What is a Low Noise Block?

Low noise block (LNB) or low noise block downconverter is a receiving device usually mounted on a satellite dish for satellite signal reception (for example, satellite TV reception). It is used in the RF front end of the satellite receiver. The dish receives the microwave signal from the satellite, amplifies it, and downconverts the received high-frequency RF signal frequency into a low-frequency IF signal. The dish is connected to the LNB via a feedhorn (a small horn antenna) and a short waveguide (a hollow metallic tube for conducting RF signal). LNB which is available with an integrated feedhorn is called LNBF.  

The LNB device typically consists of filters, a low noise amplifier (LNA), a mixer, a phase-locked loop local oscillator, an intermediate frequency (IF) power amplifier, and DC-DC convert circuit.

In the down-conversion operation, the satellite signal received by the dish is sent to the LNB via feedhorn & waveguide. The input high-frequency RF signal first goes through a bandpass filter that only allows the intended band of microwave frequencies to pass through it. Since the satellite signals travel from a long distance, they are weak in signal power level. Hence a low noise amplifier (LNA) is used to amplify the incoming RF signal with low noise. Then, the output of the LNA is sent to the mixer.

The in-built phase locked loop local oscillator (LO) works as a stable frequency source and produces the LO signal. The phase-locked oscillator circuit typically uses a phase detector, a feedback circuit, and a stable reference frequency source (typically 10 MHz) to maintain the required IF frequency.