RTL SDR Expansion Card

Albert · September 26, 2023, 12:51am

Hello,
Over the past few weeks I’ve been working on an RTL SDR expansion card.
The plan is to post updates here.

I previously posted about this project here, after reverse engineering the RTL SDR schematic and getting most of the way done with designing a board.
Since I can’t find the chips online, the plan is to take the rtl2832u and r820t2 chips out of a cheap RTL SDR, which unfortunately raises the barrier to entry by quite a bit.
For that reason, among others, I don’t plan on selling these (at least not at a reasonable price or in large quantities) unless I get access to the chips. Either way, I’ll release the design so those crazy enough to desolder QFN chips can make one.

Since that first post, I finished and ordered a version of the board for testing:

…And desoldered (and measured) every component other than the QFN chips on the donor RTL SDR (sorry about the uncleaned flux)

TODO:

Design and print a case
Desolder the rtl2832u and r820t2
Wait for the parts to arrive
Solder the parts to the board
Test it
Release the design as open source, probably after I get it working

Kyle_Reis · September 26, 2023, 1:54am

You’ve probably already seen this, but @John_Mayson shared that the internals of a Nooelec NESDR Nano 3 (which uses the same rtl2832u and r820t2 chips) already fit mostly inside of the USB-A expansion card without any soldering required.

More info here.

It does stick out a little bit, although if you removed the USB-A connector from the SDR and soldered a USB-C connector in its place (instead of just using a small adapter) it would probably fit fully within an expansion card (it might fit within a headphone expansion card, the hole looks to be roughly the correct size). That would still be simpler than what you’re doing.

Albert · September 26, 2023, 2:29am

I saw that in the SDR idea thread, but it didn’t have photos and I couldn’t find it on archive.org, so this is the first time seeing just how much smaller the NESDR nano 3 is compared to the card. Knowing that a NESDR Nano 3 can fit within the card slot actually inspired me to go ahead with this project.

That would still be simpler than what you’re doing.

That is true, but I’m more doing this because I find it fun, and because I get to learn about designing PCBs. I also prefer an SMA connector since it gives me more flexibility with antennas.

Another thing to point out here is that I’m only putting components on one side of the board. The Nano needs to cram things in on both sides, and it looks like it uses a linear regulator for the 1.2v rail which would generate more heat.

AquilaFasciata · September 28, 2023, 8:29pm

I can’t express how much I would like this. Keep the updates coming!

Albert · September 30, 2023, 10:59pm

This is more of a minor update, but there won’t be much else to say until the PCBs arrive.

For the case, the SMA connector is the only thing I need to modify the example case for. I’m not quite sure how that connector will fit physically, so I’m just getting the example case printed for now.

I desoldered the chips a few days ago, and managed to clean up the board a bit too:

I’m somewhat worried that I damaged the chips with the amount of heat applied, but I do have a few extra SDRs ready for sacrifice if anything goes wrong.

Here’s the back for reference, already flipped and aligned with the front:

Bonus 3D render with component models:

AquilaFasciata · September 30, 2023, 11:52pm

Considering it’ll already have a connector on it, why not have the SMA on a small jumper wire? Alternately, you could look at using an SMC connector or a smaller one.

Kyle_Reis · October 1, 2023, 12:14am

Fair enough. I also prefer an SMA connector.

When I get my Framework 16 I plan to do a similar mod to what I linked (since I don’t have much experience with soldering) and 3D print an enclosure with room to contain the SDR with an SMA adapter connected. It’ll stick out byt most of the time I probably won’t have it in my laptop so that won’t be a big deal.

Albert · October 11, 2023, 11:54pm

The boards are finally here:

I really should have chosen faster shipping. Anyways, the other parts have been here for a while, so I have everything I need to start soldering it together tomorrow.

Kent_Brockman · October 13, 2023, 2:49am

Looks awesome!! Looking forward to seeing the final product

Albert · October 14, 2023, 12:35am

Yesterday, I soldered the QFN chips and the oscillator with hot air, then that evening I got the rest with my soldering iron. The QFN chips were a lot easier to do that I expected. The USB connector was the hardest part, especially with a soldering iron.

