Down Converter
Understanding Downconverters
Downconversion is the essential first step in most receivers. The incoming RF signal is too high in frequency for practical filtering, amplification, and digitization. By mixing it with a local oscillator, the signal is translated to a lower IF where processing is simpler and more effective.
Downconverter Components
- LNA: Amplifies the weak received signal while adding minimal noise.
- Image filter: Rejects the image frequency before mixing.
- Mixer: Multiplies RF with LO to produce IF output.
- LO source: Provides the reference frequency for mixing.
- IF amplifier: Boosts the IF signal for further processing.
Key Specifications
- Conversion gain/loss: Net signal gain from RF input to IF output. Active downconverters have gain; passive mixers have conversion loss.
- Noise figure: Total noise added by the downconverter chain.
- Spurious: Unwanted mixer products that appear at the IF output.
f_IF = |f_RF - f_LO|
Conversion loss (passive mixer): 6-8 dB typical
Conversion gain (active downconverter): +10 to +30 dB
Image frequency: f_image = 2 x f_LO - f_RF
(for low-side LO injection)
Frequently Asked Questions
What is a downconverter?
A downconverter translates a high-frequency RF signal to a lower IF using a mixer and LO. This makes the signal easier to filter, amplify, and digitize. Every superheterodyne receiver uses a downconverter as its first frequency conversion stage.
What is conversion loss?
Conversion loss is the signal power reduction from RF input to IF output in a passive mixer. Typical values are 6-8 dB. Active downconverters with built-in amplification have conversion gain instead of loss.
What is an LNB?
An LNB (Low-Noise Block downconverter) combines an LNA and downconverter in a single unit mounted at the antenna feed. It amplifies the weak satellite signal and converts it from Ku/Ka-band to L-band IF for transmission over coaxial cable to the indoor receiver.