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Given a quiet local environment and several hours of
integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? George |
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On a sunny day (Thu, 15 Jan 2004 19:57:06 -0000) it happened "George Dishman"
wrote in : Given a quiet local environment and several hours of integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? George Not sure what you mean, but if the 'integration time' is longer then milliseconds, you for sure will not be able to detect those milliseconds (as modulation in amplitude). You will detect there is something there (average power). So you mean measurement time? No idea, depends on how strong the signal is, and I dunno that. |
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![]() "Jan Panteltje" wrote in message ... On a sunny day (Thu, 15 Jan 2004 19:57:06 -0000) it happened "George Dishman" wrote in : Given a quiet local environment and several hours of integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? George Not sure what you mean, but if the 'integration time' is longer then milliseconds, you for sure will not be able to detect those milliseconds (as modulation in amplitude). You will detect there is something there (average power). So you mean measurement time? No idea, depends on how strong the signal is, and I dunno that. If you already know the period of the pulsar it is possible to integrate the signal and raise it above the noise background. Probably would employ an electronic device known as a phase-locked loop, which can "lock-on" to a weak signal if it is set close to the signal frequency ( the weaker the signal, the closer the PLL has to be set to the exact signal frequency ). Andy |
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On a sunny day (Fri, 16 Jan 2004 00:37:24 -0000) it happened "andrewpreece"
wrote in : "Jan Panteltje" wrote in message ... On a sunny day (Thu, 15 Jan 2004 19:57:06 -0000) it happened "George Dishman" wrote in : Given a quiet local environment and several hours of integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? George Not sure what you mean, but if the 'integration time' is longer then milliseconds, you for sure will not be able to detect those milliseconds (as modulation in amplitude). You will detect there is something there (average power). So you mean measurement time? No idea, depends on how strong the signal is, and I dunno that. If you already know the period of the pulsar it is possible to integrate the signal and raise it above the noise background. Probably would employ an electronic device known as a phase-locked loop, which can "lock-on" to a weak signal if it is set close to the signal frequency ( the weaker the signal, the closer the PLL has to be set to the exact signal frequency ). Andy Yes that is one method, to try to lock a PLL to it, however PLL does not like noise at all. So then your loop filter for the PLL will have to have a low bandwidth. Normally, the way I see it, if a signal (noise with some of the pulsar pulsing in it at frequency x), you would take n samples (so for a fixed sample frequency a specific time), run a FFT on it. In the resulting frequency spectrum you would see a peak at say 1 kHz (if the pulsar pulsed once every millisecond). The more samples you have (the longer your data, *receiving time*), the better the result of the fft. You need at least a (Nyquist) 2 x the pulsar frequency to make anything out. So integration is (in this example) not the right word. This is the way *I* would look for a signal (more advanced algos exist). When using a non digital setup, you are heterodyning , down mixing from some short wavelength, and then use a AM detector, it would show a 1 ms pulsar as a 1 kHz beep (on a speaker), slower ones do 'phhs', 'phhs', 'phhs', with each 'phhs' for one revolution of the pulsar (not sure if it pulses twice or once per revolution, may depend on the angle dunno). It is a typical sound, I have heard it. And noise... The filter after the AM detector is a lowpass (you can call that an integrator), and it must be able to pas the 1000 Hz as in this example, a 1 Hz low pass would give you no pulsar 'sound'. It WOULD give some DC level that you could detect. As good narrow band filter (once you find the frequency) (parametric equalizer for example) could filter out more noise and you'd have a nice reference that you could then sync you watch to.. Pulsar is supposed to be very stable.... Note no PLL here, but could be used of cause. This is all I know about pulsars, but the electronics I can expand on if you wish. JP |
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"Jan Panteltje" wrote in message
