# Center Freq and Bandwidth Functions

Question asked by REID_GE on Jul 21, 2008
Latest reply on Jan 20, 2010 by teka
Note: The purpose of all of this is to get Q by doing Fc/BW. Please let me know what i am doing wrong and where my syntax is incorrect. If I am thinking about it wrong please let me know. Thank you in advance for your help.

I have S11 data over a range of frequencies, 10 Hz - 150 MHz. After loading the data into ADS from an Agilent VNA I plotted S11 in dB. Note: DUT is a 1-port device.

Graph of S11:

I would like to use ADS to grab the center frequncy. From my understanding I must use the following:

center_freq(db(S11),3)

Is this syntax correct? For some reason I am getting 42.47 MHz if I use this method. Which is obviously wrong according to the above graph.I am expecting to get around 63 MHz. I don't know what is going on. Is it because I am using S11?

If I change the syntax to the following:

center_freq(db(zin(S11)),3)

Then I get a value of about 63 MHz. Why? The graph of zin(S11) is below.

Graph of zin(S11):

I also have question about the bandwidth function. i tried using the following syntax:

bandwidth_func(db(s11),1)

I thought this would provide the 3dB bandwidth. From my understanding, stating '1' tells ADS to evaluate it as a bandstop (as you can see from the first graph). I end up getting 18.32 MHz as my bandwidth. I don't think that is right.

When i use this syntax:

bandwidth_funct(db(zin(s11)),1)

I get 4.8 MHz as the bandwidth. It seems to look like a much better answer but after ooking at the 2nd plot above, and having the syntax '1' stating that it is a bandstop raises eyebrows.

Also, if I have a S11 plot that does not go down as low as -3db then I can assume that I cannot use the function to calculate the bandwidth correctly?

For example, the following S11 plot.

Zin plot

I get a bandwidth of 0 and a center frequency of 30 MHz with this syntax:

center_freq(db(S11),3)
bandwidth_func(db(S11),1)

But I get a bandwidth of 327 kHz and a center frequency of 62.84 MHz when I use this syntax:

center_freq(db(zin(S11)),3)
bandwidth_func(db(zin(S11)),1)

Why is that?