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#1
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I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. Doesn't sound to bad really. A one second scan across the entire 100MHz is fine really (depending on what your doing) - the PC is a perfect storage scope. To convert a linear I/Q baseband sample to a log scale is no problem at all in software. The software cud easily deal with any variation in VCO/Mixer level differences across the whole band. Best to have the PC control the vco though, then as you say, you can zoom in on a desired freq range etc. Clive |
#2
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wrote in message news I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. The problem I'm having seeing how it can work is this. If you take 44100 measurements per second, that's one measurement every 27uS. But you've low pass filtered your baseband at 1KHz. It would take at least one of those cycles to measure the envelope amplitude with any degree of accuracy but you're allowing just 2.7% of a single 1KHz sample, how does that work? Or am I looking at it too simplistically? I certainly agree that the PC can make a nice storage and display device, specially if 'scopes aren't so easily available. Seems to me a question of how much of the signal chain is implemented in analogue and how much in digital. I Just think it saves an awful lot of hassle to add that little extra analogue stage before you go digital, i.e. 2nd IF and detector. Log could be done on the digital side no problem if desired provided enough ADC resolution was available. VCO/Mixer level differences could still be compensated in software, and the PC control the VCO. Hans G0UPL http://www.hanssummers.com |
#3
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wrote in message news I still think you stand a fair chance of problems, when essentially you're doing your signal strength measurement in baseband and sweeping at rates in the same frequency range. The aquisition time for the signal strength measurement needs to be much shorter than the sweep, or the signal strength will change while you're attmpting to measure it. Yes Sweep rate can be a problem, but it's all down the baseband bandwidth and what kind of update rate you want/need. If you want to sweep across a 100MHz within say a second, assuming your using 44100 sampling rate, that's 44100 samples you'll get across the 100MHz sweep in that one second - 2.26Khz wide freq segments (non-overlapping), but you'd need a baseband bandwidth of 1KHz to overcome the anti-alias problem. The problem I'm having seeing how it can work is this. If you take 44100 measurements per second, that's one measurement every 27uS. But you've low pass filtered your baseband at 1KHz. It would take at least one of those cycles to measure the envelope amplitude with any degree of accuracy but you're allowing just 2.7% of a single 1KHz sample, how does that work? Or am I looking at it too simplistically? I certainly agree that the PC can make a nice storage and display device, specially if 'scopes aren't so easily available. Seems to me a question of how much of the signal chain is implemented in analogue and how much in digital. I Just think it saves an awful lot of hassle to add that little extra analogue stage before you go digital, i.e. 2nd IF and detector. Log could be done on the digital side no problem if desired provided enough ADC resolution was available. VCO/Mixer level differences could still be compensated in software, and the PC control the VCO. Hans G0UPL http://www.hanssummers.com |
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