PCB trace for carrying 24MHz clock signal

时间：04-04 整理：3721RD 点击：

Hi!
My work is generally limited to power electronics and therefore I do not have much of experience with high frequency signal transmission.
However, an application requires carrying 5 digital lines, from one portion of a 4-layer PCB to another portion. Length of traces will be approx 50mm. Frequency is 24MHz.

1. Do I need some special treatment for routing the lines or can I just route them simply as I have been routing other traces?

2. I used a simple microstrip impedance calculator software an it shows that at 24MHz the capacitance is approx 2pF/inch. For two inch trace it should be 4pF. This should be 1.8Kohm impedance at 24MHz, but the software shows an impedance of only 70Ohm. What am I missing?

3. Please give some guidelines how to route the traces with minimum mutual interference. Signal voltage = 3.3V

Thanks

You are missing the basics of transmission line theory. Characteristic transmission line impedance Z0 = √L/C.

50 mm is rather short. Worst case your transmission line is unmatched at both ends, e.g. shorted by a low impedance source and open at the load side. A fast edge causes reflecting waves running multiple times back and forth, slowly decaying. In practice, a CMOS gate driving the line has already 10 - 30 ohms output impedance which might be sufficient to limit the ringing. Inserting a series resistor that adds up source impedance to the transmission line impedance can absorb multiple signal reflections completely, at least in a point-to-point connection.

You are confusing Reactance with Impedance.
Both are measured in ohms but they are different things.