pcb rules for EMI immunity

[bmandl]

Hello.
I am designing my own functional generator with MAX028 IC and I am sceptical about designing rules for PCB. What should I pay attention to? I am planing to make it for at least 10Mhz and above and I don't know how to start. How should I place polygons on two sided board, on which side to put components (SMD and throuhole), how to place everything in a box and how to connect BNCs on chassis and then coaxes to PCB? These are all questions that bothers me.
I am asking you guys for advices. Thank you.

 

[tpetar]

See this materials:

A Practical Guide to High-Speed Printed Circuit Board Layout
**broken link removed**


High-Speed Layout Guidelines - TI SCAA082
**broken link removed**



PCB v2.0 will be better then v1.0


Best regards,
Peter

;-)

 

[bmandl]

Thank you, I will read these. What about grounds? What is diference between signal ground and power ground? Are they phisycal the same ground, but different in schematic? Where to put each other on schematic?

 

[bmandl]

What would you advice to me for this specific design? I can read all the pdfs and guides but I still don't know how to design my PCB. What should I pay attention to on this my schematic? Should I add something to schematic? What is the point of signal ground and ground in Altium? They are connected together on PCB, so there is really no difference between them.

 

[DMC Engineering]

One thing you can do right off the bat (and I would do this before starting the actual placement/layout phase of the design) is set up the design rules for the project using a known good template. Sunstone circuits, which is a fabricator based out of Oregon, has an Altium-specific design rule download on their site. At the very least, this will make sure you're not trying to do anything in your design that an "ordinary" vendor won't be able to manufacture. If you're not sure exactly how to use the design rules, or want to know a little more about how Altium uses them, there's a good overview here.

After looking at your schematic, here are a few things I can comment on that will help you with EMI-related design decisions
- It looks like you're using an external +/- 5V power supply. Make sure you physically separate the board connectors from any high-frequency components of your board (oscillators, fast-switching logic gates, etc). This will reduce the amount of noise pick-up between the two regions of the board (if you're using a switched-mode power supply, there's always a chance that could "jump" over to other traces in your design).
- If you can swing the extra cost, use internal layers for your Ground and Power planes. Punching straight down to the planes reduces the trace length from components to their supplies (great for EMI reduction) and the planes will inhibit EMI pickup.
- When you're done routing the board, add ground pours to the top and bottom copper layers. Again, this will inhibit EMI pickup.
- Route your critical traces and components first (high-frequency lines, analog circuitry, SMC connectors, etc). The pushbuttons on your schematic are very low priority compared to the oscillator line for your PIC

Even though the datasheets linked above might seem generic, make sure you stop a few times during the layout phase and check to make sure you're not violating any of the rules. Designing PCBs is part art and part science, but if you ignore the best practices, you're setting yourself up for failure.

 

[FvM]

Designs like the present one are best with a common, continuous ground plane, alternatively meshed ground traces and copper pours.

Maxim circuit examples have placed a 50 ohms series resistance before the output connector, which is strongly suggested for medium speed generators and also reducing the risk of IC damage by an inadvertendly shorted output.

 

[bmandl]

Thank you very much, but I really don't know where to start. I don't know, which components to mark as digital and which as analog in this schematic (It is not mine). Should I use rooms for separating different parts of PCB? Is there any option, that I can separate critical parts in early stage before PCB drawing in the time of schematic drawings and Altium would make groups himself, which I can then just arrange on the board and connect them? Where to put AGND and DGND symbols in this schematic? How do I connect BNC's? Do I isolate them from chassis and connect shield on PCB or connect them on both sides? Where to put ground from shield of coax on pcb? On the supply ground or where? There is so much that I must know, before start PCB part of the project. It is in no help for me to read tons of PDFs on that theme if I don't know what to do in this specific case.

 

[bmandl]

Ok, I managed to somehove place all of the components on the board in sections (digital, analog, power supply) and I made space between them. Unfortunately, I don't have an option for buried and blind vias, so I decided, that 2 layer board is better option than 4 layer without blind and buried vias. Now, can someone please have a look at my board placement and tell me, what I have to make better, is anything critical, and so on. Any advices in routing part, maybe? Oh, and I added 50Ω resistor in series with max038 output. The clearance between resistor and output of max is far long. What it is better, to place resistor close to max038 or close to output connector?

 

[bmandl]

Finished. I hope someone can look at it and comment my potential mistakes. Thank you.