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What frequencies are when the pcb design get tricky?

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The project on my hand now is a 20MHz SPI bus shared between four devices,now I have problems with this project and look for general guidelines. 1.How to prevent long traces from radiating when frequencies increase? Are striplines and coax the way to go? 2.Do you have any guidelines as far as trace length vs frequency? I assume that about 3 inch traces are fine with 20MHz, but what is the general case? 3.What is the RF characteristic impedance of a typical microcontroller output stage, anyway? Please feel free to tell me anything you know and I am missing.

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Jun 26 at 19:25
Giovanni Romanelli

At my work, the guideline is, if the electrical length of a trace is longer then 1/10 wavelength, you need to treat it as a transmission line. At a minimum, this means you must terminate with a resistor matched to the impedance of the line. How do you figure out what resistor value to use? You estimate what the impedance will be during design, and then you adjust the value to minimize ringing during DVT.

Jul 5 at 11:07
ondrasebesta

For your question 1,Are there guidelines as far as trace length vs frequency? I assume that 3 inch traces are fine with 20MHz , but what is the general case? If the electrical length of a trace is longer then 1/10 wavelength, you need to treat it as a transmission line. At a minimum, this means you must terminate with a resistor matched to the impedance of the line. How do you figure out what resistor value to use? You estimate what the impedance will be during design, and then you adjust the value to minimize ringing during DVT.

Oct 3 at 08:02
cgr-pcb

Dimensions which is above 0.1 wavelength of the highest frequency or harmonic. That doesn't mean the circuit will stop working at 0.2 wavelength. It depends on the sensitiveness of the circuit.

Nov 10 at 17:16
sean-maloney
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