Why is NF critical for LNBs?

The LNB is the first active component in the satellite receive chain, so its noise figure directly sets the system noise temperature per the Friis cascade equation. For a satellite system with dish antenna noise temperature of 30 K, an LNB with 0.3 dB NF adds only 21 K of noise temperature. An LNB with 1.0 dB NF adds 75 K. The total system noise temperature (T_sys = T_antenna + T_LNB + T_rest/G_LNB) determines the G/T figure of merit that governs receive sensitivity. Since satellite signals arrive at extremely low power levels (minus 130 to minus 100 dBm at the dish feed), even 0.5 dB improvement in LNB NF can make the difference between reliable reception and signal breakup.

How does the universal LNB work?

A universal Ku-band LNB covers the full 10.7 to 12.75 GHz satellite band using two local oscillator frequencies. The low band (10.7 to 11.7 GHz) uses LO = 9.75 GHz, producing IF = 950 to 1950 MHz. The high band (11.7 to 12.75 GHz) uses LO = 10.6 GHz, producing IF = 1100 to 2150 MHz. The receiver selects the LO frequency by sending a 22 kHz tone on the coax: tone present = high band, tone absent = low band. Polarization (vertical or horizontal) is selected by DC voltage: 13 V = vertical, 18 V = horizontal. A quad LNB has four independent outputs supporting four receivers simultaneously. A wideband LNB uses a single LO at 10.41 GHz to cover the entire band in one IF range, but requires a wideband receiver.

What are the key LNB specifications?

Noise figure: 0.3 to 1.0 dB (equivalent noise temperature 20 to 75 K). Lower is better for weak signal reception. Gain: 50 to 65 dB to overcome coax cable loss (20 to 40 dB at L-band for typical 30 m runs). Input frequency range: 10.7 to 12.75 GHz (Ku-band universal), 3.4 to 4.2 GHz (C-band). Output IF: 950 to 2150 MHz (L-band). LO phase noise: affects carrier-to-noise ratio, typically minus 80 to minus 90 dBc/Hz at 10 kHz offset. Image rejection: greater than 30 dB to prevent interference from the image band. Cross-polarization isolation: greater than 25 dB. DC power consumption: 100 to 300 mA at 13/18 V supplied via coax from the receiver.

Satellite Component

LNB

/ell-en-bee/ — Low-Noise Block Downconverter

Mounted at dish focal point. Amplifies weak satellite signals (−130 to −100 dBm) with NF 0.3-1.0 dB. Downconverts Ku-band (10.7-12.75 GHz) to L-band IF (950-2150 MHz) via coax to receiver. Universal LNB: dual LO (9.75/10.6 GHz), 22 kHz tone selects band, 13/18 V selects polarization. Gain: 50-65 dB. G/T figure of merit determines system sensitivity.

NF: 0.3-1.0 dB

Gain: 50-65 dB

IF: 950-2150 MHz

Understanding LNBs

The LNB is arguably the most critical component in a satellite receive system. Positioned at the dish's focal point, it is the first electronic device to process the incredibly weak signals that have traveled 36,000 km from a geostationary satellite. The signal power at the dish feed is typically −130 to −100 dBm (femtowatts to picowatts), making the LNB's noise figure the dominant factor in system sensitivity.

The LNB performs two essential functions: low-noise amplification (50-65 dB gain) to boost the signal above the noise floor of subsequent components, and frequency downconversion from Ku-band (10.7-12.75 GHz) to L-band (950-2150 MHz) for transmission over standard coaxial cable. Direct transmission of Ku-band signals through coax is impractical due to extreme cable loss at those frequencies.

LNB Equations

System noise temperature:
T_sys = T_ant + T_LNB + T_rest/G_LNB
T_LNB = 290×(10^NF/10−1) K
NF=0.3 dB: T=21 K, NF=1.0 dB: T=75 K

IF frequency:
f_IF = f_RF − f_LO
Low band: f_LO=9.75 GHz
10.7 GHz → IF = 950 MHz
High band: f_LO=10.6 GHz
12.75 GHz → IF = 2150 MHz

G/T figure of merit:
G/T = G_ant(dBi) − 10log(T_sys)
60 cm dish (33 dBi), T=50 K:
G/T = 33−17 = 16 dB/K

LNB Type Comparison

Type	Band	LO (GHz)	NF	Application
Universal Ku	10.7-12.75	9.75/10.6	0.3-0.7 dB	DTH satellite TV
C-band	3.4-4.2	5.15	15-25 K	VSAT, broadcast
Ka-band	18.3-20.2	Various	1.0-2.0 dB	HTS broadband
Wideband	10.7-12.75	10.41	0.5-1.0 dB	SCR/Unicable
Quad	10.7-12.75	9.75/10.6	0.3-0.7 dB	Multi-receiver

Common Questions

Frequently Asked Questions

Why is NF critical?

First active component: NF sets system noise temp per Friis. T_LNB = 290×(10^(NF/10)−1). 0.3 dB: 21 K. 1.0 dB: 75 K. T_sys = T_ant + T_LNB + T_rest/G. Signals at −130 to −100 dBm. 0.5 dB NF improvement = difference between reliable reception and breakup.

Universal LNB?

Dual LO: 9.75 GHz (low 10.7-11.7 GHz, IF 950-1950) and 10.6 GHz (high 11.7-12.75 GHz, IF 1100-2150). 22 kHz tone selects band. 13 V = vertical, 18 V = horizontal polarization. Quad: 4 independent outputs. Wideband: single 10.41 GHz LO, needs wideband receiver (Unicable/SCR).

Key specs?

NF: 0.3-1.0 dB. Gain: 50-65 dB (overcomes 20-40 dB coax loss). Input: 10.7-12.75 GHz (Ku). Output: 950-2150 MHz (L-band). LO phase noise: −80 to −90 dBc/Hz @ 10 kHz. Image rejection: >30 dB. Cross-pol isolation: >25 dB. DC: 100-300 mA via coax (13/18 V).

Related Terms

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Satellite Systems

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