... You might also wish to look at software methods to pull the signal out of the collected signal. See, for example, Scargle Periodogram methods. I did an article on this for The Orrery newsletter a while back. -- ----------------------------------------------------------------------- Greg Neill, Editor The Orrery: Models of Astronomical Systems http://members.allstream.net/~gneill/ ----------------------------------------------------------------------- |
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On a sunny day (Fri, 16 Jan 2004 13:02:49 -0500) it happened "Greg Neill"
wrote in : "Jan Panteltje" wrote in message ... You might also wish to look at software methods to pull the signal out of the collected signal. See, for example, Scargle Periodogram methods. I did an article on this for The Orrery newsletter a while back. I just did a Google for Scargle Periodogram pulsar. Very interesting, finding planets around pulsars with it. Even with missing data points. http://astrosun.tn.cornell.edu/~akgun/Grad/pulsarz.ps Nice paper too. Thank you. -- ----------------------------------------------------------------------- Greg Neill, Editor The Orrery: Models of Astronomical Systems http://members.allstream.net/~gneill/ ----------------------------------------------------------------------- |
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First I'd like to thank all who have replied, the information
provided has been very helpful. "Jan Panteltje" wrote in message ... On a sunny day (Fri, 16 Jan 2004 00:37:24 -0000) it happened "andrewpreece" wrote in : "Jan Panteltje" wrote in message ... On a sunny day (Thu, 15 Jan 2004 19:57:06 -0000) it happened "George Dishman" wrote in : Given a quiet local environment and several hours of integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? George Not sure what you mean, but if the 'integration time' is longer then milliseconds, you for sure will not be able to detect those milliseconds (as modulation in amplitude). You will detect there is something there (average power). So you mean measurement time? No idea, depends on how strong the signal is, and I dunno that. If you already know the period of the pulsar it is possible to integrate the signal and raise it above the noise background. Probably would employ an electronic device known as a phase-locked loop, which can "lock-on" to a weak signal if it is set close to the signal frequency ( the weaker the signal, the closer the PLL has to be set to the exact signal frequency ). The weaker the signal, the narrower the bandwidth of the loop filter needed to reject the noise to below the signal, and of course the initial PLL frequency has to be within about a bandwidth of the actual signal. However, AIUI, pulsar signals are broadband. As a result I had assumed the signal would also be non-coherent (i.e. not a modulated sine wave but amplitude modulated noise) but a page on "Phase-coherent De-dispersion" has made me wonder about that now. Dispersion arises because different frequencies are delayed by different amount by the interstellar medium (ISM) so if you had a wide enough bandwidth on the receiver, it could be delayed a whole cycle and the sum would be a continuous constant level. http://www.jb.man.ac.uk/~pulsar/Educ...00000000000000 Yes that is one method, to try to lock a PLL to it, however PLL does not like noise at all. So then your loop filter for the PLL will have to have a low bandwidth. Normally, the way I see it, if a signal (noise with some of the pulsar pulsing in it at frequency x), you would take n samples (so for a fixed sample frequency a specific time), run a FFT on it. In the resulting frequency spectrum you would see a peak at say 1 kHz (if the pulsar pulsed once every millisecond). The more samples you have (the longer your data, *receiving time*), the better the result of the fft. You need at least a (Nyquist) 2 x the pulsar frequency to make anything out. Imagine a simple wideband receiever with a fast amplitude detector on the output. The 'signal level' would vary on sub-millisecond timescales. Put that amplitude rather than the RF signal itself into the FFT and you would get what you describe. Note the "Square law detectors" in this: http://tucanae.bo.astro.it/pulsar/32mt/ The use of the term "Phase-coherent" when talking of de- dispersion puzzles me since RF phase information is discarded by such a detector. So integration is (in this example) not the right word. The key is what Andy said, "If you already know the period of the pulsar ..." Suppose the period is known to be exactly 1ms. By sampling the RF amplitude every 100us and adding the value to a set of ten 'bins', the pulse shape emerges from the noise: http://www.radiosky.com/rspplsr.html The trick is that the free-running clock doing the 100us timing has to stay exactly in step with the