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Meaning of db

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Meaning of db

Postby swetterlin » Wed Jan 21, 2004 2:32 am

I know db measurements in Picoscope can be changed to dbm; but if you view these as power measurements, they are power per WHAT? As near as I can tell, they are power per a unit equal to the bandwidth divided by the number of FFT points. Is that correct? Can that be shown on the spectrum graph? It would be nice to get dbm per Hz, but it certainly is not that.
swetterlin
 

Postby markspencer » Wed Jan 21, 2004 8:33 am

Hi,

This is when the voltage is set in the win.ini file and a reference to zero db is stated. This is just defining a ratio.

If you experiment with this you will see that it will effect the vertical scale on the spectrum viewer.

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Regards,

Mark Spencer
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meaning of db

Postby swetterlin » Wed Jan 21, 2004 3:32 pm

I understand that db is based on a ratio of voltages, but in order for the signal to have a certain voltage at a certain frequency, the measuring device must be admitting signal in a certain bandwidth around that frequency. For example, even a very strong signal if measured in a bandwith of 1/1000 of a Hz (not really practical) would have a virtually nil voltage, because the signal power within that bandwidth would be nil. Analog spectrum analyzers have selectable bandwidths, so you know what you are dealing with. I am just trying to figure out what the bandwidth is for Picoscope. My guess is that it is the total sweep bandwidth divided by the number of FFT samples.
swetterlin
 

Postby markspencer » Thu Jan 22, 2004 8:32 am

Hi,

The Bandwidth for the FFT is that chosen in the dropdown box on the spectrum viewer. The bandwidths that are available for the various units are all different. The bandwidth is always from DC to the displayed value.

The bin size for each Hz is the bandwidth/no of FFT points. Therefore the bin size can be reduced by selecting more FFT points upto 4096 having greatest accurate.

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Mark Spencer
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