How to calculate frequency of fft output, e.g. what is frequency at index 128?

[simpletk]

Here is simple FFT demo:

* first line is ignored
VU1 1 0 DC 0 AC 0.9 0 SIN(0 0.1 50 0 0 0)
RR1 1 0 1000
.control
tran 100u 100m 0
linearize v(1)
set specwindow=blackman
fft v(1)
print vdb(1)
.endc
.end

I run it from command line: "ngspice -b demo.src" and the output is:

... first 20 or so lines omitted
Index vdb(1)
0<->-2.93591e+02<-->
1<->-9.77563e+01<-->
2<->-1.06951e+02<-->
3<->-4.24296e+01<-->
4<->-2.56337e+01<-->
.... some lines omitted
124>-1.85311e+02<-->
125>-1.88034e+02<-->
126>-1.91794e+02<-->
127>-1.98066e+02<-->
128>-2.93557e+02<-->
129>-1.98610e+02<-->
.... some lines omitted
124>-1.85311e+02<-->
125>-1.88034e+02<-->
126>-1.91794e+02<-->
127>-1.98066e+02<-->
128>-2.93557e+02<-->
129>-1.98610e+02<-->

So first column is index from 0 to 512 and second column is amplitude in db, how do I find out what frequency correspond for example to index 128 or index 50?

[Holger Vogt]

What about

print frequency vdb(1)

?

[simpletk]

It works, thanks