Low Noise Amplifiers and RF Filters

5. Low Noise Amplifiers and RF Filters


SDRs are powerful & flexible platforms to learn, experiment and explore RF communication. They are highly capable in their own right but their performance can be enhanced by incorporating additional components such as Low Noise Amplifiers (LNA) and RF Filters. These components play a critical role in improving the overall performance of the radio, including signal quality, sensitivity, and noise reduction.

Low Noise Amplifiers

In an SDR setup, the LNA is particularly important when receiving weak signals, such as those from distant stations or low-power transmissions. A good LNA can significantly improve the sensitivity of the radio, allowing weak signals to be detected that would otherwise be lost in the noise. LNAs are usually placed as close as possible to the antenna to amplify weak signals from antennas, enabling them to be processed by downstream components in the receiver. When used properly they can help improve the signal-to-noise ratio (SNR) of the received signal. The SNR is the ratio of the received signal power to the overall noise figure, and it is a measure of the quality of the received signal. A high SNR means that the received signal is strong and clear, while a low SNR means that the received signal is weak and noisy.

It’s important to note that LNAs have their noise figure much like any other RF component. Adding an LNA to your RF chain will introduce additional noise to the overall noise figure of the setup but LNAs are designed to amplify the signal while adding as little noise as possible so they have a high gain and a lower noise figure. The gain is the ratio of the output power to the input power, while the noise figure is a measure of the amount of noise added by the amplifier. The overall noise figure of the setup is a critical parameter since it determines the minimum detectable signal (MDS) level. The MDS level is the minimum signal strength that the SDR can detect and demodulate, and it is determined by the noise figure of the system. Therefore, to achieve optimal system performance, it is essential to use low-noise amplifiers that have as lower noise figures as possible.

Best practices for using LNAs

  • Choose the right LNA: The choice of LNA depends on the frequency range of the signals you want to detect. Different LNAs are designed to operate at different frequencies, so it is important to choose the right LNA for your application. Wideband LNAs have a much wider frequency range, often a few hundred Mhz wide but with lower overall gain. They can be used as general-purpose LNAs and are especially helpful in mitigating transmission line loss when placed on the antenna side. On the other hand, some LNAs build for specific frequency ranges such as VHF HAM Band, UHF HAM Band, HF Band, and ISM Band and usually only have a few Mhz of narrower bandwidth. They can be used to improve reception, sensitivity, and selectivity of that specific frequency range and tend to be more expensive than wideband LNAs.
  • Use a high-quality LNA: The quality of the LNA can have a significant impact on the performance of the SDR. It is important to use a high-quality LNA with a low noise figure and high gain.
  • Use proper shielding: LNAs are sensitive to electromagnetic interference, so it is important to use proper shielding to reduce EMI.
  • Use proper grounding: Proper grounding is essential to reduce noise and interference in an SDR. Make sure that the LNA is properly grounded to reduce noise and interference.
  • Placement of the LNA: The best placement for a LNA is generally as close to the antenna as possible, to amplify the signal before any losses occur in the transmission line. This is called the “front-end” or “antenna side” placement.

LNAs can be implemented using various technologies, including bipolar junction transistors (BJTs), field-effect transistors (FETs), and operational amplifiers (op-amps). FETs are commonly used in LNAs because they have a low noise figure and a high input impedance, which makes them well-suited for amplifying weak signals. They are not even hard to make yourself and there are plenty of resources online that anyone can use to make their LNAs for their desired frequency. We will discuss how to make simple LNAs for a few of the important Bands in future articles and hopefully organize a few online workshops alongside them.

RF Filters

RF filters are electronic devices that selectively pass or reject signals in specific frequency ranges. They are used to isolate desired signals from unwanted interference or noise sources, resulting in improved signal quality and reliable communication. The filter’s design and performance characteristics determine its ability to separate signals of interest from unwanted noise or interference.

There are several types of RF filters, each with its own advantages and disadvantages.

  1. Low Pass Filters (LPF) 

A low-pass filter is a type of RF filter that allows frequencies below a certain cutoff frequency to pass through while attenuating higher frequencies. LPFs are commonly used in SDR applications to ensure that only the desired frequency band is processed, as higher frequency signals can introduce interference and degrade the quality of the received signal. However, these filters can also introduce some distortion to the signal by lowering the amplitude of high-frequency components. Therefore, careful selection of the cutoff frequency is essential to balance the trade-off between the desired bandwidth and the signal distortion.

High Pass Filters (HPF) On the other hand, a high-pass filter is a type of RF filter that passes frequencies above a certain cutoff frequency while attenuating lower frequencies. HPFs can be used to remove low-frequency noise or interfering signals from SDR applications. Additionally, HPFs can be employed for harmonic rejection in transmitters to reduce the impact of unwanted harmonics generated by the RF power amplifier.

Band Pass Filters (BPF) Sometimes, it is necessary to pass frequencies within a specific frequency range while attenuating frequencies outside this range. A band-pass filter is a type of RF filter that allows signals within a particular band of frequency to pass through while rejecting signals outside this range. BPFs are useful in SDR applications when it is necessary to isolate a specific frequency range.

Band Reject Filters (BRF) A band-pass filter is a type of RF filter that attenuates signals within a specific frequency range while allowing signals outside this range to pass through. BRF, also known as a notch filter or a reject filter, can be used in SDR applications to remove unwanted signals or harmonics. These filters can be used to eliminate noise sources that are close to the desired frequency range and are difficult to filter using other types of filters. A common example of an application for BRF is the removal of 50/60 Hz power line noise.

Multiband Filters SDRs can cover a wide range of frequency bands, and several types of RF filters can be used to pass different frequency ranges. A multiband filter combines two or more filters to cover multiple desired frequency ranges. These filters are helpful when processing signals across multiple frequency ranges, such as the filter arrays that can be found on most HF transceivers centered on each supported amateur radio band employed to mitigate unwanted interference from nearby high-power broadcast bands.

Tunable Filters Tunable filters are a type of RF filter that allows the user to adjust the frequency response dynamically. These filters are particularly useful in SDR applications where the desired frequency range can vary over time or when the user wants to explore different frequency bands. Additionally, tunable filters can be used to adapt to changing signal conditions or to improve reception in the presence of interference signals.

There are many online resources that let you design and create your own filters for any desired frequency. Such as www.rf-tools.com/lc-filter, which will not only calculate component values but also generate circuit schematics automatically. It even lets you output standard component values rather than exact values to make it easier to find necessary parts.

In conclusion, RF filters are essential components of RF systems that support the filtering of desired frequency bands while rejecting unwanted signals. The choice of filter type depends on your requirements, signal characteristics, and the application.

Written by Dilusha Samarasekara – Associate Member, RSSL

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