Homemade Interferometer
Reference Material[edit]
<latex> \documentclass[11pt]{article} \usepackage{graphicx} \usepackage{amsmath} \usepackage{fullpage} \begin{document}
\subsection*{The antenna} The interferometer consists of two dipole antennas. The dipoles are made out of single sided copper circuit boards (Prototyping Products 521. 12x12" single sided copper board;590-521). The dielectric on these boards was FR4 with a dielectric constant of K=4.3. They were milled so that only 4 strips of copper remained (each being ~5). These formed two perpendicular dipoles (one for each polarization). The dipoles are tuned to pick up 750MHz (in the middle of the ultra high frequency (UHF) band). Above and below the dipole plane are two circular metal sheets or radius ~2.5. These sheets are used to increase the frequency response of the dipoles by a few hundred megahertz.
\begin{figure}[!h]
\centering
\includegraphics[width=12in]{baoant.jpg}
\includegraphics[width=12in]{baoant2.jpg}
\caption{The dipole. Copper strips on the board are used for the dipole.}
\end{figure}
%\begin{figure}[!h]
%\centering
%\includegraphics[width=10in]{baoant2.jpg}
% \caption{The basic two-element interferometer.}
%\end{figure}
\subsection*{The Signal Path}
The interferometer consists of two dipole antennas; here we describe the signal path for one of the two polarizations available to each antenna.
The signal path begins at the sky, whose temperature at 750 MHz is approximately 10 K (obtained through models of the sky), which corresponds to a power of approximately -95dBm (P=k*T*BW). This initial -95 dBm needs to be increased to approximately -22 dBm, which is the input power that the analog-to-digital converters (ADC) need to output 16 rms-counts into the correlator. This is the optimum value we want going into the correlator.
The antennas are set up with an East-West baseline (however they can be set up any way you want, this is the easiest). Each antenna is propped up onto a ground screen ( a giant metal mesh plane), which has a circle cut-out in the middle to allow the signal-path cables to go through. The ground screen is used to reflect away anything coming up from the ground, as well as the sidelobes characteristic to dipole antennas.
The signal is first run through a balun, which turns a differential signal into a single-ended signal. Currently, ADT4-1WT 2-775MHz 50 ohm 1:4 balun SMT are being used. The baluns are placed on a baun test board. Next comes a series of amplifiers. The first is a 12.5 dB gain amp, with a noise figure of 2.5, connected directly to the balun. Following this first amp are two 18.5 dB gain amps (NF = 3.5 each), which are connected by a 40-foot LMR400 cable to a chassy which contains a 530-730 MHz filter, and three 13.5 dB gain amps (NF = 4.5 each). The three amps and the filter are connected in series by hand conformable SMA cables.
All of the amplifiers are powered by a 15V-5A linear power supply. Power was supplied to the amplifiers at the antennas with 50ft Belden CAT5e cables.
The output from the amplifier chassis is connected to the iADCs inside the ROACH board. Lastly, the iADC is also connected to a signal generator that outputs a 800 MHz sine wave. This is needed to sample the signal coming in at 800MHz, which corresponds to a bandwidth of 400 MHz. Note that since our bandpass filters go from 530-730 MHz, we are looking at the second Nyquist band (400 - 800MHz).
Note: This is the default setup. Many changes were made in the first few deployments to get the levels right.
\begin{figure}[!h]
\centering
\includegraphics[width=12in]{signalspath.jpg}
\caption{The backend amplifier and filter chain.}
\end{figure}
\subsection*{The Correlator} The correlator was built with CASPER architecture. Specifically, we used a 4 input ROACH correlator, known as a Pocket Correlator. This correlator took advantage of iADC's which can be sampled up to 1GHz with two inputs or 2 GHz with one input (interleave mode). The digitized signal then goes through a Polyphase Filter Banks (PFB) which consists of an FIR filter and an FFT. Then the signals are quantized to 8bits (4bits real and 4 bits imaginary). Finally, all the signals are cross multiplied to give the cross correlations.
The scripts used to collect the cross correlations and initialize the correlator can be found [here].
\begin{figure}[!h]
\centering
\includegraphics[width=12in]{corr.jpg}
\caption{The CASPER ROACH board with iADC's used as the correlator hardware.}
\end{figure}
\subsection*{Test Equipment}
The test equipment we used to verify power levels and spectra were:
HP Power Meter with Low Frequency power head. HP Spectrum Analyzer
\subsection*{First Trial-HFA} The first trial was run at 6pm on 10/11/2011. Unfortunately, we did not observe any fringes. After double-checking all connections, the auto-correlations did not show any convincing passbands.
\subsection*{Secong Trial-HFA} The second trial was run at approximately 2pm on 10/12/2011. The signal path had to be changed from that described above, since we found that the sky signal power had significantly increased from the standard -95dB (Note that this doesn't take into account RFI and other foreign signals). This was due to all the RFI present and the Sun. We underestimated the RFI environment. We removed all but one 13.5 dB gain amplifier for each antenna. Although this increased the power to an appropriate level for our iADCs, we still did not detect any fringes, and the auto-correlations for each antenna again did not show convincing passbands.
\begin{figure}[!h]
\centering
\includegraphics[width=12in]{hfa2.jpg}
\caption{HFA deployment!}
\end{figure}
\subsection*{Third Trial-Leuschner}
The third deployment was done at Leuschner observatory on October 18th 2011 during the afternoon. Again the signal path for each antenna was changed, as well as the correlator design from the previous deployments. We had an east-west baseline of 575 inches = 36.52 wave lengths at 750MHz. The signal path of one antenna was as follows:
dipole antenna->3ft sma with ~90db shielding-> AP1309C Cougar amp (12.5 db gain NF=2.5) -> AC3064 Cougar amp (18.5 db gain NF~3.5) -> 40ft LMR400 cable -> 530-730 MHz bandpass filter -> AC1309 Cougar amp (13.5db gain NF~4or 5) -> AC1309 Cougar amp -> 6ft sma (90db shielding) cable into iADC's. Power was supplied to antennas in the usual fashion (CAT5e cable).
Correlator details: running baopoco design from october14th. equalization coefficients set to 500. int time set to default (2**28 --> 1.3 seconds). The power levels into the ADCs were ~20.5 dbm with this set up (measured with the HP power meter). Signal generator clock was set to 800MHz +6.00 dbm.
This deployment went great! We saw fringes! Although Lots of RFI is still present. More to come...
\begin{figure}[!h]
\centering
\includegraphics[width=10in]{baseline.jpg}
\includegraphics[width=10in]{blofringes.png}
\caption{A baseline at Leuschner Observatory and our first fringes!}
\end{figure}
\subsection*{Fifth Trial-Leuschner} Fifth deployment of BAOBAB-4 at Leuschner on May 24th 2012. New active Baluns with Noise Temperature ~40K.
