MHD simulations

Miguel has asked for the latest in MHD simulation cubes. so, i have read data with Python, instead of C++. it is a lot faster to write the code and test it. it is also to process and update.

1. Read the density array: split the lines in parts
2. Store the data in density_data[] and reshape it.
3. plot select slices along z, and they are okay.
4. Now read density and velocities.

BITS Goa Radio Telescope : Software Correlator

Thanks to Aniket and Mandar, we will get a data acquisition card interfaced to a PC by next semester.

All the card does, is to accept AC voltages of 250 KHz bandwidth, digitize it (2-bit ADC) and sample it at Nyquist rate (500 KHz). Four samples (2x4 bits = 1 byte) are then packed together on the fly to form one byte. The resultant one-byte is stored on a PC for processing. So, the pipeline looks as below


--- signal ---- >>-- ADC -->>--Linux PC-->>- FILE
(0.5 V AC________3-level____bit packing
0.25 MHz band)___2-bit_________program


ADC has two comparators (NE 521 ?). Depending upon the input, one of the the following 00 (-2), 01 (-1), 10 (+1), 11 (+2) is the output of the ADC.

The sampler signal of 0.5 MHz samples the ADC output voltages. Four of the samples are fed to the acquisition card through data cables.

The data rates are slow, 500 k Bytes per second. Given the modern computer disk rates, it is possible to sustain a on-the-fly bit packing program in PC. The program accepts 4 bytes, and based on a precalculated table, stores corresponding 1-byte output onto a file.

LO and Sampler Frequency Solution

We have our RF band as 73 - 74.6 MHz. Using one of the frequencies given by the Oven Controlled Crystal Oscillator, we would like to generate LO (of) such (frequency) that, our band is folded at some suitable IF. The end of the IF band should be the sampler frequency, again one of those given by the oscillator.

The first image is one such combination for the RF band and LO (plotted very quickly using PLOT program in Mac).


Given the 0.26 MHz band ending at 4.096 MHz beyond LO (70 MHz), the IF band looks as given in the second figure. The IF band now falls between 0 MHz and 4.096 MHz, although mostly empty (0-3.75 MHz), due to our RF filter.

The band is now sampled with 8.192 MHz, the sub-harmonic of 16.384 MHz from the oscillator.

Since both LO and sampler frequency are derived from the same ref, there should be more stability in the system.

One can have another combination of such a LO and sampler frequencies. As an exercise, try these two frequencies and draw the plots: LO (76.8 MHz) and Sampler (5 MHz).

Oven Controlled Crystal Oscillator MCOCXOW

I looked for crystal controlled COs. Here is one data sheet from Golledge. Excellent stability, we do not know the price. It is their featured product, from price and availability point of view.

BITS Goa Radio Telescope

when i spoke about our plans for hardware for our radio telescope, there was some feedback from my supervisor (Deshpande, RRI). he was of the opinion, that

1. our telescope was a fantastic idea, and could eventually do international science using 4 element dipoles. we have to plan and execute it well, of course.


2. in the beginning, we could make images using 2 antennas only, with Mandar and Aniket's data acquisition cards. this means, as soon as we demonstrate 2-slit interferometer, we can think of maps in the second semester.


3. we should think of using 2 bits and less bandwidth. eventually, we will have 4 element dipoles. so, we will have to store 4x2 = 8 bits = 1 byte at nyquist rate. if we opt for smaller bandwidth (0.25 MHz), we could be saving 0.5 MBytes per second on the disk, with each byte being a sample each from 4 elements.




In the immediate plans:



4. we should converge using "Mini Circuits" ICs for our RF components. See their website



5. also, for LO stability, either
   
   

   a) opt for GPS receiver, and use its 10 MHz signal for LO

   
   

   b) use a "oven controlled crystal oscillator"

   
   
   the second option is better for us for now. we will continue to explore the second option for better eventual stability of frequency signal. for better mapping, we need our sampler and LO to be really stable.
   
   

Amateur Radio Telescope at 61 MHz

We finally tested the interference environment around BITS using a TV Yagi antenna. This was at 3 pm on Monday 13th November, 2006.

The frequency region around 60-70 MHz appeared the most calm of all, particularly 61-62 MHz. We need about 500 KHz bandwidth, and we will keep it on 61 MHz. The band at 120-130 MHz appeared to be good as well. It is possible to utilize the same antenna and receiver system at 122 MHz as well, just in case there was intermittent terrestrial source at 61 MHz.

Making an Amateur Radio Telescope

Radio Interferrometer

Today I detailed the entire telescope project with Anita. The standard components will be:

1. Two antennas, separated by about 100 meters. To begin with, of course, we will start with one antenna. Signal frequency of 40/60 MHz +- 5 MHz
   
2. Pre-amplifier will be required in at least one case, to boost the signal by 20 dB and carry it over 100 meters. Signal freq 40/60 MHz +- 5 MHz.
   
3. RF to IF conversion followed by an appropriate filter: 0-2 MHz band chosen.
4. IF amplification, possibly two amplifiers back to back for 40 dB or so gain: signal freq. 0-2 MHz.
5. Phase shifter and adder (with 90 degrees phase difference), followed by a detector. The output is a amplitude time series at the rate of 1/16 second.

This kind of project requires a team of students, possibly 8 students working on different aspects simultaneously. Complex job, but it will be fun if we invest their time in it...