TV Fool Noise Figure Accounting
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 2-Dec-2013, 8:07 PM #1 mmbridges Junior Member   Join Date: Nov 2013 Posts: 8 Noise Figure Accounting I am trying to understand how the addition of a low noise figure (NF) preamp can improve the overall system NF. I have a Zenith DTT-901 Set Top Box (STB) that I cannot find the NF for so it so lets assume it is 6dB. What is the formula that shows how the gain and NF of a preamp improve the overall system NF when placed in front of the STB. (this assumes the preamp NF is lower than the STB NF) I have read that as long as the preamp gain (dB) is sufficiently larger than the STB NF (dB) then the over all system NF (dB) will be determined by the pre-amp NF (dB). I am an equations kind of guy so would like to see exactly how as the pre-amp gain increases the system NF approaches the pre-amp NF. Thanks!
 2-Dec-2013, 10:59 PM #2 GroundUrMast Moderator     Join Date: Oct 2010 Location: Greater Seattle Area Posts: 4,681 Have you seen this thread yet? http://forum.tvfool.com/showthread.php?t=109 __________________ If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.) (Please direct account activation inquiries to 'admin')
 2-Dec-2013, 11:40 PM #3 mmbridges Junior Member   Join Date: Nov 2013 Posts: 8 Missing Details Thanks for the reply Gmast. I have seen this thread. In particular: Scenario 3 (rooftop antenna followed by pre-amp) indicates (Note that his scenario assumes that the gain of the pre-amp is more than enough to overcome the losses of the cable, splitter, and receiver noise figure that come after it in the chain.) the example suggests for preamp gain that is "more than enough", all of the above losses are cancelled out. It is the "more than enough" part I am interested in getting more details on. Is the degree to which these losses + NF are overcome a function of the preamp gain? If so what is the formula? In other words if the preamp gain exactly equaled the referenced losses + receiver NF, would the net NF exactly equal to the preamp NF? If not what is the formula that determines how different it would be?
 3-Dec-2013, 6:45 AM #4 GroundUrMast Moderator     Join Date: Oct 2010 Location: Greater Seattle Area Posts: 4,681 You can estimate the losses in your cabling and splitter(s). Worst case, RG-6 should have no more than 6 dB loss per 100' at the top end of the UHF band. 2-way splitters should not have more than 4 dB insertion loss, 4-way splitters should not have more than 8 dB insertions loss and 8-way splitters should have no more than 12 dB insertion loss. Many splitters will be labeled, indicating loss at each port. Simply add the length of cables starting at the antenna, all the way through to the tuner you are estimating distribution losses for, then (Total Cable Length / 100') * 6dB + Insertion Loss of the Splitter(s) in the Path = Estimated Antenna to Tuner Distribution System Loss Given that the base thermal noise power of a 6 MHz channel is -106 dBm (http://en.wikipedia.org/wiki/Johnson...3Nyquist_noise), we can estimate the equivalent power level of the noise in a tuner if we know it's noise figure... -106 dBm + Tuner NF. In your case, assuming an estimated NF of 6 dB, the equivalent baseline noise level in the tuner is -100 dBm. The antenna has a baseline noise level of -106 dBm for the same 6 MHz channel, so if a preamplifier is placed in the system, close to the antenna, both the signals and noise received along with the baseline noise of the antenna will be amplified. For a 20 dB gain amplifier with a 3 dB NF, we would expect signal + received noise to be increased by 20 dB, and we would also expect the baseline noise of the antenna to be elevated to a power level of -106 dBm + 20 dB = -86 dBm Presuming that the preamplifier has a lower noise figure than the tuner... As long as the loss in the Antenna to Tuner Distribution System does not attenuate the amplified baseline noise level of the antenna to a level equal to or less than the equivalent baseline noise level in the tuner, the preamplifier NF dominates the net system NF. In other words, if we had 14 dB of Antenna to Tuner Distribution System loss, the power level of the amplified baseline noise of the antenna would be attenuated to -86 dBm - 14 dB = -100 dBm which is equal to the tuner equivalent baseline noise level in the tuner of -100 dBm -- which would make the superior amplifier NF of no benefit. If we could reduce the distribution losses by 3 dB or more, the low NF of the preamp would be at the threshold of fully dominating the net NF of the system. __________________ If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.) (Please direct account activation inquiries to 'admin') Last edited by GroundUrMast; 3-Dec-2013 at 6:47 AM. Reason: sp.
 3-Dec-2013, 12:10 PM #5 ADTech Antennas Direct Tech Supp   Join Date: Jan 2010 Posts: 2,843 If you want to calculate the actual numbers, use a system "cascade noise" calculator. There are plenty of them on the internet. __________________ Antennas Direct Tech Support For support and recommendations regarding our products, please contact us directly at https://www.antennasdirect.com/customer-service.html Sorry, I'm not a mod and cannot assist with your site registration.
 4-Dec-2013, 3:00 AM #7 GroundUrMast Moderator     Join Date: Oct 2010 Location: Greater Seattle Area Posts: 4,681 If it's not obvious to everyone at this point... My math skills are limited to the level of a graduate of senior level public high school algebra circa the mid 70's. I admire @mmbridges apparent skills and have to presume that those skills have been honed by significantly more education than mine. Not to discourage or minimize the value of accurate evaluation... Such skill and ability is vital in applications such as high reliability terrestrial microwave or space communication links. However, given that OTA grade amplifiers are not provided with the necessary precision specifications, it seems to me that I'm stuck with what are relatively 'crude' estimations of expected performance. @mmbridges, welcome, and thank you for your contribution(s) to the forum.

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