After checking for shorts, I plugged the board into a USB hub (plugged into my dock) so that if something went wrong, the hub would be damaged instead of my laptop. Surprisingly, it just worked! No issues so far. It does get very warm at times, but the board isn’t ever too hot to touch, even after letting it run for a while in the laptop without direct airflow.

This morning, I removed some material from the case to fit the SMA connector, which can be seen above. The top of the connector overlaps with the laptop slightly, but it doesn’t prevent the card from being fully inserted.

This doesn’t have shielding around the board, and putting it into a computer causes some amount of interference. But the goal of this project was to create a RTL SDR that fits in my laptop and works reasonably well, and I’m very happy with the result.

Next steps:

Design and print a better case
Release the design (which I want to clean up a bit first)
Work on impedance matching the antenna input properly, if I can get access to the necessary equipment. It works fine as-is, but the performance could probably be improved.
Assemble a second one (because why not, I have the parts to make four)

MJ1 · October 14, 2023, 12:52am

Very nice!

Are there components on the back? I didn’t notice any pics in the thread of the backside. You could use hot plate reflow for all parts that are on the top only.

Albert · October 14, 2023, 2:09am

There aren’t any parts on the back, so yeah that would probably be better, but I don’t really have the equipment for that. I’ll ask around, but for the second one I’ll probably just finish it with the iron (I did the hot air section of it earlier today).

James_Blackwell · October 29, 2023, 8:33am

If you do decide to sell these I’d put my name down!

Albert · October 30, 2023, 5:56am

I still don’t really have plans for significant production. I would consider doing small batches of a revised design given enough interest, but it would still involve soldering the main chips by hand.

Anyways, I do have extra parts… If you’re willing to pay like $100 for what is basically a $25-30 dongle, but in a form factor that fits in your laptop and with somewhat reduced performance due to the probable impedance mismatch, well, DM me and we can work something out. For the moment, I have an extra board already soldered together and enough parts for two more. If not, no worries. I’ll release the design either way.

PS: I’ve been relatively busy recently, so it’s been a while since I posted an update. If all goes well I should have more to say about the case soon.

Colin_Downey · December 18, 2023, 6:59pm

@Albert, this project you have going here is awesome. If you want or need any help i’d be happy to do whatever I can.

Keep us up to date, cant wait to see it finished!

Albert · February 4, 2024, 12:15am

Hi everyone,
Sorry it’s been so long. Designing the case took longer then expected, then getting it printed took longer than expected, then getting around to writing this took longer than expected. I’ve also been more busy with work and other obligations than when I started the project.

Anyways, the case is finally designed, printed, and tested!

Source

Here’s most of what you’d need to make your own. There is still some documentation to write, mainly about which components need to be ordered separately vs removing them from the donor SDR.

The Case

The case went through multiple redesigns. It’s been long enough that I don’t fully remember the details, but I’ll summarize the designs. The goal was to fully enclose the board. I also wanted to be able to remove the board, which excludes anything involving glue.

Abandoned designs

The first design was heavily based on the example designs from Framework. I added a cover to the top and a cover to the front (the side with the SMA connector), which would hold on the top cover. The idea was to hold the front on with a nut on the SMA connector.

After getting most of the way done with the first design, I realized that the SMA connectors I used didn’t have any threads exposed when in use. This meant I couldn’t add a nut. After this, I redesigned it so that the top and front were connected, but soon realized that the SMA connector could not physically be put into the top of the case without adding a large hole, which would partially expose the board.

Final design

To fix the problems in the previous designs, I wanted to remove the USB-C connector side from the bottom and attach it to the top, then attach the SMA connector side to the bottom.
This is similar to the original framework modules, with the main difference being the original modules split the usb c side along the connector.

I added some very thin tabs to keep the top from coming off vertically, and two detent things to keep the cover on. I was hoping the sides of the case would bend outward slightly when putting the top on, enough to keep the top from easily sliding out but not enough for the case to break. I also added some vent holes to the bottom, which unfortunately made the whole thing significantly more difficult to print.

