What is a Low Noise Amplifier?

A low noise amplifier (LNA) is an electronic device that amplifies weak signals at its input without adding significant noise. These amplifiers are typically used in receivers. For LNAs the most important specifications are:

  • Gain
  • Noise Figure
  • Linearity

Gain

The LNA’s gain refers to its ability to increase the value of the input signal. It is most often specified in dB. Typical gain values for an LNA are between 10 and 30 dB.

Noise Figure

Noise figure provides a measure of the noise contribution due to the LNA itself. In the picture below, the LNA amplifies both the signal and noise present at its input equally. In addition, the output includes noise due to the LNA. This in turn reduces the signal-to-noise ratio. A good LNA contributes very little noise to the overall picture.

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Linearity

The linearity of an LNA is a measure of its ability to amplify the signal without distortion. When an LNA is operating linearly, the output power in dB is the sum of the input signal and the gain.  However, as the input signal level increases beyond a certain point, the output starts to level off and the LNA is no longer linear.

In the picture below, the LNA is operating linearly when the signal present at its input has an amplitude of -60 dBm. However, as the input signal is increased to 0 dBm, the device is no longer linear, the output signal is distorted, the gain is no longer 20 dB and the output signal amplitude is only +5 dBm.

picture2

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Airband Filter

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A new Airband bandpass filter centered at 128 MHz. The filter has a bandwidth of 20 MHz.  The filter is designed to reject strong signals in the adjacent FM band. Here are the specifications:

Frequency (MHz) Typical Attenuation (dB)
78 43
88 35
98 25
108 10
118 4
120 4
125 4
128 4
138 4
150 11
200 39

 

The filter is available on Tindie 

FM Notch or FM Band Stop Filter

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An FM notch filter is used to reject strong FM signals in receiver applications. A good FM Notch filter will reject FM signals and have low insertion loss for signals outside the band.

This is a 9th order Filter with excellent rejection in the FM band. The filter works up to 3 GHz with minimal out-of-band insertion loss. This product is ideally suited for Ham radio applications and Software-defined radio receivers where typically there is minimal internal filtering. In the absence of a notch filter, FM signals can saturate the receiver. The specifications of this filter are comparable to ones on the market that cost at least twice as much.

SPECIFICATIONS

Frequency (MHz) Typical Attenuation (dB)
55 1
75 4
80 8
88 60
98 85
108 25
150 0.8
200 0.5
1000 0.3
2000 0.9
3000 1.8

In Japan the FM band extends from 76 to 95 MHz while in North America it spans 88 to 108 MHz.

NOTE: Connectors are SMA-F

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Considerations when using high-gain LNAs

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Some of our LNAs provide in excess of 40 dB of gain and operate over a wide frequency range. The primary application of these products is to amplify weak signals by increasing the signal-to-noise ratio at the receiver.

One important consideration when using these LNAs is that they amplify everything across their frequency range of operation. If your receiver does not have any front-end filtering, you might find yourself having to deal with a large assortment of strong unwanted signals. Most software-defined radios are wideband and therefore susceptible to these strong signals. A strong signal present at the input of a receiver can cause it to saturate and produce garbage at the output.

When using these high gain LNAs we recommend using a bandpass filter for your frequency range of interest. We have a number of such filters for GPS, 426, 433 MHz, 915 MHz and 2450 MHz bands: http://stores.ebay.com/gpiolabs/Bandpass-Filters-/_i.html?_fsub=28020158018

and we are looking to add new filters. If you would like to see us develop a filter for your application, get in touch.

Precautions when using LNAs

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Our LNA products use a micro-USB connector for powering the device. The choice of this connector was based on the popularity of USB and the reasoning that it’s easy to find cell phone chargers. The only negative with this connector is that it is easy to rip them off the board if handled improperly. Once the connector detaches from the board, in most situations, the hardware cannot be repaired.

There are a few precautions that users can take to prevent damage to the board.

  1. When setting up to use the LNA, we recommend connecting the SMA connectors to cables prior to connecting the USB power cable. This is to prevent potentially torquing the board.
  2. Similarly when you’re done using the LNA and if it’s not part of a permanent fixture, detach the USB power cable first and then take the SMA connectors off.
  3. Finally, avoid dangling the LNA with the USB cable while it’s connected.

All our RF products can be found here: http://www.ebay.com/sch/iseeabluewhale/m.html?_nkw=&_armrs=1&_ipg=&_from=