From: Terry Pinnell
Subject: Re: How to interpret AC Analysis?
Date: Sun, 22 Sep 2002 16:51:50 +0100
NNTP-Posting-Date: Sun, 22 Sep 2002 15:51:56 +0000 (UTC)
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"Kevin Aylward" wrote:
>"Terry Pinnell" wrote in message
>> CircuitMaker, like most other Spice programs I've seen, includes an AC
>> Analysis (or 'Bode Plot') in its repertoire. But, until Win Hill and
>> John Woodgate set me straight in a recent thread, I'd assumed it was
>> something of a no-brainer to use. I'd thought that, for any circuit, I
>> could simply select an AC Analysis on the output, set the x and y
>> scales respectively to 'Log' and 'Magnitude in Decibels', and I would
>> be able to read off the circuit's '-3dB point' (or points) with
>> However, as John puts it, I now see that "You must refer the output
>> levels to the output voltage, not the input voltage."
>I have not seen this post, and don't know to what he is refereeing.
>> So, how exactly do you go about doing that in practice please? After
>> pretty extensive reading, I've still seen no documentation that covers
>Set your AC input voltage to 1V. The plot at any node is the gain
>referred to that input voltage.
>> Is it perhaps a matter of first using Transient Analysis to determine
>> the circuit's output at the 'base frequency' (say 1 Hz in a low pass
>> filter), and then applying this in a calculation after the AC
>> Analysis? Or is there some reasonably straightforward procedure that
>> professionals follow, albeit not quite as simple as I thought?
>I think they're leading you up the garden path. It is as you stated at
>the bigging. Its a no-brainer, although a decent spice automatically set
>the correct defaults scales modes for dc, ac, trans and noise graphs:-)
I may not have explained it too well, but I don't think there's any
doubt about it!
Maybe after reading the relevant posts from about
onwards, you'll see. And I posted an illustration of the 'no-brainer'
approach I was using at
It clearly gave the wrong answer - as I soon found when I breadboarded
it. With hindsight it then became embarrassingly obvious that the
cut-off point should be independent of the gain setting.
But I'm sort of pleased to see that maybe it's not so obvious after
That still leaves me looking for 'the right way' to approach such
Hobbyist, West Sussex, UK
Terry Pinnell, Hobbyist, West Sussex, UK