
Building a software-defined radio from scratch - dominicgs
http://electronics.kitchen/misc/freesrp/
======
laobeylu
Hey! I'm Lukas, the guy who made this. Just discovered someone posted it on
Hacker News, pretty exciting for me. If you want to ask me any questions or
make comments, I'm here and on reddit:
[https://www.reddit.com/r/electronics/comments/4uic5r/sdr_pla...](https://www.reddit.com/r/electronics/comments/4uic5r/sdr_platform_ive_been_working_on/)

~~~
xemdetia
Just a technical writing tip, when you are using any sort of acronym that's
the subject of the piece it's helpful to do something like SDR (Software
Defined Radio), then use SDR through the rest of the text. Either that or do
the blog-style hyperlink the first acronym to the wikipedia page or give a
footnote.

~~~
laobeylu
Thanks -- you're right. I've added a link and spelled it out completely in the
first sentence now.

------
ericjang
This is a really impressive project, and I am all the more in awe that this
guy is only 18 years old.

The project is on Github:
[https://github.com/FreeSRP](https://github.com/FreeSRP) Reddit discussion:
[https://www.reddit.com/r/electronics/comments/4uic5r/sdr_pla...](https://www.reddit.com/r/electronics/comments/4uic5r/sdr_platform_ive_been_working_on/)

He's selling a small batch of these for $350 apiece - calling it now, he's
going to be very successful in his career.

~~~
laobeylu
Thank you for the kind comment. Not selling them yet, though, and the small
batch might end up being a little more expensive than that (but I'm really
trying to keep it under $400).

------
noonespecial
_I also had done more revisions of the board and spent way more money than I
initially planned for... even though in hindsight, having only three revisions
for a fully functioning, moderately complex design, with no previous
experience working on something like this, probably is not that bad._

Are you kidding? That falls into the realm of superhuman. I've seen 20 year
veterans blow a 100 grand and get half that far. I think even Woz would do a
double-take.

~~~
bsder
> Are you kidding? That falls into the realm of superhuman. I've seen 20 year
> veterans blow a 100 grand and get half that far. I think even Woz would do a
> double-take.

A high density BGA through an OSH Park board with a homebrew reflow oven and
only 3 revs and a stainless stencil on only the last pass? Yeah, that's really
quite good.

You practically can't avoid making a mistake on a PCB of that complexity.
There are simply too many details buried on too many datasheets and one of
them is going to drop.

If you have the time, the best thing I have found for catching mistakes is to
have a design review. Even if the person doesn't understand electronics, the
fact that you have to walk through every part and _explain it_ to somebody
means that you will catch some really stupid things.

However, if you're good and the board isn't too expensive, it's often cheaper
just to fab the board. You're time is worth something, and you can be
designing something else while the PCB is being fabbed.

~~~
laobeylu
That's actually sort of what I did. I didn't explain it to someone else, but
went through it in my head, talking to myself explaining the design.
Explaining it to someone else must be even better!

~~~
noonespecial
Or just a rubber duck.

[https://en.wikipedia.org/wiki/Rubber_duck_debugging](https://en.wikipedia.org/wiki/Rubber_duck_debugging)

Personally I use a cat. Its Steve-Jobs-like attitude seems ideally suited for
the task.

------
xt00
Awesome project man. I do EE HW design for a living and this I can attest is a
hard project. I have a bladeRF and use it regularly. One thing I try to
instill in the people that work on my team is that the best attribute you can
have and try to foster in your career is not giving up on hard things. So it's
great that that is something you have.. To stick with this project for a long
time and putting in the time to make it happen. MIT will be a challenge but
I'm sure if you stick with it you will end up doing great. Go for something
tough and glad you are coming to the US. I have to apologize for politicians
that say we don't like foreigners. We do like foreigners and we need them. You
will be a great addition to the many awesome people coming here to find there
path to great success!

