Skyworks LNA

January 15th, 2014 Leave a comment Go to comments

I originally starting working on this device about the same time as the Sky65116 power amplifier.  The idea was that I should put a PA on the transmitter, and a low-noise amplifier (LNA) on the receiver.  In this post, I’ll discuss the why and how of LNAs, as well as the construction and evaluation of the Sky65047 400MHz to 3GHz LNA board shown below.  I’ve had these boards sitting on the bench for quite some time, and I finally got all the random-value passives that I needed gathered and the equipment necessary to measure it.

Finished LNA

Finished LNA

Low noise amplifier — why?

The goal of any receiver is to increase the strength of a desired signal from the antenna to a useful level.  Often, the point is driving a speaker.  Any receiver is simply an amplifier if we ignore selectivity, and some receivers do this to an extent.

I’ll discuss AM radio as an example because there’s a more direct connection between the input radio power level and the audio power level.  Let’s assume that the station we’re listening to is producing -70dBm of power at your antenna.  It’s irrelevant for the discussion, but for the purposes of illustration, and assuming a 50 Ohm antenna system, that means that the peak-peak voltage is less than 200 uV, or .0002 volts!  Then, let’s assume that we want 1 watt out of a speaker, or 30dBm, meaning that the receiver needs 100 dB of total system gain.

While I’m sure there are amplifiers that are capable of this much gain, it’s unlikely that it’s going to be appropriate for this application.  It is much more common to have a chain of amplification.  Even though I said I was ignoring selectivity, we still want to amplify only the desired signal.  It is the goal of our amplification chain, therefore, to preferentially amplify the signal and not the noise.  The ability to do this is quantified as the Noise Figure or Noise Factor (NF; these are equivalent, but noise figure is in dB) of a device.

The total noise figure of a receiver is dominated by the first amplifier in the chain.  This is the reason that we develop specialized now-noise amplifiers.  If you spend the time, energy and money on a LNA it will pay off by lowering the noise figure of the entire receiver.  On the other hand, if you take a crappy amp and put it in front of a good receiver, you’re likely to reduce its performance.  This is formalized by Friis’ equation:

friis

F4+1 should read F4-1 Thanks to Texane for spotting the error!

 

The F terms are the noise figure of the subscripted amplifier stage, and the G term is the gain of the stage.  You can see the the noise figure of any given stage is divided by the product of the gain of every prior stage.  Therefore, a good LNA has low noise, and high gain.  A simpler way to express this relationship, especially if you’re considering adding an LNA to an existing receiver is this other version:

friis2

With this version, if you have the LNA specification for the receiver you’re working with, you can see if your proposed addition is worthwhile.  I had hoped to talk about specific numbers and bring up the MRF49XA receiver that I wrote about here and here, and a few of my commercial ham transceivers, but they only seem to specify sensitivity.  I’ve seen someone off-handedly convert sensitivity to noise figure, but I can’t find the equation anywhere.  If anyone knows how to do this, let me know.  The point remains that you should never put an amplifier in front of a receiver unless its noise figure is better than the receiver.  You should be able to measure the success of your experiment by comparing the SNR before and after the addition.  If it deteriorates, then the additional amplifier is increasing the noise more than the signal of interest.

Low noise amplifier — how?

I’m not going to explain how to make a low noise amplifier, at least not at any substantial level, because I don’t know.  I’m not actually an electrical engineer, and I’m certainly not designing silicon.  If I want an LNA, I’m going to digi-key like every other mortal.  However, I will discuss how I selected this chip, designed the PCB and evaluated its performance.

I begin any project like this in basically the same way: I go to Digi-key and start parametrically searching.  In this case I’m looking for a 400MHz amplifier, so I selected all the devices that were in stock, in small quantities, that cover the frequency I want.  That left me with on 13 pages of results!  Ok, next, I decide that I’m only interested in amplifiers with a noise figure less than 1dB.  Good, now we’re down to 15 results.  I can actually think about these choices now.  If I’m being honest, I sort by price next.  If I wanted to do the best job, I’d sort by gain.  Sorting by gain, the top two choices are $12 and $20 each, with gain values of 24 and 22 dB respectively.  In contrast, the one I picked is $0.56 and 15.7 dB of gain.  I like my way better.  :)  I should say that if you need the best LNA, it don’t matter how much it costs, and in this case, you’re probably not shopping at digi-key.

