Archive for category Radio Control

Skyworks LNA

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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SKY65116 Amplifier

Amplifier breakout board

I’ve finally gotten around to assembling a breakout board for the Skyworks SKY65116 UHF amplifier.  It’s really amazing how the state of the art in RF ICs has advanced.  They can still be on the expensive side ($6 at digikey), but still relatively cheap when you consider the cost of all the support parts that it takes to build an amplifier from a RF transistor.  This particular amplifier has a 50 ohm input and output, and 35dB of gain.  It works from 390Mhz to 500Mhz, which means its perfect for the 70cm ham band.  The breakout board is stupid simple, copied directly from the evaluation board schematic in the datasheet,  but I’ll include schematic and design files anyway.

Source for the amplifier test

This is the video transmitter from my first person video experiments.  The performance was pretty terrible, even after I tested it using different receive antennas.  I’ve even purchased a receive-side amplifier to try, but haven’t done anything with it yet.  Anyway, the transmitter had a built-in antenna, so I wasn’t sure how I was going to add an amplifier.  I ended up assuming that the output would be roughly compatible with an 50 ohm load.  I unsoldered the antenna and installed a bit of thin coax to the antenna port.  I scratched off some of the solder mask on either side of the board near the antenna port to make sure I had a solid electrical and mechanical ground connection.  The transmitter is pretty crappy, and the prices you can find online are COMPLETELY RIDICULOUS!  I wouldn’t pay more than $20 for it.  I think that’s about what I paid, it was on clearance.

Amplifier test configuration

Testing configuration

This image is the testing configuration I used.  The camera, power board and transmitter are in the top of the image, and are exactly as I used them for first person video.  The added coax can be seen going into the amplifier on the left.  Coming out of the amplifier is the cable going to the oscilloscope or spectrum analyzer.  The amplifier wasn’t inline all the time, though.  I measured the output power from the transmitter at about 25mV into 50 ohms using the oscilloscope.  Using Minicircuits’ handy table that comes out to be about .01 mW, or -19 dBm. A measurement from a spectrum analyzer verifies the -19 dBm measurement from the o’scope (see below for image).

NTSC modulated spectrum (click for source)

I’ve attached a very nice graphic from wikipedia that describes the components of modulated NTSC video.  There is something happening here that isn’t obvious, so I’ll explain it.  In the spectrum analyzer image, below, you’ll notice that I’ve labeled the luminance and chrominance carriers.  The luminance carrier is really the main carrier for the entire signal.  It comes from black and white TV era.  There are significant DC components in NTSC video, so this carrier is very important.  Notice, in the graphic above, that the luma carrier is 1.25 Mhz above the lower edge of the band.  This is because NTSC video uses what’s called VSB, or vestigial side band, which means that the lower half of the signal is attenuated.  This reduces the spectrum necessary to transmit video.  The choice was made to include the carrier and 1 Mhz with of lower sideband while removing the rest.  Later, when color TV was added, they needed a way to encode color.  This is done by adding another carrier and encoding hue and saturation by modulating the phase and amplitude of this carrier.  All this is explained at length, and probably much better, in the wikipedia article on NTSC.

Source spectrum

In the spectrum image I’ve included above, it’s clear that the little transmitter uses AM rather than VSB.  You can tell because AM modulated signals are always symmetrical with respect to the carrier.  If it was VSB, the spectrum on the left side of the carrier would be suppressed.  You may notice that the left and right side don’t look 100% alike.  This is because it takes time for the analyzer to sweep the band (it does this 30 times a second), and it will be analyzing the spectrum of a different part of the image as it scans.

Source signal through unpowered amplifier

Well, that was an unexpected tangent!  Back to the amplifier…  In the above image I have the amplifier in the signal path from the source to the analyzer.  It’s disconnected from any power.  I’m a little off on the “-60 dBm” text, it’s closer to -64 dBm.  I was interested in seeing how much RF would leak through an unpowered amp.  It appears that the amp provides a little more than 40 dB of forward isolation between the input and output when it’s unpowered.

Amplifier powered on

Finally, this is the spectrum when the amplifier is powered on.  I had to install 40 dB of attenuation on the analyzer to capture this image.  The peak of the carrier is almost 5 dB lower than the top line, so it’s about 36 dB stronger than the input.  This is inline with expectation, as the amp specifies +35 dB gain.  The resulting signal is +15 dBm, which is a modest 32 mW of power.  The hope is that through a better antenna and some amplification I can get better performance from the video link.

