The Basics Of LNA Use In GPS Reception

This application note is about LNA's used in GPS reception. The purpose of this application note is to give the reader an understanding of the basic properties of LNA's and help in defining the right amplifier for his or her application.

Basic LNA Terminology

Noise Figure (NF)
Noise Figure is a ratio that indicates how much noise power the LNA will contribute to the total receiver noise. The Minimum Discernable Signal (MDS) is the weakest signal a receiver can decode.
The more noise the LNA contributes, the higher the noise floor and the less sensitive the receiver is. At the systems level, a poor LNA degrades the MDS. While LNA NF is not the only factor that drives the MDS, it is an important consideration since the noise figure of the first stage in a receiver chain is the single largest contributor to the system noise figure.
A typical state-of-the-art, single stage commercial grade LNA at L-Band (GPS) has a noise figure between 0.5 and 1 dB. A multi-stage GPS LNA with filtering has a noise figure between 1.0 and 2.5 dB.

Gain
Gain is the ratio of input to output power. For a single stage LNA, this is typically 15 dB. Typical GPS LNA's use two or three gain blocks and yield 25 dB to 50 dB of gain depending on the user's requirement. Unlike NF, a low or high gain does not indicate a good or bad LNA.
It is important to specify the amount of gain that is required rather than to go for the highest available gain: more gain will produce more intermodulation products in LNA and receiver. Not enough gain can cause the GPS signal to be below the MDS level of the GPS receiver.

Second or Third Order Intercept Point and 1 dB Compression Point
These terms define the LNA's behavior when either multiple (possibly strong) signals or at least one strong signal is presented at its input terminal. Multiple signals in an LNA can mix and generate a set of new signals. Some of these can fall inside the GPS passband and cause interference. A single strong signal that is fed into the LNA could drive the amplifier into compression and may suppress the GPS signal(s) at the output of the LNA. GPS reception may be lost. The theory of all of this is beyond the scope of this article, but a well-designed LNA should be able to keep going with as much as -15 dBm of power at its input terminal.