Of course, before I hit “buy,” I read the data sheet.  First and foremost, I’m looking for a relatively simply evaluation circuit.  I’m the RF engineering equivalent of a script kiddie, and I’m not ashamed to admit it.  It’s a bonus if they provide matching circuits or component values for my frequency.  Luckily, the Sky65047 has both.  There is some weirdness, however, with the 450MHz version in the data sheet and application note.

915 MHz evaluation circuit

915 MHz evaluation circuit

There is a gotcha lurking in that data sheet, though.  The specifications presented on digi-key aren’t for 400 MHz.  The NF at that frequency is 1.2; a bit worse than I had hoped, but the gain increases to 20 dB.  Normally, the fact that this is a DFN (Dual, Flat No-Lead) package with very fine-pitch leads would mean that I would just skip over it.  I’ve developed the skills to work with these packages lately, and I’ve had good results, so it wasn’t a deal breaker.  A side-benefit is that it goes up to 3 GHz.  I’ve been thinking about building a HRPT (High-resoultion picture taking, for weather satellites) receiver, which is right-around 1700MHz.  There is a matching circuit for both 450MHz and 1700MHz in an app note, so I went for it.

Basic general schematic

Basic general schematic

The next step in the process was designing a PCB.  The topology used for each of the evaluation circuits is slightly different, so I tried my best to design a board that would work with all of them.  I think I mostly succeeded, at least for 400 and 1700 MHz.

Breakout board design

Breakout board design

There are only a few point I’d like to make about the PCB.  First, the most important thing about RF design is the minimization of parasitics.  It’s not shown in the above image, but I like to keep the solder mask off of the RF portion of the board.  It might be a bit ridiculous in this application, but the idea is that it would change the permitivity, and therefore, the calculations for the characteristic impedance.  It’s ridiculous in this case because those traces are way too short to be striplines, and with traces this short, the impedance doesn’t really matter.  Also, notice that I have the ground vias practically on top of the SMD pads.  These are to minimize the length and inductance of current return path.  There are no breaks in the ground plane under the RF section of the circuit; the only trace on the bottom layer only crosses a DC trace, and is very short.  Finally:  tons of vias!

Construction and evaluation

It seems with RF projects that you get the great pleasure of ordering a stupid number of weird-ass values of capacitors and inductors.  It took quite a while to collect all the pieces that I needed, I only did so about a week ago.  I was impatient and started soldering some parts onto the boards months ago, and forgot what I was doing in the mean time.  This becomes obvious later.

450MHz version

450MHz version

Something strange is happening in this example circuit.  The input matching network is completely non-sensical.  First of all, L1 is specified to have 4.3pF. Nope.  Second, C3 is shown as an inductor and has 30nH.  Hmmm, also no.  I emailed Skyworks to ask for clarification, and they never got back to me.  I had to just guess for what these are really supposed to be.  I started by assuming that L1 is actually an inductor of 4.3nH (pH inductors can’t really be bought), and that C3 is a capacitor of 30nF.  Below is the same board as in the image is from the head of the post, built with the assumptions I just mentioned.  Play spot the changes! :)

Assembled amplifier

First attempt

Noise figure

Now that we have a circuit, it’s time to determine whether it’s performing to expectation.  Because the noise figure is this amplifiers reason for being, lets first discuss the concepts of measuring it.  There are three primary methods used to perform this measurement, which are summarized nicely by this Maxim application note:

  • Noise figure analyzer such as the Agilent N8973A.  The good news is that this method is the simplest and best for measuring very low noise figures; the bad news is that it’s almost $40,000.
  • The gain method:  This method is the easiest to perform with more commonly available equipment.  The downside is that it’s very difficult to measure small noise figures.  This will be discussed in great detail later.
  • The Y-Factor method: Requires an excess noise ratio (ENR) source in conjunction with a spectrum analyzer.  This is much more affordable than a noise figure analyzer, but these still go for around $1000 on ebay.  There are some ~$300 ones, but they are of unknown quality.

You should know that I’m an insufferable cheapskate, so the noise figure analyzer and Y-factor method are non-starters.  Because we’re stuck with the gain method, but what kind of conclusions can we draw given the equipment on hand?

The definition of noise factor is the ratio of total output noise power divided by the output noise that is contributed by the input assuming a perfect noise-free amplifier.  We can measure the total output noise power within limits, and we can make assumptions about the input noise because we control it.

