Why a good LNA is key to a viable antenna front-end
The need to amplify the received signal presents a dilemma. On one side, the unamplified signal is too weak to be useful; on the other hand, amplification increases the signal magnitude, but also degrades SNR and thus potential link performance. This dilemma is resolved to a large extent by choosing an amplifier that contributes as little noise as possible. The front-end low-noise amplifier (LNA) has two parameters of primary interest: how much noise it adds to the signal, and how much gain it can provide. LNAs fabricated with highly specialized analog processors do one thing well
Figure 2: The Skyworks Solutions SKY67180-306LF is a two-stage, 31 dB gain LNA for 1.5 to 3.8 GHz with 0.8 dB NF; the first stage is optimized for low noise figure while the second stage provides additional gain. Image source: Skyworks Solutions
TV, and about the same for cellular links. There are extreme examples, of course: signals are still being received from the Voyager 1 and Voyager 2 spacecraft, both launched in 1977 and now over 11 billion miles from Earth. These signals arrive here from their 23 watt transmitters with signal power of less than an attowatt (a billionth of a billionth of a watt) and an SNR of only a few dB. To compensate for this to some extent, their data rate is now throttled down to around 100 bits/second (bits/s), down from the several kilobits/second (Kbits/s) rate when much closer
with much higher received signal strength.
LNAs to the rescue There’s an engineering cliché that originated in the early days of “wireless” and is still true: if it wasn’t for noise, the challenges of most system designs would be much, much easier. This is true of a receiver’s antenna link for a simple reason. The amplifier, which is needed to “gain-up” the weak received signal, contributes its own noise to that signal, as does any interconnection cabling between antenna and receiver front-end.
Although antenna transmitter and receiver functions adhere to the principle of reciprocity, their actual challenges diverge
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