Degrees Kelvin vs dB (C/Ku bands)
- Thread starter adrian
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Kelvin = 290 * (10 ** (dB / 10) - 1)
and
dB = 10 * log(1 + (Kelvin / 290))
Values can be confirmed using the following table
http://www.modcoinc.com/charts/chart7.htm
and
dB = 10 * log(1 + (Kelvin / 290))
Values can be confirmed using the following table
http://www.modcoinc.com/charts/chart7.htm
Am I reading that chart correctly?
So a .3dB (or .30) would be 20.74 K ?
Most top quality C-band LNBs are 15K - 17k correct? Under .25dB.
So a .3dB (or .30) would be 20.74 K ?
Most top quality C-band LNBs are 15K - 17k correct? Under .25dB.
The actual C/N ratio change from 15K to 20K is very small. The real spec you should worry about if freq stability. You also have to factor in how they measure the LNB. There are usually three measurement points in the spectrum. Low middle and High. Most manufactures sell an LNB based on the Highest noise figure. Others sell based on the average of all three test points. It has been several years since I have worried about C-band LNB's but back in the day we used to see noise figures ranging as much as 30 degrees from low to high, while other brands were far more flat line. If a manufacture test an LNB at 14, 20, 25, the average is 19 but 8 transponders above that. Is that legal, not sure, but I know it is done.
IMO, anything under 25 yields the same PQ. Your dish is a far more critical of a factor these days than the LNB.
IMO, anything under 25 yields the same PQ. Your dish is a far more critical of a factor these days than the LNB.
Thanks for that info; I know when I upgraded my BUD setup a while back I added the "high-stability" models to my corotorII; 15deg C and I think .5dB Ku. They have served very well.
Okay, obviously you guys know way more about this than I do - but I thought dB was a ratio and kelvin a temperature? Either I am wrong or I don't understand the concepts involved (or, more likely, both). Can anyone clarify for me?
dB is a logrithmic value. It simplifies power values for Signal-to-Noise ratios (SNR).
For example, if the Signal strength is 60 dB and your noise strength is 40 dB, the SNR is 20dB (60 - 40). Its not 60 / 40. The reason is that the ratio is measured in units of Power (Watts). 60dB is 1MW and 40dB is 10kW. If you divide 1MW / 10kW, the ratio is a value of 100. Take the log of 100 (2), and then multiple by 10 (20).
So instead of having to do a bunch of complex math, you only have to do additions and subtractions when using dB. Its a property of the log function that allows this to happen.
The Temperature charts show the how much noise level you can expect at certain temperatures, which would be subtracted from the Signal or SNR values.
You can find out more about dB at the link below...
http://en.wikipedia.org/wiki/Decibel
For example, if the Signal strength is 60 dB and your noise strength is 40 dB, the SNR is 20dB (60 - 40). Its not 60 / 40. The reason is that the ratio is measured in units of Power (Watts). 60dB is 1MW and 40dB is 10kW. If you divide 1MW / 10kW, the ratio is a value of 100. Take the log of 100 (2), and then multiple by 10 (20).
So instead of having to do a bunch of complex math, you only have to do additions and subtractions when using dB. Its a property of the log function that allows this to happen.
The Temperature charts show the how much noise level you can expect at certain temperatures, which would be subtracted from the Signal or SNR values.
You can find out more about dB at the link below...
http://en.wikipedia.org/wiki/Decibel
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C band and Ku band LNBs are measured on a different scale because when Ku band LNBs first came out, they would have been over 200K. Since C-band LNBs were well under 85K at the time, it was thought that people would think the ku lnbs were not going to work well. Most people did not realize that the receive frequency is part of the equation when calculating the noise temp of an LNB.
The industry used a dB rating to avoid confusion. a 1.5db Ku-LNB sounded far better than A 220k (or what ever the temp would be)
The industry used a dB rating to avoid confusion. a 1.5db Ku-LNB sounded far better than A 220k (or what ever the temp would be)