We can use a 50 Ohm terminator at room temperature for our noise source.  The noise power can be derived from theory and should be equal to -174dBm.  A perfect amplifier would amplify this noise without adding any of its own, and the noise power would increase linearly with gain.  This means that to get noise figure (NF) all we have to do is a little accounting: NF = P + 174 – G, where P is the output noise power, and G is the gain; all of these quantities are in dBm.

Measuring the noise figure

Measuring the noise figure

The gain method equation can also tell us what the minimum gain and noise figure we can measure on a spectrum analyzer when it is limited by its noise floor.  Working the problem backward, let’s say that my analyzer has a displayed average noise level (DANL) of -154 dBm at 434 MHz with 10 Hz RBW and 3 Hz VBW.  At that power level, the noise figure calculation would be NF = -154 + 174 - G = 20 – G.  This means that a perfect amplifier (NF = 0) with 20dB of gain wouldn’t change the apparent noise level of the analyzer at all.  It also means that we aren’t capable of measuring the noise figure of any amplifier where NF < 20.  Unfortunately, that also means that I should only see 1.2dB of difference in noise for this amplifier.  This is reasonably close to what I observed.  In the above image, the pink trace is with the amplifier connected and the yellow trace is with the spectrum analyzer input terminated.  There’s technically a little more than the 1.2 dB of additional noise, but you can see that the variation in the noise floor is more than that, so there’s basically zero confidence in the measurement.  At least I know that it isn’t a complete disaster, because that would show up.  Also, note that, for now, I’m making assumptions about the gain using the data sheet values.

Gain

Remember that the noise figure calculations depend on knowing the gain value for the amplifier.  I had used the data-sheet values of 20dB in the above example.  When it came to actually measuring the gain, things got weird.  There is where my subtle lies come apart.  When I recorded this video I had forgotten about the assumptions I made in the construction section.  Remember that I said this project sat on the shelf for a while?  While writing up this post I rediscovered those problems.  Now things make are starting to make a lot more sense.  My conclusions in the video aren’t really accurate.  I was right in that there was a matching problem in the circuit.  I was able to later discover that the “ruler effect” only affected in the input section.

At least now I know that one of the assumptions I made in the construction section was demonstrably false.  I started with C3, and replaced it with a 30 nH inductor.  This completely eliminated the ruler effect, and improved the gain from around 12 up to 17 dB.

Gain after C3 fix

Gain after C3 fix

Not content to assume that this is the best possible performance; I also tried replacing “L1″ with a capacitor.  I figured that it might be a capacitor because 4.3 pH isn’t really an inductor value, you can’t even buy them that small at digi-key. I had a 4.7 pF cap handy, so I tried it to see what happens.  Gain improved again from around 17 to almost 20 dB, and the shape of the trace in low frequencies improved.

Performance after changing L1

Performance after changing L1

Unfortunately, the current draw never really changed from the ~7.5mA that I was seeing in the video.  I figured that it would be prudent to re-measure the noise figure with the now higher gain.

Second attempt at measuring noise figure using the gain method

Second attempt at measuring noise figure using the gain method

Now, the results from the gain method are clearer, though still not that meaningful.  At this point, it’s just shy of 5dB difference.  The math would indicate that the NF is around 4.59 dB, but confidence in this number is still very low.  If it were true, it isn’t even close to data sheet performance.

Input return loss

The last quantity I’d like to measure is input return loss.  This is, quite simply, the amount of signal applied to the input that is absorbed by the amplifier.  As this number gets larger, it means that less signal is reflected back toward the source, and that’s a good thing.

Measuring input return loss

Measuring input return loss

A common way to perform this measurement is through a directional coupler (DC).  I’ll discuss this more in a later post, but briefly, a DC has three or four ports.  A three-port DC has an input, coupled, and output port, a four-port DC has coupled ports for forward and reverse coupled ports.  A perfect DC places some of the power from the input and none of the power from the output onto the forward coupled port.  If you hook it up to the spectrum analyzer “backward,” you can sample the energy reflected by the device under test (DUT).

Really cheesy schematic for reflection bridge

Really cheesy schematic for reflection bridge

I apologize for the crudity of the above diagram, but I just wanted to show how you’d hook-up a directional coupler to measure input return loss (or reflections).  I’m not so familiar with the schematic diagram for a directional coupler, but I assume that the arrows indicate the direction of coupling between the ports.  This is a four-port DC, so it’s a little different than what I have.  In the above example the tracking generator of the spectrum analyzer is represented by the AC power source.  The majority of the forward power through the coupler is passed directly through to the DUT, and a small fraction is absorbed by the 50 Ohm load.  The power reflected by the DUT is mostly returned to the tracking generator, but a fraction of it is sent to the spectrum analyzer input.  Ideally, none of the forward power ends up on the return coupled port.  This measure is called either isolation or directivity.

