RECEIVER SIGNAL LEVELS
Signal power is commonly measured in dBm - decibels above or below 1 milliwatt (mW). Television receivers will process signals from about 316 µW (microwatts) = -5 dBm to about 0.0003 µW = -65 dBm. A -65 dBm signal should come-in good for a short cable run to a single television, but may be too weak for long cable runs or signal splitters unless an antenna preamp is used.
A television's required minimum signal depends on the receiver sensitivity and local conditions (electronic noise pollution). Minimum signal can vary from as high as -55 dBm for a TV with poor sensitivity in a noisy environment, to as low as -75 dBm for a TV with good sensitivity in a low or no noise environment. Most televisions and conditions will require a signal level of at least -65 dBm.
ESTIMATE a TV's MINIMUM SIGNAL
Sensitivity depends on receiver Noise Figure (NF), a measure of the receiver's noise, and the minimum signal-to-noise (S/N) ratio required. Television receivers typically have a noise figure of 10 dB, but it can be as low as 6 or 8 dB, to as high as 20 dB. A minimum S/N of 30 dB is required in most cases due to interference from other signals, multi-path, and electronic noise.
Another factor is background noise, also called thermal or kTB noise, and is based on receiver noise bandwidth (6 MHz for an RF channel), and temperature. Temperature has a minor effect, and it is standard practice to use 300°K (80°F / 27°C), or 290°K (62°F / 17°C) for calculations. Noise is random with an average deviation of about ± 5 to ± 10 dB from the average level. Peak noise will be greater.
Pmin = Minimum signal level required. |
k = Boltzmann's Constant = 1.380 622 x 10-23 Watt sec / °K.
T = Temperature in Degrees Kelvin (°K).
B = Receiver Noise Bandwidth in Hertz (Hz).
NF - Noise Figure (ratio).
S/N - Minimum Signal-to-Noise (ratio).
°C = Degrees Celsius
°F = Degrees Fahrerhigh
°K = Degrees Kelvin
In the laboratory a television could receive a signal as low as -83 dBm, using a receiver with a noise figure (NF) of 8 dB, and a signal-to-noise (S/N) ratio of 15 dB. Noise would have to be minimized by protecting the TV receiver from outside interference by cabling a single signal (1 RF channel) from a lab signal generator directly to the TV receiver.
Over-the-Air Digital TV (OTA DTv)
TV Signal Strength Needed
© Copyright 2017