I designed a PCB where I have a local oscillator Input to an IC. The LO signal (2.48 GHz) has a power of -20 dBm. I put a cut SMA cable on the ground plane and measured a signal level of -60dBm. Is this normal?
Electronic – highfrequency signal on ground
groundgroundingRF
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2) I highly recommend AGAINST cutting ground anywhere near high-speed signals. Stray capacitance really doesn't have too much of an effect on digital electronics. Usually stray capacitance kills you when it acts to create a parasitic filter at the input of an op amp.
In fact, it is highly recommended to run your high-speed signals directly overtop of an unbroken ground plane; this is called a "microstrip". The reason is that high frequency current follows the path of least inductance. With a ground plane, this path will be a mirror image of the signal trace. This minimizes the size of the loop, which in turn minimizes radiated EMI.
A very striking example of this can be seen on Dr. Howard Johnson's web site. See figures 8 and 9 for an example of high-frequency current taking the path of least inductance. (in case you didn't know, Dr. Johnson is an authority on signal integrity, author of the much lauded "High-Speed Digital Design: A Handbook of Black Magic")
It's important to note that any cuts in the ground plane underneath one of these high-speed digital signals will increase the size of the loop because the return current must take a detour around your cutout, which leads to increased emissions as well. You want a totally unbroken plane underneath all your digital signals. It's also important to note that the power plane is also a reference plane just like the ground plane, and from a high-frequency perspective these two planes are connected via bypass capacitors, so you can consider a high-frequency return current to "jump" planes near the caps.
3) If you have a good ground plane, there's pretty much no reason to use a guard trace. The exception would be the op amp I mentioned earlier, because you may have cut the ground plane underneath it. But you still need to worry about the parasitic capacitance of a guard trace. Once again, Dr. Johnson is here to help with pretty pictures.
4.1) I believe that multiple small vias will have better inductance properties since they are in parallel, versus one large via taking up approximately the same amount of space. Unfortunately I cannot remember what I read that led me to believe this. I think it's because inductance of a via is linearly inversely proportional to radius, but the area of the via is quadratically directly proportional to the radius. (source: Dr. Johnson again) Make the via radius 2x bigger, and it has half the inductance but takes up 4x as much area.
Personally speaking, if you don't have an antistatic mat to sit the electronics on then it's a bit pointless. The idea is that the person (the potential generator of static) is equipotentially bonded to the equipment that is to be worked on thus equalizing voltage levels. It's of secondary importance that both you and the equipment are earthed. That mat serves as a go-between; you connect to the mat and the equipment is sat on the conductive mat.
Regards safety issues the earth point needs to be checked at regular intervals. If earth becomes disconnected at the socket then there is no safety issue but if earth gets disconnected downstream of the socket then there will be a capacitive connection between the supposed earth wire and the AC power.
This, once caused me a problem and although I could only feel a slight tingle, it did bust some chips when I connected my oscilloscope (which happened to be earthed correctly. I measured the voltage and it was 115Vac - exactly half of the 230Vac on the bench - this is due to the net coupling (via capacitance) to live and neutral being the same i.e. it formed a potential divider.
In short, my experiences tell me that using an antistatic wrist strap without an antistatic mat is pointless. The wrist strap has to connect to the mat and the mat has to connect to ground/earth to be properly secure against static issues.
Best Answer
At 2.5 GHz, the wavelength in free space is only about 120 mm or 5 inches. That means if your distances exceed about 12 mm or a half inch, you don't have a lumped system anymore.
It's not clear what exactly you did, but it seems that you are measuring some signal at -40 dB where you expected none at all. First, none is never realistic. Second, this not being a lumped system, exactly how you measured it is significant.
The fact that a 2.5 GHz signal shows up somewhere else nearby attenuated by 40 dB doesn't by itself seem surprising.
You say you measured this "on the ground plane". This brings up the question of how exactly you measured this, which you haven't stated. You also seem to be under the false assumption that every place on the ground plane should be at the same voltage all the time. That's the desire, and is often a useful approximation, but at 2.5 GHz you can't just wave your hand at such things. The exact geometry matters, as does the route of the return current to this oscillator.