Fobos SDR: High-Quality Radio for Hobbyists, Researchers and Professionals | Hacker News Reader

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

hackster.io

22 pointsteleforce13 comments

Stupid question: what can you actually do with SDR? Everything I have seen talks about things like listening to telemetry from satellites or power meters which is cool but also not that interesting in a practical sense. Is there more to it than that?

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.

That makes a lot of sense. In industrial applications or when designing products I can see why it would give you all the flexibility.

I should have phrased my question a bit better: why is it so popular in the hobby space? Reading power meters seems like it would get boring real fast.

Another good question. I don't super understand a lot of the "I downloaded a software, bought a radio, and look it works". But I do like, for example, things like [1]. The reverse engineering side, the legal hacking side, that's where it gets interesting. The Flipper Zero is a great example of that, though I've never used one.

Another example I thought of on the industrial side is old NASA equipment like in the curiousmarc series. Any time they change anything, even frequency by a small amount, it involves effort like swapping out crystals and tuning filters and junk. In SDR, it's just a variable.

[1] https://www.washingtonpost.com/news/speaking-of-science/wp/2...

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

rtlamr! https://github.com/bemasher/rtlamr

Using a 2.4 Ghz transceiver chip for IF is an interesting choice. It will definitely keep image frequencies well away. 8)

Separate I/Q inputs might be useful, but I would opt for a wide band 90 degree phase shift network to tie them together and simplify signal processing downstream.

Note this an approximately $600 receive only board.

Not sure I understand the I/Q comment. What would the phase shifter do to the signal? Just overlap I on top of Q?

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

I understand that. Why do you think this isn't already a good IQ stream? The max2830 has the 90 degree hybrid built in (like every IQ demodulator I've ever seen)

Because at baseband they go around the max2830. A 90 degree hybrid for 100 khz all the way to 25 mhz isn't going to be small, so I can see why they didn't include it.

Ah. I missed that. But you wouldn't put a filter, you'd do what's called digital downconversion or DDC. This is doing the IQ demodulation in software. It's not a phase shift, it's a different type of block.

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.

Reply on news.ycombinator.com