The toaster oven approach is understandable, especially with the OSHpark 4 layer RF boards, it’s very difficult to get things soldered to a ground plane even with thermal reliefs. And then the “hand editing” of parts is rough too, but that is where the hot air device is handy.
The SAW filter before the LNA mmic chip (a Mini Circuits PMA2-162LN+) is because so many of the 1090 MHz ADSB systems get overwhelmed by local cell phone signals. The receiver dongles made by Flight Aware use single chip Software Defined Radios (SDRs) that are typically direct conversion recipes with A2D converters and digital samplers. It’s pretty easy to swamp the first mixer. Mixers without some degree of Band Pas Filtering (BPF) often struggle. The Flight Aware ADSB receiver dongle does have an LNA in it, and there is a SAW filter after the LNA which is part of why it works so well. They also have a discrete component BPF you can put before the dongle if the local interference is too great. It is a most elegant solution.
The SAW after the LNA was an experiment. Once you have the gain of the amplifier, tossing another 2.5 dB is not an issue. And if the LNA chip mixed any of the input signal with any residual power supply noise, it could produce some out of band signals that might confuse the SDR.
As for the noise figure, the LNA chip specification is .5 dB. Another learning experiment here is how well to SAW filters work in very low signal strength environments. While the loss is specified, there is no mention of noise when the input is at -120-ish dBm. Most of the time you see discrete component filters or cavity filters before the LNA, and then the SAW filter is downstream in the signal chain where there is more amplitude available. I don’t have a noise figure meter, they are a bit pricy. And I’m pretty good at coming up with excuse to by test equipment.
I know a company that has one, and hope to give this a test someday. will advise.
There are good LNAs available in this frequency range, so in addition to just wanting an LNA this was a great time to learn about living in a 1 GHz world. It was a good time to experiment with how to put parts on both sides of a PCB, ground plane design for RF, tightly packed 0402s, switching power supply noise management in sensitive RF systems and the like.
@BobZ Indeed 0402 are a pain, but at these high frequencies they seem to be required. My standard go-to size used to be 0805, but I’ve gotten good enough that it is now 0603. For people starting out, I tell them 0805 with the “hand solder” footprint and then go from there. One lesson learned is that if you need a Hi Q inductor, you will want to investigate going to an 0603 part instead of a 0402. The larger size allows for a bigger wire so the Q goes up and the DC Resistance goes down.
This board had to be pretty much all 0402 for the LNA. The switcher is mostly 0603. I do like placing an 0402 in a .001uF to .1 uF range very close to the input and output side of switchers, and then put the larger caps next. This helps maintain a low Equivalent Series Resistance (ESR) across a broad frequency range. On this board, both the switcher and the LDO linear regulator have variations on a chain of caps from .001 to 100 uF values to really provide as close to an ideal cap as you can get. A the LNA chip there is a 9.1 pF close to the power input which is crazy until you realize it’s not.
It’s easier to remove parts than add them, and the cost to toss a few more caps on the board is in the noise. This is why “hobby boards” are more fun than “work boards”.
I realize we’ve moved into a bit of an electronics discussion, but, when your circuit board is in a high frequency or very log signal level world, the electronics and the layout become integrally entwined.
Gratuitous pix of the LNA frequency performance and schematics, just to show the crazy problem of getting clean power supplies. Also a pix of the conducted noised from the 5V switcher before it goes into the linear 4V regulator. All examples of when, in some designs, the PCB layout is part of the circuit.
Gain and filter characteristics. Note the small bump in the purple trace, which seems to indicate that the part is working with a noise floor less than the -120 dBm limit of the spectrum analyzer.
Here is the conducted noise at the output of the 5V boost-buck switching regulator as it goes into the 4V linear LDO regulator. Pretty standard performance, most of the noise is in the 1 MHz to 150 MHz range. The key concept here is how much the regulator (blue) is above the background (yellow). This is where the PCB becomes part of the circuit, as can be seen by some of the blue spikes.
LNA schematic (there have been changes). Left to Right analog signal chains are so easy to grasp, so hard to get right. Digital goes everywhere but noise is so much less of an issue. There have been some posts on what the style or “look and feel” of a schematic should be. I don’t claim this is perfect, but it probably not too bad of an example.
Switching supply schematic. Filtering the input power to keep the switcher noise from radiating out the power leads, multiple stages of filtering with a spread of capacitor values and sizes, and a two stages- the switcher on the bottom of the PCB, and the Low Noise LDO on the top with the LNA. Probing with the spectrum analyzer at each stage shows the importance of the surface mount ferrite beads and verifies the design. Note that the switchers, the LDO regulator and the LNA are all on their own 3 layer ground planes. The interconnection of power and ground between the ground planes is critical.
Bonus gratuitous pix: The 3D print design for the chassis which includes board spacers:
Apologies for the circuit design tilt here, but, when you are in low signal level or high frequencies, the PCB layout is now part of the circuit. Little side projects like this help you learn.