------
batbomb
I think the USRP1 motherboard, when I originally used one in 2008, used to be
$700 complete/$550 for the motherboard. The price now is $719, but that
includes the enclosure, cables, power supply, etc...

I actually cut my teeth on Python learning GNURadio.

~~~
laobeylu
That's true, I'll update the USRP price on the website.

------
jaytaylor
I am curious; what would one actually do with this device once it is working?

This is a really neat sounding project.

~~~
simcop2387
With 56MHz bandwidth, a lot. You could record and analyze most 802.11
connections (typically 20MHz to 40MHz, except for newer AC hardware that can
go to 160MHz).

You could also record the entire FM band in your area at once, all of it. Same
with AM and a lot of other bands. It'd also probably work really well for
recording things like fire and police bands, with trunking and being able to
determine an entire conversation at a time that normally need 2 or more
cheaper SDRs to record the trunk and then the channels. That would normally
mean you'd probably miss some channels that are at the other end of what
you're listening to. It'd also mean you can monitor the entire bandwidth of
many of the ISM bands to determine what some misbehaving device is doing.

So many things that I'd love to do with it! I really want one now.

~~~
userbinator
_You could also record the entire FM band in your area at once, all of it._

I'm not very familiar with RF beyond the basics, but could you explain simply
how does that work? Don't you need to tune into a specific frequency to
receive a signal?

~~~
tonmoy
Basically a traditional radio would multiply a signal with the frequency you
want to tune to and give you the resulting signal which is an audio signal.
With an SDR, you can record the actual RF signal and then store/multiply
whatever you want to. So you can store everything and later tune to any
frequency using software later!

------
asimuvPR
Amazing project!

1\. What are your plans for it? Aside from high altitude balloons.

2\. Do you plan to turn this into a sort of "shield" add on?

3\. What will you work on after this project is done?

~~~
laobeylu
Thanks!

Well, I'm currently working on a new version that I'm hoping to sell. I also
still need to work on the software.

This won't be an addon, but the next revision will include an expansion header
so you can add addons. Should be pretty neat to make an LCD+battery pack
"shield" so you can use the SDR as a stand-alone spectrum analyzer.

What I'll actually use it for: I built it because I wanted to learn more about
signal processing and the different protocols that common devices use, so I'll
be tinkering with all those protocols.

No idea what I'll do after this, maybe that LCD addon I was talking about?

~~~
dbyte
Really an amazing job. Keep feeding your curiosity, it is really worth it and
try to travel a bit while explaining your project. Array processing is
something that deserves a better look. Many SDR solutions still lack a bit
under this point of view. Yet that will be quite useful to have. :)

------
btashton
Lukas, your schematic link seems to be broken on the project site. I have been
thinking about doing a similar project with this chipset actually. I really
want to put down about or 8 of these ADI parts and actually make lower cost
phased array development environment. One day....

~~~
laobeylu
That'd be a cool project... The link is working for me, the schematics should
be here: [https://s3.amazonaws.com/freesrp/prototype-r2/freesrp-
sch.pd...](https://s3.amazonaws.com/freesrp/prototype-r2/freesrp-sch.pdf) Are
you sure it's broken?

~~~
DanBC
Your link here, and the link in the article, both work for me.

This is a really nice write up of an impressive project!

------
mschuster91
Could this project, in theory, be done with a 2-layer (front and back) board?
I'm thinking of building a small SoC board... but I don't want to shell out
hundreds of bucks (and days of shipping time) for prototype PCBs, but hand-
etch them instead.

~~~
kersny
Its been done, but AFAIK not with hand etching: [http://hforsten.com/making-
embedded-linux-computer.html](http://hforsten.com/making-embedded-linux-
computer.html)

I don't think you can do high frequency RAM lines with hand etched boards or
BGA escape routing without vias, but two layer prototype boards are pretty
cheap these days.

------
enesunal
Is it the truth, or just pure genius? I choose second! What an amazing job,
well done!

------
Filter
SDR: Software-defined radio

