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Old 9-Mar-2014, 12:21 AM   #2
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 153
Quote:
Originally Posted by stvcmty View Post
Without preamplifier: NM at TV = NM in the air + antenna gain – cable loss – splitter loss

With preamplifier: NM at the TV = NM in the air + antenna gain – preamplifier noise
(Assuming the selected preamp has enough gain to overcome cable loss and splitter loss in the system).
As applies to the input of TV tuner, the above is pretty much correct.

There is a bit more.

Primarily, the signal-to-noise ratio (S/N) at the tuner demodulator is what matters. Signal-to-noise ratio is related to what is referred to as ‘noise margin’.

Often a large contributor to S/N is the tuner noise figure. Typical tuners have noise figures that range from ~6 to 8 dB.

So, even with a very short transmission line, adding a preamp can improve overall S/N ratio.

This improvement comes about via the gain of the preamp. The preamp raises the signal (actually signal plus preamp & other noise) to high enough level that the tuner noise becomes a smaller fractional contributor to total noise.


Quote:
Overload is another problem. If a preamplifier is overloaded by signals in the air, it will make noise at harmonics of the overloading signals that could drown out desired channels.
Yes.

Another topic that also could be quite technical.

If the preamp overloads, it generates harmonics and additional intermodulation (IM) distortion.

Harmonics occur at multiples of fundamental (2x, 3x, etc)

IM involves other products of frequencies (frequencies & frequency multiples added and subtracted.

Depending on frequencies involved, the harmonics and IM may, or may not, interfere with desired signals.

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With typical preamps and typical tuners, the tuner is more likely to be the first to overload.

Similar to preamps, harmonics and IM are generated within the tuner.

Low gain preamps help to avoid overload, but in some (maybe few) cases low gain may not be optimum for S/N improvement.

To optimize, one must know the signals and noise levels throughout and select gain accordingly.
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