All of that was based on the SelfTapping printable version of the example case, but I wanted to try removing the need for screws entirely. Since I had the top cover, it seemed like it could hold the board down from above while some plastic posts held the board in place horizontally. After making the necessary modifications, I printed out one of each. The screwless design worked better than I expected, so that’s what I’m using.

There were some complications while printing the cases, but overall I think they turned out fine. If I print any of these again I would change the orientation, disable the raft, and I might have messed up the nozzle size somewhere. Given that this was roughly my third time 3d printing anything, I’m happy with the result.

In terms of the case design, a few things could be improved. Mainly, the whole thing could be designed to 3d print much better. It also seems like geometry of the parts seem to be slightly broken. They 3d printed fine, but the slicer didn’t like the models when exported as some formats. I may come back to the case design in the future, but for the moment, I’d like to call this this project done.

Final thoughts

There are a few things that could be improved that I haven’t mentioned yet, mainly the stability of the SMA connector. Since the SMA connector is only connected to the board, all force put on the SMA connector will be applied directly to the board. Another minor concern is heat, but only if you were to insulate the module by throwing the laptop on a bed, for example. Otherwise, I’m happy with how it performs thermally, though there is some room for improvement.

Overall, I am very happy with how this project turned out. The goal was to make a RTL-SDR with reasonable performance, in the form of an expansion card. I also wanted it to be something I would reasonably use, and could toss in my backpack and forget about when not in use. The result easily covers that criteria. It’s been working great for when I occasionally want a SDR. If I want anything more advanced, I’ll typically get out my SDRPlay, but the convenience of an SMA connector on the side of my laptop outweighs the advantages of any of my other RTL-SDRs.

blinry · February 5, 2024, 11:08am

Hey @Albert, that’s extremely cool, thanks for publishing all the design files under CC BY!

I’d love to build one! Couple of quick questions:

Which specific cheap “donor” RTL SDR did you use? Can you recommend a model?
What kind of antenna would you attach?
I guess the rtl2832u and r820t2 chips are the two components removed from the donor? Or did you take more?
Would it be an option to order the PCB with many of the remaining parts pre-assembled?

Yeah, a table of the parts and where you got them would be a great piece of additional documentation! Can be really rough first, and be polished later.

Albert · February 5, 2024, 7:12pm

Hi, I’m happy to hear someone is interested in making one of these. I would wait until I have the time to document what components are needed from where, as there are some important details, but to answer your questions:

You should be able to use pretty much any model depending on how many components you want to take from it. I would go with the cheapest one you can find with a TCXO. I’ll put more information with the component documentation.
Whatever antenna you want. I’ve mostly been using the dipole that I got with one of my other RTL SDRs. You could probably get a 433 or 900 mhz antenna that mounts directly on the SMA connector, just be careful not to put too much force on the connector.
The only components that are strictly necessary from the donor are the RTL2832U and the R820T2, however finding a TCXO with the correct frequency is not easy (though not impossible) and there’s an inductor in the RF section with an inductance too low for me to reasonably measure. That should give you an idea of the minimum, but there’s quite a few other passives that could be reused if desired.
Yes, however you should take into account soldering the QFN chips. If you use hot air, it can be easy to knock off nearby components.

It’s also worth noting there’s some complications relating to the package of inductor L3. In short, I would advise choosing a component for the physical space, then modifying the board layout for the correct footprint, rather than finding a component that matches the current footprint.

Kyle_Reis · February 7, 2024, 12:32am

Just to note the R820T2 has been discontinued and replaced with the R860, however since the difference is purely relating the manufacturing (the end product is the exact same chip) it is fine to use a donor board with an R860.

Ryan_Dewsbury · February 8, 2024, 8:42pm

For your casing design it might be worth it to think about some heat sinking. These radios can run pretty hot but it doesn’t take much to sink it away somewhere.

Often the Nano sized SDR’s will have a rather large USB A connector to help with heatsinking.