Tuesday, July 17, 2007

Data Acquisition Block Diagram

The data acquisition of a band-limited signal has the following broad steps:

  1. DC removal

  2. 2-bit ADC (0.5 MHz speed)

  3. Collect 4 such samples at one time (for simplicity, copy the same signals 4 times)

  4. Build a sampler, of 0.5 MHz.

  5. One could take 10 MHz signal, and use every 20th pulse (simple counter) for sampling.

  6. Put together four 2-bit samples on a bus with an isolator for i/o.

  7. Feed the signal to the data acquisition card sitting in the PCI slot of a PC.

Wednesday, July 04, 2007

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.

Tuesday, July 03, 2007

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).

Using the Crystal Oscillator

Our Band is 73-74.6. How do we use a Oven Controlled Crystal Oscillator, referred in the earlier post? We need to generate an LO signal (between 70-80 MHz) to bring RF signal to an IF of 2-5 MHz. We also need to use one frequency for our sampler.

We have a few frequencies at our disposal, 10, 12.8, and 16.384 MHz, etc. We should use 10 MHz for sampling, after some downconversion (0.5 MHz for 0.25 MHz band). This would be easy, using a digital counter. How do we go about LO? we need one frequency such that a bandwidth of 0.25 MHz in the above band gets down converted at IF.

It depends upon a combination of IF, and bandwidth we choose. The problem is, if the band has a lot of interference, we should be able to switch to some other location nearby (within 73-74.6). A smaller bandwidth makes this possible, while bringing down data rates.

I have a couple of solutions, I will detail two in the next post.