraries. This only needs to be compiled once.
• Run make in AO/_home_cima_Wapp_Bin_Sources_Lib_linux and copy all the object files (*.o) to AO/_home_cima_Wapp_Bin_Sources_Lib_linux to AO/lib
• Run make in AO/_home_phil_vw_pnt_exportCode and copy libazzatoradec.a in AO/_home_phil_vw_pnt_exportCode to AO/lib

Then the datarecorder is compiled by running make in the datarecorder2 directory and should produce the setispec_dr executable.

## DDC

The DDC is a decimating quadrature downconverter. It receives samples of the IF from the ADC at a rate of 4 8-bit real samples per cycle at 200MHz, and outputs 1 16-bit complex (8-bit real, 8-bit imaginary) sample of the baseband signal per cycle. The incoming IF is mixed down with 200MHz, put through a decimating symmetric low-pass filter. The filter downsamples by 4 and has a -6dB cutoff frequency of 80MHz. Filter specs:

• Filter type: Windowed FIR
• Filter order: 63
• Cutoff frequency: 80MHz; -6dB
• Windowing: Tukey, alpha=0.5

Input: ADC block, 4 parallel data streams

Output: XAUI

General description:

For normal operation, the register "ddc_en" must be set HI, "beam_switcher/switchyard_BRAM" must be filled, and "beam_switcher/max_addr" set.

It may be that for the XAUI link to be set up properly, the "tx_nvalid" pin needs to be toggled. See point 3 below.

1) Snap blocks

2) Functionality of we pins

3) "tx_nvalid" (Transmission NOT valid pin) Sometimes when establishing a link, the RX XAUI block reports "rx_valid" = 0. This can be fixed by toggling the "tx_valid" port on the TX XAUI block. The "tx_nvalid" provides a manual NOT enable register which when toggled HI, toggles "tx_valid" LO in line with the sync pulse.

## PFB

The first stage of data processing is a coarse channelization handled by a 4096-channel polyphase filter bank (PFB). A general description of the PFB algorithm and architecture can be found here. The PFB takes a 16-bit (8 bits real, 8 bits imaginary) input from the DDC per cycle which is passed through a 4K-channel, 8-tap FIR filter that allows bit growth from 8 to 18 bits (18.17 signed). The post-FIR data is then fed into a 4096-point biplex pipelined complex FFT with programmable downshifts, which maintains 18.17 signed data at the output.

Input: XAUI

Output: gpio (data [0:35], sync [36] leftlink)

General description:

"fft_shift" must be set for all operation modes.

For simulation mode, MUX select sync_period registers must be set.

Default output method is gpio links.

## Corner Turner

The corner turner is a 4Kx32K matrix transpose that bridges the PFB and the FFT. By buffering each PFB frequency slice in DRAM, the transpose converts frequency-domain data back into time-domain data for use in the FFT. No data processing is done except for reordering, so it takes 36-bit data from the PFB, and feeds 36-bit data to the FFT. 32K samples of each of the PFB's 4K channels is buffered for use in the 32K FFT.

Input: gpio (data [0:35], sync [36] rightlink) or XAUI (port 0)

Output: gpio (data[0:35], sync [36] leftlink) or XAUI (port 1 & 2)

General Description:

For normal operation no registers need set. For test operations, the appropriate MUX selector pins, data, and sync_periods need set. The default is for data to be sent and received over gpio links.

## FFT

The FFT block implements a 32768-point FFT with programmable downshifts. It takes 36-bit data from the corner turner and outputs 36-bit data to the thresholder.

Input: gpio (data[0:35], sync [36] rightlink) or XAUI (port 0)

Output:

        FFT: gpio (data[37:72], sync [73] leftlink) or XAUI (port 1)
CT: gpio (data[0:35], sync[36])


General Description: In order to feed the thresholder with the output of the CT, the data input to FFT block (from CT) is passed through block.

## Thresholder

The spectrometer's thresholder module compares the power of each frequency bin with a scaled local average and reports those bins that are over the noise floor. It is comprised of three subcomponents: a threshold computation (averaging plus scaling), the actual thresholder, and a reporting interface to the postprocessing software backend. It takes 36-bit data as complex samples from the PFB and FFT, and reports 32-bit data out as power squared (r^2 + i^2; from here on "power" refers to power squared).

