Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: How to interpret AC Analysis?
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sun, 22 Sep 2002 14:51:04 +0100
NNTP-Posting-Date: Sun, 22 Sep 2002 14:51:13 BST
"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:-)
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.