High Frequency Printed Circuit Boards: Explained

[jeff1evesque]

The connection pads on low frequency printed circuit board (PCB) create enough capacitance so as to make it unusable at high frequencies. The back side of the PCB is often solid conductor to provide shielding.

Question
Can someone explain to me why the capacitance makes it so high frequencies do not function in circuit boards?

Thanks,


JL

 

[negitron]

Capacitive impedance has an inverse relationship to frequency. The higher the frequency, the lower the impedance through a given capacitance. Thus, at high frequencies, PCB design needs to minimize stray capacitance to maintain signal isolation.

 

[jeff1evesque]

Capacitive impedance has an inverse relationship to frequency. The higher the frequency, the lower the impedance through a given capacitance. Thus, at high frequencies, PCB design needs to minimize stray capacitance to maintain signal isolation.

How are pads low frequencies?

 

[negitron]

Low frequencies, below a few MHz, don't generally require special design considerations with respect to stray capacitance.

 

[berkeman]

How are pads low frequencies?

Not sure I understand the question as stated, but the bigger the pad, the higher the capacitance, because capacitance ratios linearly with area and inversely with spacing (between the pad and the GND plane, for example).

 

[jeff1evesque]

Not sure I understand the question as stated, but the bigger the pad, the higher the capacitance, because capacitance ratios linearly with area and inversely with spacing (between the pad and the GND plane, for example).

I am just trying to understand the basic concepts. That is If the pads have low frequency running, then the capacitance reactance will be higher, indicating greater impedance- more opposition towards the sinusoidal current. But I cannot see how this makes it impossible for high frequencies.

 

[negitron]

What happens is high frequencies can get capacitively coupled from one pad to an adjacent one where you don't want signal getting in.

 

[berkeman]

I am just trying to understand the basic concepts. That is If the pads have low frequency running, then the capacitance reactance will be higher, indicating greater impedance- more opposition towards the sinusoidal current. But I cannot see how this makes it impossible for high frequencies.

The pad capacitance is in parallel with the signal path, not in series with it. So the lower parallel impedance attenuates the signal.

You also have to deal with transmission line effects at higher frequencies, when the signal trace length gets to be some fraction of a wavelength of the frequency of the signal. For digital signals, you deal with the rise and fall times, which typically are in the couple nanosecond range.

 

[vk6kro]

Capacitance itself doesn't introduce any losses, but shunt capacitance (to the back plane) will cause tuned circuits to require smaller inductors. At UHF and above, these are often just a short piece of track on the board and making them even shorter makes them less efficient.

Using Teflon (PTFE) boards reduces the capacitance to the back plane from any given size track. So tracks on teflon board can be bigger and more efficient for tuned circuits.

If there is less capacitance per square inch, say, for a given impedance line, the tracks can be wider. Having bigger tracks, means that skin effect is reduced too.

Probably more important, though, is that Teflon has lower loss than cheaper boards inherently, and also because it does not absorb water from the air. So, it doesn't heat up and dissipate the power of signals passing along the tracks.

Also, Teflon has a Dielectric Constant that stays fairly constant with frequency. So it is easier to design for.

So, like most things, you get what you pay for.