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Posts Tagged ‘Radio Control’

Skyworks 65116 update

January 9th, 2014 No comments

It has been a while since I played with the Sky65116 amplifier boards that I built and wrote about.  Since getting my own spectrum analyzer, I’m now able to make much better absolute measurements.  The DSA-815-TG analyzer is specified as having 1dB of uncertainly across the span, and the SA that I have at work is essentially uncalibrated.

Video modulator output

Video modulator output (click for enlargement)

Not only do I now know that the absolute power out of the video transmitter at the fundamental frequency is about 5.5dBm, I can also see the first and second harmonics.  I don’t know why I didn’t see them with the other analyzer, but they are disturbingly large.  I believe the FCC requirement is that these harmonics should be more than 40dB below the fundamental.  By this criterion, the transmitter should not even be sold in the US.

The SKY65116 amplifier has close to 36dB of gain, and a 1dB compression point of 32.5dB.  The 1dB compression point is specified as the output power level at which the gain is reduced by 1dB.  The easiest way to show this is with the graph from the Sky65116 data sheet, shown below.  You can see that as the output power begins to approach 32dB gain drops quickly.  The goal is to not force the amplifier to operate in this region.  If you do, you’re likely introduce harmonic distortion and other nasty nonlinear effects (i.e. intermodulation products).

Gain vs. Pout curve

Gain vs. Pout curve

So, knowing that the output is +5.5dBm, and we want about +30dBm out of an amplifier with a gain of 36dB, we need to introduce 11.5dB of attenuation (30dBm – 36dB – 5.5dBm = -11.5dB) between the transmitter and the amplifier, at a minimum.  It’s easy to make 14dB attenuators with standard value resistors (4150 ohm and 1120).  I’ve been playing around with QUCS a lot lately, so I’ve provided a model for the attenuator.  It’s just about the most boring S-parameter model you’ll ever see…  Perfectly flat response at -14dB, but that’s what we’re after.

Simple 14dB attenuator model

Simple 14dB attenuator model

A while back, I had a bunch of these simple 5-pole filter PCBs made up.  I just left them blank until I needed them, and I made two of them into 14dB attenuators using the circuit above.  I had two extra poles, so I filled one with a 0 ohm resistor and the other with a 1uF DC-blocking capacitor.  The capacitor reduces the low frequency performance, but only less than about 1.5MHz.  It’s worthy the trade-off in my mind.

Constructed attenuators

Constructed attenuators

I went ahead and covered one of the attenuators with copper sheet just to make it more of a completed package.  I’m sure I’ll need to use it many times in the future.  I left the other open so I could unsolder it and make it something else if needed.  An interesting thing happened when I installed the cover.  The small ripple in the attenuation (around 500 MHz, see below) occurred only after I installed the cover.  I assume this is due to parasitic capacitance between the components and the copper covering.  It’s still only about 1dB of ripple, so I’m satisfied with it.

14dB attenuator performance

14dB attenuator performance

Anyway, back to the amplifier.  I’ve now got about -8.5dBm going into the amp ( +5.5dBm – 14dB), so with its 36dB of gain, I should expect to see +27.5dBm out of the amp.  That’s getting very close to the maximum input power on my SA (+30dBm).  It’s always better to be safe with these things, so I used the other 14dB attenuator between the amp and the SA.  Now, I should expect to see +13.5dBm on the input.  I maxed-out the input attenuation on the analyzer (another 30dB) and gave it a shot.  Note that the internal attenuation is calibrated out of what’s shown on the display, and I told the SA about the other 14dB of attenuation, so the power values shown on the display are referencing the amplifier’s output.

Transmitter after amplification

Transmitter after amplification

In the above image, you can see that we’re getting 27.5dBm out of the amplifier!  I love it when a plan comes together!  This is the value I calculated, right on the nose.  I promise that I didn’t work the math backward! :)  Again, it’s so painfully obvious that the transmitter is AM, rather than the VSB signal that it should be.

Close-up of the signal

Close-up of the signal

Now, just for fun, let’s dive back into the video signal coming out of the transmitter.  In the image above, I’ve put some markers on the various carriers present in the signal.  The luma carrier is in the center at 433.85MHz, which is where we expect it to be.  Marker 2 is at 437.45MHz, which is 3.6 (let’s call it 3.57) MHz away, matching exactly where the chroma carrier is supposed to be.  There’s no audio carrier, which isn’t a surprise because there’s no audio, though I wouldn’t be surprised to see the carrier.  Marker 3 is 19 MHz away; I have no idea what this is or why it’s there.  It’s not supposed to be.  Same with marker 4.  Oh, well…  that’s what you get with a shitty transmitter.

amplifier gain v. frequency

amplifier gain v. frequency

Now, what about those pesky harmonics?  The Sky65116 is a 390-500 MHz amplifier, so my hope is that the reduced gain by the first harmonic will attenuate the harmonics enough to bring them into compliance.  The graph above is the gain v. frequency graph from the data sheet.  It’s neither encouraging nor discouraging.  It’s difficult to infer what’s going to happen at 800 MHz when the graph stops at 500 MHz.  In the image below, it appears that I lucked out.  The first harmonic is 42dB down from the fundamental.  If I were selling a product, there’s no way I would send this out for compliance testing.  It would just be too risky, I’m not that confident in my measurements.  By my math, it would cost less than $2 in parts (single unit quantities) to make a decent low pass filter.  That’s the right thing to do.  When I modeled it (in QUCS, again), I calculated that the harmonics would be within compliance even without the rolloff of the amplifier.  With the rolloff, the harmonics would be well below the noise floor.

Harmonics after amplification

Harmonics after amplification

I’ve had a ton of fun redoing this experiment with my new spectrum analyzer.  I’m going to write lots more about the analyzer in the future, and I’m really looking forward to it.  Coming soon is an exploration of the skyworks low noise amplifiers.  Between these two products, I expect to have a solid video link over 1000 feet or so.

SKY65116 Amplifier

September 15th, 2011 1 comment

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. Read more…

Transceiver enclosures

April 6th, 2011 No comments

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. Read more…

Brushless Mud Bug

February 8th, 2011 1 comment
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. Read more…

Security Camera Repair

March 28th, 2010 No comments

Unsuspecting security camera

Well, as a (perhaps welcome ;)) deviation from the spectrum analyzer posts, I’ve spent a little time working on repairing a security camera I’ve been hanging onto for a while.  I’ve been toying with the idea of installing it near the radio control flying field as someone in the club knows the owner of a nearby business.

Read more…