A word about the legal implications.  Ham radio people are notoriously concerned with the rules of everything they do, so I feel obligated to mention them.  In the U.S., at least, 434 Mhz is a commonly used ATV (amateur T.V., or “fast scan TV”) frequency.  There is some concern due to the proximity to the  “satellite only” frequency band of 435 Mhz to 438 Mhz.  This means that the carrier is sometimes shifted to 433.92 Mhz, as this transmitter is.  Some of the sidebands still end up in the satellite only band, but with much lower power.  Because this amplifier only outputs +15 dBm I’m very unlikely to upset anyone with its use, though I should think about adding an overlay with my call sign to the video at this power level.  Maybe I’ll have a new 8-bit microcontroller project…

eagle files

gerber files

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Transceiver enclosures

Transciever enclosure

It has been a while since I finished the transceiver modules, and I how now used them in an actual application.  But, before I talk about that, I’d like to show some pictures of the process I used to put them into enclosures.  I had some of these cast aluminum enclosures lying around, so I thought I’d use them.  They’re a little on the heavy side, as the completed weight is around half a pound, but it’s well within the carrying capacity of my Kadet.

Before diving into the process of cutting the holes, I want to show some images of the transceiver board with the RFI fence installation process.

Cutting copper sheet

For use with the spectrum analyzer project, I found some sheet copper at the craft store.  It was sold at a local crafts store, and I think it was for etching.  I chose the thickest one they had.  So far, the best way I’ve found to cut it is using an exact and straight edge.  I tried scissors, and it didn’t really work.

RFI Fence

Once I had a strip of copper cut, I cut openings for the power and control traces and soldered it onto the PCB.  I also soldered it onto the SMA connector.  Once all that was finished, I soldered on a lid.  Lots of solder flux helps here.

Once the board was prepared, both by soldering on a fence and replacing the pin header with a right-angle one, I began to prepare the enclosure.  I was intending to drill a hole for the SMA connector, then cut a hole for the digital connection.  The SMA connector hole was trivial to make, though the connector I soldered onto the board was a little short.  I ended up having to use an O-Ring from the hardware store (look in the plumbing section) to hold it in.  When an antenna or cable is screwed on the O-Ring compresses, having the nice side-effect of sealing it.

DB-9 template

For the digital connection, I decided on using a DB-9 connector.  I figured it was a prolific connector, so I should have lots of connectors laying around.  That didn’t turn out to be as helpful as I had hoped, but I’ll get into that later.  To create nice holes for the DB-9 connectors I decided that I could use an old PCI bracket as a template.  I lined the bracket up against the side of the enclosure and traced it.  On the black box, I traced it using a knife, and on the grey box I used sharpie.

DB-9 template using sharpie

Once the outline was traced onto the box, I drilled holes for the retention screws.  Then, I drilled out as much as I could of the trapezoid shaped interior.  I most used the drill press, then the dremel with a router/cutter bit.  I made sure to leave a margin inside the perimeter to remove with the files.  I had avoided purchasing a set of jewelers files for a while, I think I assumed that they were expensive.  They’re not, you should get a set.

Finished penetration

In the photo above, you can see the finished penetration for the DB-9.  I beveled the inside edge to make room for the fillet on the connector that I had.  The black box got a male DB-9, and the grey box got a female one.  The holes need to be about the same size, as the male shroud always has to fit over the female connector body.

DB-9 connector installed

In addition to the RF and digital connectors, I needed a way to securely mount the internal circuit boards.  The way I chose to do this was first to drill holes in the bottom of the box, then “countersink” some screws into it.  I have countersink in quotes because I don’t have a countersink bit, so I used a larger drill bit.  You can see the results of this in the headline picture of this post.  Though I think it looks pretty good, I still decided to buy a drill & tap for 4-40 screws after building the black box.  For the grey box, I used the tap and screwed directly into the box.  This requires slightly less hardware and looks pretty good, I think.

Breadboard transceiver circuit

For whatever reason, the board I built for the black box using some veroboard-style construction didn’t work the same as the breadboard.   Because I was under time constraint (I was planning on flying one of the transceivers over the weekend.  I decided to put it back on the breadboard and use it as the base station.  This version uses an FTDI cable to connect to my computer.

Flyable transceiver module

For the grey box, I used an extra ATMega48 breakout board I had.  This one worked just fine in the enclosure, so I flew it.  I also built a power regulation/distribution board, seen on the right of the photo.  This concludes this article.  Now that I’ve got at least one flight worthy transceiver I can test them in flight.  That’ll be detailed on a future post.