Input return loss

Input return loss

In the above plot, the magenta line is the returned power when the DUT is an open connection, and represents the maximum returned power (100 percent reflection or 0 dB input return loss).  The yellow line is the return loss from a 50 ohm terminator, and shows the directivity of my DC.  This is what a perfect termination would look like.  The teal trace is the reflected power from the LNA.  You would subtract the teal trace from the magenta to find the input return loss.

In our case, the best performance is at 382 MHz, with 16 dBm and worst (in this plot) is at 334 MHz with about 10 dB input return loss.  This is near enough to the data sheet specifications (14 dB) for my needs.

Conclusion

So, I’m really shocked that this ended up being so freaking long.  I feel like I had a lot of ground to cover, and to do it any justice I had to take my time.  If you’re still reading at this point, I’m humbled.   I sincerely hope it was worth it.  If you have any questions, please don’t hesitate to comment.  I try really hard to contribute meaningfully through the comments.



 

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  1. Aaron Holtzman
    January 17th, 2014 at 08:25 | #1

    Sometimes when a reference design is tweaked for a different frequency band an L turns into a C because the matching strategy changes. The only way to be sure about the values is to put them onto a smith chart and see what is happening. I recommend the freeware version Smith32 for doing matching.

    For NF measurements you need a good lab LNA to compensate for the usual poor noise performance of your spectrum analyzer. You can get wideband 1-2 dB NF LNAs from mini circuits for less than $100. Also, Agilent has a good paper on making noise figure measurements here: http://cp.literature.agilent.com/litweb/pdf/5952-8255E.pdf

    BTW your layout is pretty good, though C1 should be moved down onto the transmission line as the pad is acting like a stub.

    You need to finish building your SSA so you can use the VNA part to characterize your LNA. How is that going?

  2. January 17th, 2014 at 11:45 | #2

    @Aaron Holtzman

    Very good points. I didn’t change the frequency band when building this board, though I may in the future. I’m aware of what smith charts are and what they do, but I don’t understand how to use them yet. If you know of a really good tutorial with examples to work, I’d love to know. I’ve discovered that I have to do things to understand them, that’s why I enjoy doing this blog so much. I knew what noise figure was before doing this post, but I feel like I understand it now.

    I see what you mean about C1, I was trying to split the difference between the ground and the signal, but I understand why that’s not really a good idea.

    With the SSA, I’m still interested in working on it, though I’m thinking hard about how it’ll fit into my lab now. I’m thinking that it makes sense to build it with a focus it to be more of a VNA and 1G+ SNA rather than specifically a spectrum analyzer. It really doesn’t change that much, except that it changes the RBW filters that I would try to build into it. Also, it motivates me to think more about the OSL and port switching.

  3. EB4FBZ
    January 20th, 2014 at 01:23 | #3

    I agree with Aaron. You need to lower your equipment noise figure to reduce uncertainty in the measurement. When switching between hot/cold state, please don’t simply turn the LNA off, as the SA input will be unmatched and that could vary the cold measurement.

    Besides C1 placement issue, you absolutely need low loss components at the input. X7R capacitors are absolutely forbidden, you need NP0/C0G or high-Q capacitors (ATC) if possible.

    The capacitor placed at L1 looks like a 1206 capacitor. I would not use a bigger than 0603 package to reduce parasitics, 1206 has a big parasitic inductance.

  4. January 20th, 2014 at 08:48 | #4

    These are fair comments. I’m not switching hot/cold as in the Y-method. I’m measuring the analyzers noise level terminated with a 50 Ohm load, then attaching the powered amp with its input terminated.

    I think I understand how to use another LNA to increase the measurement certainty, but as I see it, there are many ways to do it. I’m assuming you’re referring to an LNA such as ZX60-P103LN+, with NF=0.6 and 13dB gain. I would leave this connected to the input of the SA for the whole test. When its terminated with 50 ohms, its theoretical output noise power should be -160dBm/Hz, below the noise floor of my analyzer. However, when it’s connected to the DUT, it’s input signal (noise from DUT) gets another 13dB boost, and we’re at -139.8dBm/Hz which is easily measured. Is that what @Aaron and @EB4FBZ suggest?

