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.
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).
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(4x150 ohm and 1x120). 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.
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 about1.5MHz. It's worthy the trade-off in my mind.
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.
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.
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.
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.
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 lowpass 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.
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.