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Brushless Mud Bug

Mud Bug flying

Mud Bug flying

I maiden’d my new plane, the Mud Bug, last weekend!  I was a ton of fun.  Now, I’ll tell you all about building it, and converting it to use brushless motors.

laser cut precision

Laser cut parts are a joy to use

All the parts in the kit are laser cut, and fit perfectly.  The design of the plane is such that the shape of the wing is created almost exclusively by bending the wing’s top skin into shape.  There are only a few ribs, and no bottom skin at all.  It’s remarkably light.

Motor stick mount

Motor stick mount

The kit calls for a stick-mount geared, brushed motor.  These are getting pretty out-dated, and aren’t very efficient.  I really like working with brushless motors, so I had to devise a way to mount it.  I chose the E-flight park 250 motor because it was pretty inexpensive and only a few dollars and grams more than the park 180.  It’s possible to mount it inside of a carbon fiber tube (using glue), but you need to use their tube, and it wasn’t in stock.  I just decided to use the cross-mount adapter and make a plywood plate that mounts on the stick.  Where the stick mounts, I added another, smaller, piece of ply to help support it.  It ended up working perfectly.

Front scab plate

I cut a pair of sheets of 1/64″ ply to use as scab plates on the balsa firewall and fuselage parts.  I had read online that those pieces of the original kit were somewhat weak.

back scab plate

back scab plate

This is the scab plate on the back of the fuselage portion of the motor mounting.  These are only there to spread out the loads transmitted from the stick to these parts.

Park 250 mounted

Park 250 mounted

The motor mounting process went beautifully.  At first everything fit so tightly that I didn’t need any glue.  Half way through the first flight it had vibrated enough to polish the wood parts that they could slip.  I just put a few drops of thin CA, and it was fine.

In flight

In flight

The flights went great.  That park 250 has waaaaay more power than you need with a 7×6 APC Slo-fly prop  (which is the only prop I’ve tried).  The plane flies easily at 1/4 throttle.  At full throttle it gets very small very fast.  When flying slowly, it’s also very agile.  It’s possible to complete an entire circuit in 1/3 the length of the runway.

first landing

first landing

Even though it sports GIANT tires, it’s still easy to nose-over during landing in even short grass.  Given that it has almost no mass, no damage was sustained.

I give it 2 thumbs up!

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Spectrum Analyzer IF Amplifier

IF Amplifier cooling down

I’ve decided to hold off posting spectrum analyzer modules until they’re complete.  I’ve been collecting tons of pictures along the way, so hopefully the post for each module will be interesting and visually appealing.  With that in mind, I can’t post about any of the modules with the exception of this one.

For whatever reason, my kit didn’t come with any voltage regulators, and this is the only module besides the mixers from my last post that don’t use any.  The IF Amplifier, as the name implies, amplifies the Intermediate Frequency on the analyzer.  I made a simplified version of the block diagram from Scotty’s site.  I’ve also indicated the modules I’ve finished in green.

Block diagram, green modules are completed as of this post.

As you can probably see, there’s a lot to be done.  Luckily both DDSs, the Log Detector, ADC, Control Board, and one PLO are done once I get some parts to replace the missing ones.  I also need some parts for the Master Oscillator.  Once that (actually quite small) order comes in, I’ll be almost finished with the boards.

Anyway, back to the amplifier.  I was one of the first people on an order of boards back in August ’08 because I agreed to look-over a new revision of the design files.  Of course, by having me look them over essentially guaranteed that there would be errors.  As it happens there was an error on this board.  The engineering change order (ECO) is luckily quite simple, the only problem was that a short section of the border was missing.  Here is a photo of the completed fix:

Fix for the slight error,

You should be able to see that along the border there is some solder wick saturated with solder bridging the gap, and connecting to the capacitor.  Also, on (at least) the 2 resistors (R3 and R4) there are tiny balls of solder.  These happen during the reflow stage when a bit of solder squeezes out from underneath the device.  I haven’t gone over the board picking all these out yet in this photo.  I think it’s important to remove all of these because they may cause shorts.  Also, you may be able to see a slight, shiny, residue around everything.  This is the solder flux that’s included in the solder paste.  I remove this later with some 99% rubbing alcohol.

Completed IF Amplifier

This is the completed amplifier board.  There are actually 2 amplifiers here, mirrored.  In practice, the output of one will be the input of the other.  This serves to roughly double the gain.  Finally, notice on the right side, that many of the parts have been omitted.  On the schematic, Scotty simplified this section.  Technically, he has 2 zero-ohm resistors (basically jumpers) in addition to the capacitor.  I chose to just jump the capacitor across and leave the resistors out.

Anyway, I enjoyed building this one.  I hope you’ve enjoyed reading about it.  I can’t wait to finish the other ones!

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