Electrical – the effect of PCB trace length to a capacitors frequency response

Creating Delay

To create a delay you want to pick a filter with a large group delay at your frequency range.

Microwaves101 explains it to some extent.

Why a longer line helps

If you are using a longer line and dropping your noise floor you are actually creating a very basic isolator. Now the microwaves101 explanation of an isolator is a tied to an easy to implement microwaves component, the circulator. Now this is a relativly complicated device to think about implementing if you are used to frequencies that Microwave engineers consider DC(like 500kHz). The device that approximates an isolator for you is a diode. If you connect power to a line with a diode that is idea, any reflections/ringing/feedback will be blocked and dissipated by the diode.

What the Cable Does.

As you increase the cable length, you increase cable loss. Lets say you have a number like 1dB loss (which is a gain of -1dB). If reflections are your primary source of noise then a reflection off of your load will travel back to antenna and then back to the load again. This means just from the cable you have caused a 3dB loss(half power). This neglects the fact that the reflection is a rather large loss on both interfaces also. This "isolates" in the respect that as the line gets longer your reflected signals have a larger and larger loss, at-least 3 times what your signal feels.

How Do I Fix It?

This is probably a sign that your load, whatever you are measuring with, is miss-matched, A decent match, giving a reflection of around -20dB coupled with the loss hitting the antenna again should leave your reflections relatively small.

You can fix this with a tuning network, which adds discrete components nearby to attempt to create the correct impedance. You can correct this by fixing any flaws in your layout also. Probably both (based on experience with my own boards).

Another way to approach this is to really cheat with your line connecting the boards. If you are operating over a small frequency range(ie. Able to treat the signal as a single frequency), then you can use stub tuners(which have been mentioned in this thread, just not by name) where the length of the stub acts as a capacitor or inductor(this is very frequency and fab dependent). You can also have a lot of fun by using a length of cable that is a quater-wavelength long. This means that when a signal is reflected, it will return to the antenna with a 90 degree phase shift, then return to the load completing a 180 degree phase shift. This cancels itself out in stead state(not perfectly, but it gets the job done).

Summing it Up

At least you jumped in head first. There are a number of other things that can cause a miss-match, with more application specifics they can be looked into also. For example, a COAX cable causes a missmatch due to its shield having currents on the outside and inside that do not sum to the inner conductors current. I hope this has helped some, and I hope microwaves101 can be of some help. It is by far not the perfect learning resource, but someone is trying.

My favorite electronics book is "High Speed Digital Design: A Handbook Of Black Magic". I highly recommend this book. It seems expensive, but it is totally worth the money. This book has 12 pages on choosing a bypass cap! The author, Howard Johnson, also teaches some classes with decoupling caps as one of the topics.

Some important things that I've learned over the years, and have been backed up by this book, is that the "standard practices" with decoupling caps are almost always wrong and there is more art than science when it comes to choosing and routing them.

There are lots of calculations that you can do regarding decoupling caps, but much of those are not accurate due to many things. The caps themselves are vary wildly (especially the higher dielectric caps like X7R). The PCB layout changes things greatly (and you'll need to think in 3-D for this one). Temperature and voltage will change the behavior of the caps. A single cap will behave as both a "power supply smoothing cap" and a "AC signal return bypass cap". Etc.

What Johnson did was, after a lot of experimentation, figure out that inductance is the most important factor and it swamps almost every other consideration. So the goal when selecting and placing decoupling caps is to use a lot of physically small caps, with the highest practical value, and route them so the total inductance is as low as possible.

The ideal would be to use lots of 0.1 uF caps in an 0402 package. Place them under the chip on the back side of the PCB. The cap be routed as in the image below. And the vias go directly to the power/ground planes (not to the chip's power pins, as that would usually increase the inductance). If you place the cap under the chip then sometimes you could share the same via without any issues.

The reason why a 0.1 uF cap was chosen is because it is the highest practical in an 0402 package. The reason why 0402 was chosen is because it is the smallest practical size, and you want to use a lot of them to get the effective ESL/ESR down. Of course all bets are off if you have a 2 layer PCB without power and ground planes.

I don't want to belittle the use of the math, that is important, but the complexity of power supply decoupling and AC return paths often makes the math not so practical in the real world. In the real world, a "rule of thumb" really helps. Of the many rules of thumb for this topic, it has only been Howard Johnson that has proven the other rules don't work and provided this better rule. My experimentation and experiences has shown this to be true.