Fobos SDR: High-Quality Radio for Hobbyists, Researchers and Professionals

At nearly 600 euro (in NL) I would really like to see how it stacks up against other SDRs like the RTL-SDR V4, HackRF One and the RSPDX R2 for instance.

I think there are a few ways to approach this question as an engineer that makes SDRs professionally (in a group, not just me, I'm not magic). I could probably fill a decent sized book with all I've learned of practical applications but I'll focus on a few high level points.

The real reason is flexibility. Let's say you have a product that receives your specified signal at your specified frequency. Something like a cut down 802.15.4 (thread, zigbee, etc type systems). Then you deploy your system and find out the spectrum is really cluttered and oh if only you could switch frequencies. SDR. Or you realize you didn't take security seriously and need to change the low level processing to fix that [1]. SDR. Maybe you want the ability to listen around the 2.4 GHz range for sensor messages then transmit them over WiFi or Bluetooth. So even the same frequency range, SDR lets you switch protocols.

In the same way that software was revolutionary in letting the phone I'm typing this on kinda replace a TV, camera, mail system, etc, SDRs take that to the next level with flexible hardware interfacing. It probably makes more sense to think of it but as telemetry or meters, but the ability to talk to satellites and power meters with the same hardware. This saves on development cost because there's only 1 product, system cost because only 1, unit cost because economies of scale, etc.

There is certainly a trade with efficiency across size, weight, power, and cost, but it's usually worth it. Even if you do a fixed frequency 2.4 GHz you can have a lot of flexibility. But often it's trivial to be frequency flexible.

[1] this is an often overlooked point to a lot of development. In the same way software allows security patches, SDRs do too. There's so many times wireless protocols get revised to increase security or dropped because they're insecure. SDRs allow that to be fixed as simply as a desktop or mobile app update.

I’ve seen people use an SDR dongle to figure out the frequency and data being transmitted by electricity meter outside their homes.

If you only have a real signal, you're going to have to delay it 90 degrees to get the imaginary part before you do any frequency shift or filtering if you want to avoid image frequencies, negative frequencies, etc.

Digital Signal processing is far easier if you have a complex signal. Without it, any frequency shift results in 4 output frequencies, f1+f2, f1-f2, f2-f1, and -(f1+f2), it gets geometric quite quickly.

Things are much simpler when you have a good I/Q stream to work with. You do a complex multiply and it's always f1+f2