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Old 19-Apr-2017, 6:17 PM   #15
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
FYI, the TVFOOL NM calculation assumes that a preamp will be used. If you look at a report you will see that the difference between the NM and the signal power is a constant of about 91 dB. This allows for the minimum required SNR of 15 dB above the Thermal Noise Floor which puts the a 0 dB NM signal at -91 dBm.

If you do not use a preamp, then you must allow for the minimum required SNR of 15 dB and the NF (6 dB average) of the tuner, placing the minimum required signal at -85 dBm (+6 dB NM) where most tuners drop out. With a preamp, the tuner NF is buried in the amplified Thermal Noise Floor, because the amplification is prior to the tuner input, and the tuner noise is internal.

These two diagrams show why a signal of at least +6 dB NM is needed at the tuner input to allow for the tuner NF when a preamp
is not used.

Version 1 is usually how it is shown, but Version 2 is more accurate because it shows the tuner noise added to the Thermal Noise
Floor before adding the minimum required SNR of 15 dB.




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Last edited by rabbit73; 11-Sep-2017 at 3:03 AM.
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