unknown signal for the integration period or the pulse will just smears over the bins and give the average in all. This is where I was wondering if timing the sampling with an atomic clock could allow longer integration periods and hence a smaller antenna. This is the way *I* would look for a signal (more advanced algos exist). When using a non digital setup, you are heterodyning , down mixing from some short wavelength, and then use a AM detector, it would show a 1 ms pulsar as a 1 kHz beep (on a speaker), slower ones do 'phhs', 'phhs', 'phhs', with each 'phhs' for one revolution of the pulsar (not sure if it pulses twice or once per revolution, may depend on the angle dunno). It is a typical sound, I have heard it. And noise... The filter after the AM detector is a lowpass (you can call that an integrator), and it must be able to pas the 1000 Hz as in this example, a 1 Hz low pass would give you no pulsar 'sound'. It WOULD give some DC level that you could detect. As good narrow band filter (once you find the frequency) (parametric equalizer for example) could filter out more noise and you'd have a nice reference that you could then sync you watch to.. Pulsar is supposed to be very stable.... They can be more stable than atomic clocks! Note no PLL here, but could be used of cause. This is all I know about pulsars, but the electronics I can expand on if you wish. If you have any information how broadband de-dispersion might be performed in the RF stage, say by a configurable flat gain frequency-dependent phase shifter, I would be very interested. George |
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On a sunny day (Sat, 17 Jan 2004 09:29:28 -0000) it happened "George Dishman"
wrote in : the signal, the closer the PLL has to be set to the exact signal frequency ). The weaker the signal, the narrower the bandwidth of the loop filter needed to reject the noise to below the signal, and of course the initial PLL frequency has to be within about a bandwidth of the actual signal. In some video applications and servo applications, you let it lock in frequency first (high bandwidth), then phase lock with lower bandwidth. I have even seen servo system where a third even lower bandwidth / high gain loop then takes over (if phase locked), to further reduce phase errors. If some transient kicks in, the whole sequence starts again... However, AIUI, pulsar signals are broadband. As a result I had assumed the signal would also be non-coherent (i.e. not a modulated sine wave but amplitude modulated noise) eh, no, it wil always be a 'superposition', unless there is some effect in the RF or IF stages like amplitude dependent gain (say non-linearity). There will be a whole spectrum of stuff around that center 1 KHz (in this case), if it comes from a jet, it may vary over time (not likely that jet, when sweeping across your receiver will be a smooth thing), but that spectrum could be anything: say the jet consists of some small sub-beams, then you get 1KHz, but also harmonics, and the phase and frequency of these may well change over time. but a page on "Phase-coherent De-dispersion" has made me wonder about that now. Dispersion arises because different frequencies are delayed by different amount by the interstellar medium (ISM) so if you had a wide enough bandwidth on the receiver, it could be delayed a whole cycle and the sum would be a continuous constant level. Then the higher the pulsar freq, the more that could happen? I will have to read that, OK. http://www.jb.man.ac.uk/~pulsar/Educ...00000000000000 OK, later now, read that. Would it make sense that if you know the phase versus frequency curve, then you can make a compensator for that? So that would be a filter with a phase characteric that shifts in such a way that (for the bandwidth used) zero phase error remains? Makes that sense? Imagine a simple wide band receiver with a fast amplitude detector on the output. The 'signal level' would vary on sub-millisecond timescales. Put that amplitude rather than the RF signal itself into the FFT and you would get what you describe. Note the "Square law detectors" in this: http://tucanae.bo.astro.it/pulsar/32mt/ The use of the term "Phase-coherent" when talking of de- dispersion puzzles me since RF phase information is discarded by such a detector. Have not read it (yet), but I think I pointed out that 'phase coherent' here refers to the sidebands, well, for all I know, over a relatively short period of time (in the pulsars life, so for a long time for us), if nothing physically changes (in that time) for the beam(s) emitted by the pulsar, the sidebands should be phase coherent. Not sure that is what you mean, I will read that stuff. So integration is (in this example) not the right word. The key is what Andy said, "If you already