Thread: Noise Threshold
View Single Post
Old 27-Jan-2010, 11:52 PM   #2
Join Date: Dec 2009
Posts: 632
Hello and welcome!

The Noise Margin, is a measure of how strong a signal is above it's operating threshold. It is a measure of how much signal loss you can tolerate before the signal reaches an unusable state.

It's similar to SNR, but it is different in the sense that Noise Margin is relative to the operating requirements of the particular signal in question. For example, ATSC A/53 (our digital TV standard) has a theoretical breaking point around 15.2 dB SNR. That means that the signal needs to be at least 15.2 dB above a "white" noise floor in order to be usable. This represents the 0 dB Noise Margin point.

Other technologies operate at different SNR levels. For example, NTSC (our analog TV standard) has a theoretical breaking point around 27 dB SNR ("breaking point" for analog is a subjective and debatable topic, but that's a separate issue). CDMA phone technology can work at negative SNR numbers, and so can GPS. The SNR threshold for each technology can vary. The Noise Margin is always defined relative to that breaking point. Positive Noise Margin signals are usable, negative Noise Margin signals are not.

What would that noise threshold mean, i.e. if the noise threshold was 50 DB, does that mean I could tolerate an additional 50 DB of noise before my signal became marginal?
Yes. A Noise Margin of 50 dB means that the signal can be degraded by another 50 dB before it stops working.

The tvfool reports give you the starting Noise Margin in-the-air at your location. From there, you can add the gain of your antenna (in dBd, not dBi), and subtract all the other losses in your setup (cable losses, receiver noise figures, splitter losses, impedance mismatch losses, etc.). If your antenna is indoors, then you'd subtract the building losses first, then add your antenna gain, and subtract any losses that occur after that.

If you end up with a positive Noise Margin at the end of the chain, then you should have a usable TV signal. In most cases, you actually want to end up with some extra Noise Margin left over (say around 10 dB or more) to help the system cope with random variables (e.g., stormy weather, multipath, seasonal changes, interference, etc.) that might cause your signals to fluctuate.


Scenario 1 (indoor set-top antenna)

Starting Noise Margin = 50 dB
Building penetration loss = -20 dB
Antenna gain = +1 dB (indoor antennas are not very high gain)
Cable loss = -3 dB (short section of RG59)
Receiver Noise Figure = -6 dB (most consumer-grade receivers have Noise Figures between 6 and 9 dB)
Net residual Noise Margin = 50 - 20 + 1 - 3 - 6 = 22 dB

Scenario 2 (simple rooftop antenna)

Starting Noise Margin = 15 dB
Antenna gain = +8 dB
Cable loss = -5 dB (long section of RG6)
Receiver Noise Figure = -6 dB
Net residual Noise Margin = 15 + 8 - 5 - 6 = 12 dB

Scenario 3 (rooftop antenna followed by pre-amp)

Starting Noise Margin = -2 dB (it is possible to start with a negative NM)
Antenna gain = +14 dB (using a high gain antenna to pick up weak stations)
Cable loss = -1 dB (a short section of RG6 between antenna and pre-amp)
Pre-amp Noise Figure = -3 dB
Cable loss = --- (does not matter because of pre-amp)
Splitter loss = --- (does not matter because of pre-amp)
Cable loss (after splitter) = --- (does not matter because of pre-amp)
Receiver Noise Figure = --- (does not matter because of pre-amp)
Net residual Noise Margin = -2 + 14 - 1 - 3 = 8 dB

(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.)
mtownsend is offline   Reply With Quote