How to read AC sweep graph - Page 1
Forum section: electronics / beginners

author: Allen_Du
author_rank: Contributor
timestamp: April 04, 2022, 03:32:47 am
content: Hi I am begginer for AC cirtuit, can someone help, want to know how to check a AC sweep result is good or not.I have a simulation cirtuit AC sweep analysis result as attached, VOUT1 magnitude is higher than VOUT2, does it mean VOUT1 is better than VOUT2?does small phase mean small delay?Thanks a lot!

author: srb1954
author_rank: Super Contributor
timestamp: April 04, 2022, 04:20:15 am
content: Quote from: Allen_Du on April 04, 2022, 03:32:47 amThis is a meaningless question without you providing details of the circuit you are simulating. The simulation results could be good or they could be garbage - there is no way of knowing based on just a AC sweep graph.Please post the simulation source file (.asc) or a screen shot of the simulated circuit and then we can make some informed comment.QuoteIt all depends on what your circuit is supposed to do. All that we can tell from the supplied sweep graph is that VOUT2 is a great deal smaller than VOUT1, and has been delayed a small amount, relative to VOUT1, due to the difference in phase between the 2 graphs.

author: Berni
author_rank: Super Contributor
timestamp: April 04, 2022, 06:31:40 am
content: You need to learn what a decibel is.The dB in that graph simply show the amplitude of your signal in comparison to the amplitude on your voltage source labeled AC. So if you imagine the AC source producing a 1V sine wave at that frequency and whatever point you clicked is showing 0dB it means there is a 1V signal there too. When it shows -6dB it means its half the original signal so 0.5VThis only applies to so called "small signals" so the AC amplitude is considered to be infinitely small. This means it does not suffer any limitations such as amplifiers clipping into the supply rails, diodes distorting etc.. For this reason you should also do a regular transient simulation where you instead apply a fixed frequency sine wave of the expected amplitude and check that your output is not distorted.

author: Terry Bites
author_rank: Super Contributor
timestamp: April 04, 2022, 12:15:21 pm
content: All you can say is that where ever Vout1 comes from, its got a wider bandwith than Vout2. The magnitude of Vout2 is slighly higher in Magnitude. Tip- Plot phase in a separate window. Set up cursors so you can see the actual values. Try not to use dark colours on a dark screen.The gain an phase don't tell the whole story. As Berni says, there are other electrical quantities to be measured. Whats better than what depends on your goal. You may want a redcuced bandwith for instance.On a display dBs are used to obtain a log scale on the y axis otherwise comparing large and small signals would be a real head ache.Its important to understand how and why dB is used. It's the expression of a ratio between two voltages. 20*log(V1/V2) . eg, If V1/V2 = 2, log2 is ~0.3, *20= 6db. use 10*log(P1/P2) when comparing powers. Note that dBs are not used express precise values. They add up and subtract, eg 22db+47db=69dB much easier than 12.589....x 223.872.....Common expressions you will hear are; dB gain, dB loss or attenuation, -3dB down, dB per decade or octave and dB compression.if you want you can read the story:https://en.wikipedia.org/wiki/Decibel