Homemade 6 GHz pulse compression radar • Hacker News Board

Re: Homemade 6 GHz pulse compression radar

#21

Post by henrikf » Thu, Apr 04, 2024, 5:26 PM UTC

_Microft wrote: post #17

nick__m wrote: post #14

Why the ground planes are on layer 2 and 6 instead of 1 and 6 ? Naively, as someone who doesn't have high frequency PCB design experience, I would have placed my grounds to form a shield and put my voltage plane on layer 3 or 4. I am sure that there is a good reason behind that choice but I don't see it.

> Above is the final DDR3 routing on all the PCB layers. Layers 2 and 6 are ground, 5 is supply voltage, and others are reserved for signals. Two grounds are needed for correct impedances on the top, middle, and bottom traces of the PCB. With only one ground plane, the distance from the signal to ground would be too large on either the top or bottom layer. Is the textual description in the article correct? To me the…

That's a mistake in the text. You're correct that layers 2 and 5 are ground planes.

Re: Homemade 6 GHz pulse compression radar

#22

Post by _Microft » Thu, Apr 04, 2024, 5:26 PM UTC

nick__m wrote: post #20

_Microft wrote: post #17

Earlier quoted context omitted.

> Above is the final DDR3 routing on all the PCB layers. Layers 2 and 6 are ground, 5 is supply voltage, and others are reserved for signals. Two grounds are needed for correct impedances on the top, middle, and bottom traces of the PCB. With only one ground plane, the distance from the signal to ground would be too large on either the top or bottom layer. Is the textual description in the article correct? To me the…

thanks, layer 2 and 5 would fit the rest of the description and the reason would then be in the text : Two grounds are needed for correct impedances on the top, middle, and bottom traces of the PCB

Here are some resources on PCB layer stack-up by the way. While I'm an amateur, they didn't sound unreasonable. Chapter 10[0] has a list of links to different layer stack-ups for boards with 4 to 10 layers.

[0] https://web.archive.org/web/20200124214936/http://www.hottco...

Edit: removed wall of links

Re: Homemade 6 GHz pulse compression radar

#23

Post by henrikf » Thu, Apr 04, 2024, 5:30 PM UTC

nick__m wrote: post #14

Why the ground planes are on layer 2 and 6 instead of 1 and 6 ? Naively, as someone who doesn't have high frequency PCB design experience, I would have placed my grounds to form a shield and put my voltage plane on layer 3 or 4. I am sure that there is a good reason behind that choice but I don't see it.

Putting ground planes on top and bottom layers isn't usually done with high speed PCBs because components are there. There would need to be a cutout on the ground plane near every chip. High speed signals really need continuous ground plane and ICs, especially RF ICs, need short access to ground. Second layer is the best layer to minimize the distance from ICs to ground plane.

Re: Homemade 6 GHz pulse compression radar

#24

Post by kurthr » Thu, Apr 04, 2024, 5:33 PM UTC

nick__m wrote: post #14

Why the ground planes are on layer 2 and 6 instead of 1 and 6 ? Naively, as someone who doesn't have high frequency PCB design experience, I would have placed my grounds to form a shield and put my voltage plane on layer 3 or 4. I am sure that there is a good reason behind that choice but I don't see it.

Obviously not the designer, but my guess is to lower capacitive loading on the matched high frequency signals (those with squiggles on top and flood underneath) and make modeling and IC mounting easier without flood. Note there's not flood near those either. I'm not quite sure on the ordering in the picture but it looks like 123/654? (edit ahh looks like the text was wrong and it is 123/456 with 2&4 Gnd). I'll note it looks like the internal "low speed" digital signals are squeezed between the gnd/pwr (edit: between gnd/gnd) planes, which is probably good since they're usually the biggest source of "noise" if you keep it away from other PCBs. You'd definitely want power/gnd planes immediately next to each other since bypass caps don't work at anything close to this frequency.

Re: Homemade 6 GHz pulse compression radar

#25

Post by henrikf » Thu, Apr 04, 2024, 5:36 PM UTC

AlwaysNewb23 wrote: post #18

This is really impressive. Do you plan to use it for anything or another project?

I built it to see if I could. I didn't have any particular use case in mind.

Re: Homemade 6 GHz pulse compression radar

#26

Post by itishappy » Thu, Apr 04, 2024, 5:45 PM UTC

user_7832 wrote: post #11

itishappy wrote: post #9

Earlier quoted context omitted.

Not without significantly complicating your antenna setup (and the data processing setup too). You guessed it, getting multiple directions simultaneously is hard. Note how the current system only detects distance and speed in 1 dimension. Here's an analysis from someone smarter than me: > To enable the new features, radar systems now use multiple input/multiple output (MIMO) antenna arrays for high-resolution mapping…

So would this mean that with a few more transmitting and receiving antennas it could have comparable resolution to lidar?

In theory, though it sounds like to compete with LIDAR it will need about 1000x more antennas, with a related increase in electronics.

Re: Homemade 6 GHz pulse compression radar

#27

Post by secondcoming » Thu, Apr 04, 2024, 5:48 PM UTC

spitfire wrote: post #12

throwup238 wrote: post #2

This is great. It’s the last piece I needed for my suburban missile guidance system! This will be the last year the Joneses survive the annual block party. Can you do phased array radars next? I need the extra precision. There’s a few neighbors who don’t clean up after their dogs…

These should help. Tactical and Strategic Missile Guidance, Seventh Edition https://www.amazon.com/Tactical-Strategic-Missile-Guidance-S... Tactical missile warheads https://www.amazon.com/Tactical-Warheads-Progress-Astronauti...

Ha, I mentioned the first book the other day on here.

Re: Homemade 6 GHz pulse compression radar

#28

Post by Modified3019 » Thu, Apr 04, 2024, 5:50 PM UTC

throwup238 wrote: post #2

This is great. It’s the last piece I needed for my suburban missile guidance system! This will be the last year the Joneses survive the annual block party. Can you do phased array radars next? I need the extra precision. There’s a few neighbors who don’t clean up after their dogs…

>phased array

Jon Kraft apparently is doing a series on that: https://www.youtube.com/@jonkraft

Re: Homemade 6 GHz pulse compression radar

#29

Post by amirhirsch » Thu, Apr 04, 2024, 5:56 PM UTC

Did you test this with arbitrary waveforms? Is it possible for you to accumulate complementary Golay Codes?

Re: Homemade 6 GHz pulse compression radar

#30

Post by CobaltFire » Thu, Apr 04, 2024, 5:58 PM UTC

throwup238 wrote: post #10

gertrunde wrote: post #6

Earlier quoted context omitted.

There is this: https://www.crowdsupply.com/krakenrf/krakensdr (Although they had to take the radar elements out of the firmware/software, most likely due to ITAR - ref link: https://forum.krakenrf.com/t/where-has-the-passive-radar-cod... )

Thank you for the link! I totally forgot about kraken. I cloned their repo in November 2022, probably in anticipation of the ITAR hiccup. Hopefully the radar code is there.

I’d be interested in knowing if you’ve got that. I was going to buy one but the pull of the code put that on ice. I may pull the trigger if I can get the code!