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XRS-2
→ How it works
→ CDP
Calorimeter Digital Processor (CDP)The CDP processes the raw dataThe job of the CDP is to analyze the raw data from the detectors and report the time and energy of each X-ray that is detected. To do this it has a Digital Signal Processor (DSP) for each detector. Each channel's DSP watches the incoming data from the CAP and detects the pulses that result when X-rays hit the detector. When it detects one, it collects the entire pulse and analyzes it to determine the pulse height. The pulseheight, which is proportional to the energy of the incoming X-ray, is then sent out as data. Block Diagram of one channel
The Lowpass filter is necessary for any A/D converter, to prevent
aliasing. The A/D converter
converts the signal to digital form, and the DSP calculates the X-ray
energy and sends it out in digital form to the spacecraft. The
spacecraft stores it in a large memory bank and eventually transmits
it to the ground. Why have a CDP at all?Why not just convert the data to digital form, beam it all down and process it on the ground? The answer is that there is just too much data. Each pulseheight is sent as a packet, along with timing and other information. The packet is 64 bits in length, which is far less than the amount of raw data in the pulse (2048 samples x 14 bits = 28672 bits). You can think of it as compression by a factor of 450. For comparison, a typical MP3 file is compressed by about a factor of 10. A typical bright X-ray source will generate a few pulses per second in each detector. 5 pulses * 32 detectors * 64 bits is 10240 bits/second, which is the maximum data rate for XRS. (This is about 1/4 the rate you can get with a dialup connection.) So how does it all work?For more detailed information, follow these links:
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The CDP has a complete processing channel for
each of the 32 detectors. Each channel consists of an antialiasing
filter, an A/D converter and a DSP.