know the period of the pulsar ..." Suppose the period is known to be exactly 1ms. By sampling the RF amplitude every 100us and adding the value to a set of ten 'bins', the pulse shape emerges from the noise: http://www.radiosky.com/rspplsr.html Yes this is well known. Now later: Here your clock may help? The trick is that the free-running clock doing the 100us timing has to stay exactly in step with the unknown signal for the integration period or the pulse will just smears over the bins and give the average in all. Yes a constant clock (you want atomic clock), will reduce error. Interesting thing is all the time in this discussion I was thinking about the thread (some year ago?) in sci.crypt (or sci.physics), where NON constant sampling was used to get almost undetectable info from pictures (like Roentgen. Some prof could detect images in almost TOTAL noise with a new algo, using VARIABLE sampling. They were a bit secretive about the math, but it seemed to make sense. Applications were of cause military etc... I was thinking if this could be used to get better resolution. If an atomic clock would improve results in this (pulsar) case, I thing stability we think about here over the time of a measurement.... In the planet finding paper they talk about 200 days... (now you look for the frequency variation of the pulsar), maybe to have absolute time will help. The drift in 200 days should be factor n of a period less then period time of the pulsar? So if 1000 pulses / second, 3600 x 24 x 200 periods = 17 280 000 pulses. For 1 degree phase error x 360 = 6 220 800 000 So 1 part in 6 10^9, yes :-) I really dunno if this makes sense. This is where I was wondering if timing the sampling with an atomic clock could allow longer integration periods and hence a smaller antenna. If you have any information how broadband de-dispersion might be performed in the RF stage, say by a configurable flat gain frequency-dependent phase shifter, I would be very interested. I will read those links first, if I can think of anything that makes sense I will post it here. OK I have read those now, what I wrote stands. Your freq dependant phase shifter is the same idea, I do not see whay this should not work. To make such a filter would not be that difficult, if provided with actual degrees and MHz. Spice It does not have to work at (for example) 400 MHz (as in that link), but the receiver will have a much lower IF amplifier, with known bandwidth, and at say 40 MHz and 10 MHz bandwidth you are in normal analog TV stuff, and filters at that frequency are easy. At least I have experience with that. If it all helps, I dunno, the proof of the pudding is in the eating really. For the avid experimenter here is a great area I am sure. Digitizing at for example at 32 mega samples per second, like I am doing here for video, you could do the compensation with a DSP, or later with some algo? Maybe I got this wrong? Seems too simple, some catch somewhere? JP |
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![]() "George Dishman" writes: Dispersion arises because different frequencies are delayed by different amount by the interstellar medium (ISM) so if you had a wide enough bandwidth on the receiver, it could be delayed a whole cycle and the sum would be a continuous constant level. That's true. The "professionals" often sample a bank of narrow bandwidth channels, and then recombine these channels later using the dispersion correction formula. The use of the term "Phase-coherent" when talking of de- dispersion puzzles me since RF phase information is discarded by such a detector. George, phase coherent here is refering to the pulsar *pulse* phase, not the RF signal phase. When pulsar people talk about a phase coherent solution, they mean they have a model which can account for every pulse cycle in the data arc. Of course, just as for RF, it's possible to lose track of the pulsar cycle count, especially over large data gaps. Craig -- -------------------------------------------------------------------------- Craig B. Markwardt, Ph.D. EMAIL: Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response -------------------------------------------------------------------------- |
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In message , George Dishman
writes Given a quiet local environment and several hours of integration time, what rough size of antenna would be needed to receive millisecond pulsar signals? An acre or two of phased array dipoles should be enough to get started. Though you may need more than that to get down to the millisecond pulsars even with sophisticated integration techniques. ISTR about 20000m^2 of aerial discovered the Crab nebula at ~80MHz Regards, -- Martin Brown |
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