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High frequency circuit layout

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fergcu

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Hi everyone,
I have recently been asked to carry on a project which the original circuit designer had to leave (due to ill health). My task is to take the schematic and create a very small (fit in a pen size) circuit board. Im using a pic 18f2550 (or an 18f4550... making two to test which is most effective), and using a clock speed of 24MHz. We are going to be sending the board out to be made, so everything will need to conform to conventions. I have never actually designed a board like this before (all of my previous ones have been fairly basic pic boards with no need to consider space/EMC/conventions. And likewise i have never been taught about them.
I would be really appreciative if someone could give me some basic pointers on:
a) How to avoid EMC problems... should i thicken up certain ground tracks/avoid making loops etc?
b) Are there any conventions that need to be used for symbols etc? i have a diode silkscreen symbol, but have no idea which way round the anode or cathode is .

Any help/advice about surface mounting in general would be greatly appreciated.

kind regards

Fergus
 
You don't give any info about what the circuit is supposed to do?, but generally micro-controller circuits aren't troublesome as regards layout.

Assuming a 24MHz clock?, the ONLY part of the circuit at 24MHz is the crystal, two capacitors, and the pins of the PIC - nothing else runs anywhere near that speed.

Even better, the 18F series have PLL clock circuits, which multiply the crystal frequency - so you could use a 6MHz crystal to give a 24MHz clock.

I happily build 20MHz 16F series boards using veroboard (check my tutorial boards), I've NEVER had any layout problems (or heard of any).
 
24 mHz is not really high frequency and since the board is small, line length is not an issue. I assume you will have a multi-layer board, so if you have vcc and gnd as internal layers, and keep analog and digital grounds seperate, there should not be any problem. It is a good idea to use as much ground plane (and vcc is same as ground) as possible.

A customer of mine had one of my designs re-layed out; the switching transistor and filter cap were placed at opposite ends of the board! It sort of worked but had some hicups.
 
Have a look at the data sheet for the ADC1175. It gives various advice including how to interconnect the analogue and digital ground planes.

Also look at the National Semiconductor book "FACT Advanced CMOS Logic Databook" if you can find one. It gives useful advice re PCB design and also states that the best by pass capacitors are those with 5ZU dielectric.
 
Hey everyone,

Thank you for the advice/feedback. I will look into everything that you have all mentioned, test the prototype that is being manufactured, and get back to you if i have any problems.

Thanks again.

Ferg
 
I'd like to point out that there is a common misconception that low frequency circuits are not an EMC concern. With some bad layout practices, one can easily create an EMC nightmare with only 1 Hz clock on board. If the 1Hz clock has edge times in the ps range, it could spell trouble. 24MHz is not a terribly high fundamental but there could be significant energy in those edges.

Also be careful with the advise! what is considered no trouble for a garage shop / hobby design can be very different when it comes to passing standard EMC tests for immunity & emmisions. The OP didn't say what the design needs to be tested to. The best that can be had is good layout advise. The clock multiplication inside the device is excellent advise.
 
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