05 March 2012

LNA Functionality

The LNA (Low Noise Amplifier), marked as "Low Noise Preamp" in the picture below, is from an old Avantek DR2 radio. I don't remember the specifics (and Avantek was a little before my time) but it was probably a 4-16 T1 radio from the 1980's. It operated in the old common carrier 2Ghz band(something like 1.9 to 2.1 Ghz but don't quote me) which was auctioned off to create  "PCS" services, which I guess are no longer differentiated from the old 800Mhz cell spectrum. Now 800Mhz and 1.9Ghz is all just "cell phone" service, I never hear "PCS" referred to anymore(but I'm not cell tech).

Without looking it up in the DR2 manual(if I could find one somewhere), it seems to be happy with about 12-15VDC for power. Quick checks detailed later in this post show it provides about 20db of gain, give or take 5 or 6dB. Those numbers are in line with fuzzy memories of other brands of LNA's on some microwave radios I've worked with.

Keep in mind we're talking about 20dB gain on a small signal... say a -95dBm signal amplified to -75dBm. It's not going to add 20dB to a +24dBm output of a wifi card to give you +44dBm. The thing has a point on it's input beyond which distortion ramps up. Keep going and you're just going to burn the LNA itself out.

I often see or hear wifi stories about "putting an amp on it" for better performance in some manner. I think alot of times that is a Tim the Toolman Taylor "MORE POWER!! ARRRRRGGH!!!" type of statement. In other words, "I cranked it up to 11 !!!!".

Granted there are occasions to use the kind of power amp being referred to in those stories, but in most setups it should only be because their radio has a weak output or they have large cable losses to the antenna. For normal setups(my 14 channel RX only project here is non-normal) wifi amps need to be bi-directional because the radios have a common RX+TX input.

That is not what a unidirectional LNA is about....

Read on for some "in-action" pictures of the LNA and my long rambling attempt to convey my understanding of LNAs. You've been warned.

LNA Functionality - The Long Version

LNA stands for "Low Noise Amplifier". The idea is to amplify(with very little noise added) a weak signal right at the antenna (or ACU ) output, enough to overcome some cabling, filtering, etc losses on the way to your RX section. You're not looking to increase your antenna's reception, only to transport what the antenna is already receiving through more losses on the way into your RX section, and/or make it more easily usable by your RX section.

A good example is what's usually called a "tower- top preamp" for a receive-only antenna for 800Mhz 2-Way radio(think police/fire hand helds). It amplifies the 800Mhz band right out of the antenna before it runs through a few hundred feet of cable down the tower and into a set of cavity filters, splitters, and associated cabling losses before hitting various receivers. Even if the preamp is not "tower-top" they should still have one inside the building before running through the cavity filters splitters etc to the various receivers.

LNAs are only applicable in situations were the TX and RX are coming from different antennas, and/or operating on different frequencies and can be filtered based on freq into separate connections.

The LNA is unidirectional and is only useful on the RX path, the rest of this post is going to be written assuming TX is out of the picture (which is fine for my RX only project).

Here's what I think is a useful analogy for the LNA:
Your eyes are the RX section and a simple painting program on the computer is the LNA. You're zoomed in on a picture at 100%. As in, the paint program (LNA) is not doing any zoom(amplification).

You're trying to click on and select single pixels in the drawing. It's theoretically possible since a single pixel on screen is a single pixel movement of the cursor on screen. It requires really delicate mouse movements and is easy to mess up. It also requires that your eyes can discern individual pixels onscreen.

If you zoom(amplify by 8x, or 9db) in to 800% now you can physically whip the mouse around alot more quickly and easily while picking a specific pixel, which is now represented onscreen as a 64 pixel block.

To take the analogy further, imagine your eyes cannot see individual pixels on screen... a couple examples of why:
  • Your RX section is weak (poor eyesight).
  • Your RX section is fine but there are alot of losses by filtering, branching or etc.( 20ft from the screen, viewing off axis and lcd is dark, privacy filter installed on screen)
 Regardless of the reason, you need zoom(amplification). Say you need 1 pixel in the actual image blown up to that 64 pixel block on screen so you can see it.... that's 8x amplification or zoom, which is the same as zooming 2x three times.

2 * 2 * 2 = 8
In decibel units, ~3dB is doubling( and you just add decibels) so:

3db + 3db + 3db = 9db

So in this analogy you need an "LNA with 9dB gain".
Think of the LNA as a magnifying glass that "zooms" into the signal you already have.... well not exactly, but it's not picking up signals that weren't there at the antenna output to begin with, it's just making the signals that are there "fill up more of your screen" so to speak.

Ok enough with the analogy, you were warned but it still got out of control!

LNA Functionality - Hands On

Below is a picture of a test setup for comparing "with LNA" to "without LNA". It's shown as "without LNA", having a small SMA barrel between the two cables (skinny gray and black FSJ). The gray cable connects to a directional panel antenna on the right side, only visible as the aluminum fin looking thing. The antenna is an Andrew DL-2402 with 13dBi gain that was clamped to the table throughout the test.
Test setup for "with LNA" vs "without LNA"

I ran the spectrum analyzer for a couple minutes in Max Hold A to allow  trace A to build up across the 2.4 - 2.5 band(roughly, maybe 2.3ish to 2.6ish ), then I switch trace A to view mode. Then i put the LNA inline between the gray and black cables and allowed trace B to collect a couple minutes while in Max Hold, finally switching it to view mode also. Here's a image of the screen:

Two 10db/div MaxHold traces across ~2.4Ghz band
     "with LNA" = Upper Trace
"without LNA" = Lower Trace
The top trace is the one obtained with the LNA inline. The big humps left and right are channel 1 and 11 (which are both in heavy use at my house). The other spikes and raised areas on the top trace are not noise, those are other signals that are now amplified above the noise floor of my cheap spectrum analyzer (as it was adjusted for this test). It is possible that some spikes are transmitters that weren't transmitting while the previous trace was collecting, but not all of them. Channel 1 and 11 are comparable for sure because that's my equipment in-house over a couple minutes so they all would  have TX'd during both captures.

The spectrum analyzer could have been adjusted to improve it's noise floor, by spanning down to less Mhz across the screen and decreasing both the resolution and video bandwidth. That wasn't necessary for this test and would just cause slower sweep times. The best method would be to get a new high dollar spectrum analyzer :)

What you really have above is a perfect example of what the LNA is for.  My spectrum analyzer (with resolution+video bandwidth and span as they are set) is unable to view the smallest signals coming in from the antenna.  The LNA amplified the signals, bringing them up above the noise floor of the spectrum analyzer. It did this while adding very low noise relative to the signal amplification it provided. It did not create any signals, it just boosted signals that it could see(that my spectrum analyzer could not), to a level that my spectrum analyzer could see. It did this without introducing excessive distortion or noise.

LNA Functionality - Conclusion

Now, the way that is going to apply to my project is that the LNA gain is going to overcome the loss of the 16-way splitter so I can present the single antenna output to 14 radios.

Remember, this is going to be RX only so I don't care about TX from radios toward the LNA and antenna. I'll cover the "TX into the splitter branches" issues in another post on the splitter later. The LNA is surely directional, and a large TX signal coming into it's output may damage it anyway... I'm not sure and I'm not going to try it out :)

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