    I’m aware of the benefits of quality capacitors, and those are what I try to buy, but I don’t mention that in the article, and people should know, so thanks for mentioning it. And, yah, that’s a 1206. It’s not ideal, but that’s all I had in stock.

  5. EB4FBZ
    January 21st, 2014 at 04:20 | #5

    I know you are using gain method. I meant SA noise measurement as “cold” and DUT+SA noise as “hot”. Don’t measure SA noise with unpowered LNA, use a dummy load.

    First of all, you need to understand that you are measuring power integrated in the RBW, in case of Rigol DSA815 the minimum RBW is 100Hz.

    The specified displayed noise floor (DANL) is -135dBm (100Hz RBW, standard preamplifier on, trace average >= 50, sample detector, typical) which correspond to -155dBm/Hz. That means that the analyzer NF is arround 19dB (at 25ºC). Now you have your equipment NF to substract it from the cascade and get the NF DUT alone.

    You can’t really measure a low noise amplifier with such a high measurement setup NF. For accurate measurements you need the chain NF to be dominated by the DUT, so if the DUT gain is lower than the equipment NF plus 10dB you need to add a LNA.

    It’s easy to understand. If your LNA has 13dB gain and 1dB NF, the SA noise floor will be -154.8dBm/Hz (-134.8dBm displayed), only 0.2dB higher than a noiseless amplifier or -154.56dBm/Hz (-134.56dBm displayed) for NF=2dB.
    Due to the resolution and non-linearities of SA detector you can’t trust it, as the NF uncertainty due to this factor only could easily be higher than +/-1dB!

    If you add a LNA with 15-20dB in front of the SA, the total NF will be dominated by the DUT, so a 1 to 2dB increase noise floor by 0.95dB.
    For example, using a 20dB gain 1dB NF LNA in front of the SA with a DUT with 13dB Gain and 1dB NF, the displayed noise floor (100Hz RBW) will be -119.8dBm, or -118.83dBm if NF=2dB.

    On the other hand, gain method is not really suitable for measuring low noise DUTs because the uncertainty is very high. The measurement uncertainty will be at least twice the SA amplitude uncertainty, as you need to measure the DUT gain first and the noise power (both unrelated). That means that the final uncertainty could be as high as +/-1dB.

    For good NF measurements you need a noise generator (aka head) and use the Y-Factor method. For under 1dB NF DUTs you can’t use a SA, you need a dedicated NF analyzer and is even recommended to use low ENR heads (5-7dB) instead of typical 15dB ENR heads to reduce amplitude uncertainty due to SA detector linearity and mismatch.

    http://www.maximintegrated.com/app-notes/index.mvp/id/2875
    They asume a high gain and high noise DUT, so the DUT NF is equal to the chain NF. But there is a warning at the end of the gain-method.

    - Never measure NF if DUT gain is less than the equipment NF plus 10dB. Otherwise add a LNA.
    - Don’t use gain method if you need more than 0.5dB precision or expected DUT NF is lower than 3dB.
    - You can use Y-Factor method with a ENR noise head and SA for precise NF measurements between 5-1dB. Low ENR head under 3dB NF recommended.
    - For <1dB NF DUTs you absolutelly need a NF Analyzer.

  6. January 22nd, 2014 at 14:10 | #6

    In my case, I still have the “trial” of the 10 Hz RBW that I can work with, until it expires with no way to renew it, thanks Rigol. With it, the DANL is somewhere around -145dBm, not coincidentally the same -155dBm/Hz noise power.

    Your analysis is more or less the same as what I come to in the post, my only question was really related to the experimental methodology to be used with another LNA. It sounds like I pretty much understood how to do it.

    I realize that the gain method isn’t the appropriate tool for the precise measurement of the NF. I really wanted to know whether it was “good enough,” an entirely subjective measure. Because I believe that the system (DUT+SA) NF is dominated by the SA, it is my opinion that the LNA is “pretty good.” Of course it would be nice to have better measurements, but at the moment I have other priorities with my hobby money.

    I will keep my eye out for a better LNA on ebay, and a ENR source as well, but for the moment I’m pretty satisfied. I’m working on another post about a FM preamplifier circuit I built from a schematic I found online, including the analysis of its noise figure. In this case, I get dramatically different results. That gives me enough information to say that it’s “not as good.” :)

  7. texane
    January 31st, 2014 at 23:30 | #7

    Hi,

    Thanks for your article, it is very informative. Just to mention a
    small typo in the Friis equation, where F4 + 1 should be read F4 – 1.

    Cheers!

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