Electronic – Smith Chart Explanation

attenuatorRFsmith chart

I'm working on designing some hopefully simple RF (912 MHz) attenuators. I need a few different levels of attenuation but each different attenuator can be fixed.

I have prototyped one using just off the shelf resistors in a T-pad configuration that gives me a decent level of attenuation (pretty flat 19dB) when I perform an S21 measurement using my network analyzer.

However, the smith chart is all over the place when I measure S11.

Now I should mention that my prototype is very shotty. Basically I took apart a coaxial cable and hand soldered in some close-to-calculated carbon 5% resistors in between the two SMA connector ends.

My questions are as follows: What is a Smith chart and how do I use it to improve my attenuator+cable design? Is this a feasible method for creating basic fixed RF attenuators given that they don't need to be super accurate and only need to function over a very specific range of frequencies (905-920 ish MHz)?

As always, thanks for your help.

EDIT:

This is the SC of my taken apart coaxial cable WITHOUT the attenuator

enter image description here

This is the SC of my cable WITH the attenuator in the middle of it enter image description here

Here are the log plots of attenuation over the frequency range I am interested in:
First no attenuator: enter image description here

Second with attenuator: enter image description here

Also, another question struck me. If I'm just trying to reduce signal power at the output, does it matter where/how the loss happens? So I know poor impedance matching as indicated by my charts means a higher VSWR… but doesn't that just help attenuation? Thanks again.

Best Answer

A Smith Chart is not so much an attenuator design aid
as a means of evaluating and adjusting a design.

So - see attenuator articles below and then Smith chart articles.

Carbon resistors may be carbon film or carbon composition?

Carbon film are not suitable for UHF work as they are formed by cutting a spiral track in a carbon film cylinder, so have very substantial inductance.
Carbon composition have a solid carbon body and may be suitable for UHF work depending on other factors.

UHF Attenuators:

RF attenuator basic tutorial

Attenuator design tutorial - looks good.

Interest - Commercial products

Wikipedia

What is a Smith Chart?

Wikipedia gives a better than average concise summary:
From here

The Smith chart, invented by Phillip H. Smith (1905–1987),1[2] is a graphical aid or nomogram designed for electrical and electronics engineers specializing in radio frequency (RF) engineering to assist in solving problems with transmission lines and matching circuits.[3] Use of the Smith chart utility has grown steadily over the years and it is still widely used today, not only as a problem solving aid, but as a graphical demonstrator of how many RF parameters behave at one or more frequencies, an alternative to using tabular information. The Smith chart can be used to represent many parameters including impedances, admittances, reflection coefficients, scattering parameters, noise figure circles, constant gain contours and regions for unconditional stability, including mechanical vibrations analysis.[4][5] The Smith chart is most frequently used at or within the unity radius region. However, the remainder is still mathematically relevant, being used, for example, in oscillator design and stability analysis.[6]

Somewhat gentle introduction - 27 page powerpoint intro - still gets deep quite quickly BUT a Smith Chart can be very very useful with almost no maths or numerics involved.

Superb Smith Chart resource - essentially an index of indexes - breaks subject up into sections and provides many references for each.

Another good list of references

Smith Chart tutorial from Maxim - reasonable "dense" but looks understandable.

You'll understand this once you've read it :-)

enter image description here