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Old 15-Aug-2020, 8:36 AM   #12
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SNR is the ratio of signal power to noise power. Spectrum analyzers are deceptive in that the apparent noise floor changes with the resolution bandwidth setting. The narrower the resolution bandwidth, the lower the apparent noise floor. One can't just look at the spectrum plot and measure the signal power to noise floor. One must integrate the signal power over the bandwidth, and noise power over bandwidth, and take the ratio of the two results.

Often when people discuss preamps, they focus on a single channel of reception. This is dangerous since preamps are wide band amplifiers and there could be high power signals within the bandwidth of the preamp. One must also pay attention to what happens in the time domain. All amplifiers have a limited output voltage range. If the instantaneous input voltage times the gain of the amplifier causes the result to be outside that range, the output of the amplifier will clip. During the time that the amplifier is clipping, nothing gets through. This means that if there are any high powered signals, it can be a bad idea to try and use a preamp. In my case, I found that local FM stations were hugely more powerful than the TV stations I was trying to receive. I found that I needed an FM trap. There are two types of FM traps, and marketing seldom telly you the kind they are selling. One type kills all FM, but also kills channel 6. The other allows channel 6, but does not kill the low end of the FM band.

For antennas, I think that the shape of the reception pattern would be more important than the absolute gain. If you know you are having trouble with interference from an unwanted signal, you could pick an antenna that has a null an that frequency at an angle from your station of interest that would let you aim the null at the problem interference source. Unfortunately, antenna manufacturers no longer publish the plots of gain vs angle.

For digital television I wish I could find a low cost tool that would display the constellation plot of the signal. This can give good clues as to why you are having trouble with a signal. The closest I have seen is the HDHomeRun Tech, but that is not a full constellation plot it is a histogram of the I signal rather than a plot of I vs Q.
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