Category: Low Noise Amplifier

Jarek presented his work at the Canada Wide Science Fair on the use of Software-Defined Radio to detect 1420 MHz radio waves from outer space.

In his words “A big problem has been the high cost of radio astronomy equipment with big antennas and expensive hardware. Now with computers and software defined radio there is a new age of amateur radio astronomy growing. My project will aim to demonstrate this low cost radio astronomy”.

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.

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.

Jon has some spectrum scanning projects in which he uses our LNA and FM notch filters. It’s interesting how he puts them into die-cast boxes. This is something we get asked about all the time. You can read about the projects here and here. Jon’s wireless projects are definitely worth reading about.

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.

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.

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.

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.

Finally, avoid dangling the LNA with the USB cable while it’s connected.