#### Threshold computation

Local averages are calculated per PFB channel by taking the same data being fed to the the FFT directly from the corner turner. This way, the total power in each PFB bin can be accumulated while the FFT is being computed (via Parseval's theorem). A 36-bit power is calculated from each 36-bit complex sample received from the corner turner. The 32K (2^15) powers corresponding to a PFB bin are accumulated and divided by 32K to get the local average, then scaled according to the scalar set by the user via software. The scalar is input as a 32-bit integer, of which only the bottom 18 bits are used as an 18.9 unsigned number. The integer scalar setting is divided by 2^9, giving it an interpreted range of [0.001953125, 511.998046875]. The spectrometer boots to a default scalar of 48 (0.09375).

The small interpreted value of the scalar is misleading, due to the interaction of downshifts in the FFT. The spectral power doubles for every stage in the FFT that does not downshift, and halves for every stage that does. Thus, the power gets scaled by a factor of (2^N/2^n ; N = number of stages without downshift, n = number of stages with downshift) through the transform. A similar scaling happens in the FFT of the PFB module, but since the bin powers and local average powers are calculated from the same post-PFB data, that scale factor cancels out with regards to thresholding. Taking all of these factors into account, the threshold is set at integer_scalar/[(2^9*2^(N-n)] times the average power.

$N = \frac{N_{hw}}{2^9 \; 2^{(N_{ns}-N_{ws})}}$

where

$P_{event} \ge N

$

Nhw is integer to write to software register

Nns is number of FFT stages without a downshift

Nws is number of FFT stages with a downshift

The spectrometer boots up with default settings at 11 of 15 stages downshifting in the FFT and an integer scalar of 48. Following the above the relation, this sets the threshold at 12 times the average power.

#### Thresholding and reporting

fft_bin - 15 bits - number between 0 and 32k pfb_bin - 12 bits - number between 0 and 4096k over_thresh - 1 bit - high when there are more hits than 'max_num_hits' blank - 3 bits - no data, should e zero's event = 1 bit - will be 1 if fft_power is over threshold fft_pfb_power - 32 bits - if fft_bin is 0, then the average power is reported, if fft_bin is not 0 then the fft fine bin is reported. Note that we will lose the first fft fine bin, although we will see if there was an event.

The number of records per spectra depends on the threshold and the max number of hits (max_num_hits) we're going to allow for each pfb bin, so we won't know how many records there are going to be for a spectra. The 4096 byte packet size is somewhat arbitrary and we will need more than one packet to construct a spectra.

Note that the pfb and fft bins are flipped, so they should be interpreted as fft bins coming out as 0 to 16383 then -16384 to -1 and the pfb coming out 0 to 2047 then -2048 to -1.

The thresholding triggers "hits" for FFT bins that are greater than or equal to the threshold power. Due to bandwidth limitations in the link between the BEE2 and post-processing PC, the number of hits reported per PFB bin is capped via a software-adjustable setting. To aid the software in determining the PFB baseline, the first sample of each PFB bin is always reported, though it may not be flagged as a "hit". Data is always reported a group of 5 32-bit numbers, which include the PFB bin number, the FFT bin number, the threshold power, the FFT bin power, and flags/error codes. Reported data is buffered in FIFOs that are self- regulating; when almost full writes are halted to avoid overflows that could cause data to become misaligned. The FIFOs are emptied by the on- chip PowerPC core to be assembled into UDP packets for transmission to the backend software.

Input::

 FFT: gpio (data[37:72], sync [73] leftlink) or XAUI (port 1)
CT: gpio (data[0:35], sync[36] leftlink) or XAUI (port 0)


Output: 10 GbE

For normal operation, "thr/comp1/thr_lim" and "thr/scale/p1_scale" must be set. For simulation mode, Input MUX registers must also be set.

ADC level tests were done with ALFA set to a center frequency of 1375 MHz with the wideband filter on the afternoon of 7 June 2009.

RMS: 27.6

A 3dB attenuator was added later in the day. The new RMS should now be 19.4, but no data has currently been taken.

## 09 July 2009 Testing

Data from three snap blocks on the BEE2 and one snap block on the iBOB were captured manually during an observing session from 22:00 to 23:00 AST on 09 July 2009. The iBOB snap block grabbed 8-bit values from the ADC. The three SNAP blocks on the BEE immediately followed the DDC (mixer+dec_fir), PFB (pfb_fir+fft) and FFT (fft) blocks. These tests were taken without the datarecorder code capturing packets. In the last few minutes of the observing session, the recorder was restarted and a small amount of data was taken from which the "ALFA setup" below is derived.

ALFA setup:

  Wideband filter
Center frequency = 1374940266 Hz


iBOB setup:

  "ddc_en" 0x01


BEE setup:

  PFB "fft_shift" 0x0FFF
FFT "fft_shift" 0x6EEE
THR "thr_scale_p1_scale" 0x60 (96)
THR "thr_comp1_p1_lim" 0x19 (25)