JED - April 2015 - (Page 41)

TECHNOLOGY SURVEY A SAMPLING OF LOW NOISE AMPLIFIERS By Ollie Holt operating bandwidth. Some amplifiers may degrade in gain as the frequency increases, or may have gain "sweet spots" over the operating range where the gain increases or decreases from the specified gain. Adjusting the receiver gain as a function of the frequency to which the receiver is tuned can alleviate this issue, or the EW system designer can just select an LNA with as little variation in gain over the operating frequency range as possible. The next parameter is the noise figure. As discussed above, NF defines the amount of noise that gets added into the output of the amplifier. As the first amplifier in the RF chain, you want the lowest NF and the highest gain you can provide without distortion. Additional amplifier stages or active components cannot lower the NF; they can only add to it. In the next column, the input and output VSWR parameters define how well you need to match the amplifier to the proceeding and preceding circuits and then power, size and weight information is provided for the component. continued on page 44 The Journal of Electronic Defense | April 2015 3RD Order Intercept Point (IP3) 41 2ND Order Intercept Point (IP2) P3 1 dB Compression Point P1 Output in dB T his JED technology survey investigates Low Noise Amplifier (LNA) components. LNAs are typically the second or third element in an EW system's RF receiver chain. The first element is the receiving antenna, the second is a limiter (if needed), and the third is the LNA. Limiters are used to protect the RF front-end from high power signals that may cause damage to the front-end components. The first active component in the receiver RF chain is usually the LNA. The performance of the LNA sets the sensitivity of the receiver. If the LNA has poor performance, then the receiver will have poor performance. The components following the LNA can only make the performance worse; they cannot make up for a poorly-performing LNA. The LNA gets its name because of its desired low noise performance. It is an RF amplifier used to amplify the energy captured by an antenna. By using a low noise amplifier, the effect of noise generated in the following amplifier stages is reduced. By choosing an LNA with very low noise performance (i.e., noise created within the amplifier) and high gain, you set the performance of your receiver. Therefore the goal of the LNA is to amplify the detected signals while inserting as little noise and distortion as possible. A good LNA has a very low Noise Figure (NF), high gain, and high intermodulation and compression points. In this month's survey table, the first two columns address the LNA's operating frequency and bandwidth. The bandwidth defines the lowest to highest frequency over which the LNA was designed to perform. The next item is gain. The gain defines how much the input signal is amplified. The next parameter is gain flatness. This tells how well the amplifier does at keeping the same gain over the B A Output signal (Slope =1) P1 C 3RD Order Output (Slope =3) 2P3 2ND Order Output (Slope =2) D Input in dB Figure 1: Intercept Points

Table of Contents for the Digital Edition of JED - April 2015

The View From Here
Conferences Calendar
Courses Calendar
From the President
The Monitor
World Report
Honoring the Prowler
Technology Survey: Low Noise Amplifiers
Preview: EW Europe 2015
EW 101
Index of Advertisers
JED Quick Look

JED - April 2015