Well, before reconnecting the adapter to work laptop, I spent days and nights trying to figure out how to build Spektrum on MacOS and Pi or use it with preinstalled Processing without having to build myself. Pi is still not working but I finally figured out MacOS build so I can confirm the difference with actual scans. (The scans also help reveal some inner workings of the blue stick.)
IMPORTANT: Because the the blue stick has wideband preamp in front of a narrowband filter, the following scans are skewed by that filter, rendering them less useful to assess preamp saturation, which appears to be the problem i am combating. I wish I had an orange dongle (or a generic stick) for this task. Also keep in mind that the scans does not reflect RF spectrum received at the antenna. The result is the product of RF spectrum and the transmission function of that filter. (Actually, the result is also altered by the transmission function of the antenna.)
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The setup
In this setup, the receiver is directly plugged into Macbook Pro using a USB A to USB C adapter. Power is also plugged directly, as adapter C1 doesn’t have a power passthrough. Adapter C2 is used for a different purpose so the comparision is only between C1 and C3.
In Spektrum, default gain is 10 but very little can be seen at that level so I opened up all the way to 500. (I guess that’s the equivalent of 50 in dump1090.) -
Wideband survey
The first tests are full range 24 MHz - 1,800 MHz. The baseline clearly reveals the effect of the built-in ADS-B filter.
This baseline is saved as reference in other conditions (blue line). The next two shots show really broad and severe leaks. Not only does C1 raises noise level in the pass band of the ADS-B filter, but noise is seriously elevated even in the suppressed lower side.
The next two are taken with C3 connected, with and without monitor. They show little difference from the baseline inside or outside the filter’s passband.
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ADS-B vincinity
To further examine the characteristics, I narrowed the range to 940 MHz ~ 1,240 MHz. I also increased bin to 200 kHz to speed up scan. (Did not seem to have effect.) In addition, as I discovered Spektrum’s median hold (aqua blue), median gives a better gauge than “Video average” whatever that does.
This baseline reveals the passband of the blue dongle’s filter as ±20 MHz.
(Keep in mind that the built-in filter is designed to suppress signals outside of the passband. Judging on the baseline, it does a good job in that. But once C1 is powered up, even the apparent passband widened. This is in addition to a sharp rise on lower frequencies.)
These scans reveal some details of leakage from the “good” adapter C3. But overall, it is much smaller. (Also, C2 and C3 does not leak till an HDMI cable is attached; I believe that C3 also does not leak till the monitor is attached.)
While these results are promising, I get additional frustrations after work resumes because C3 appears “temperamental” when attached to my work laptop. Noise can change wildly when my laptop move slightly; it can reach the same level as the “bad” adapter. It seems to have the least noise when the input cable is bent at 90º against the connector. The strangest of all, there seems to be a correlation with how much activity I have on my laptop when there is no physical disturbance. If I leave the laptop for a time, noise can decrease. But when I do certain things - I haven’t deciphered which, noise increases. (This makes me suspect that this device is a spy for my boss.) Without Pi Spektrum, it is rather difficult to do live scan when the adapter is attached to work laptop. I’ll keep trying to build on Pi.