FM in matlab/simulink

Started by lolo_mk1 June 7, 2005
Hi all,

I've been trying to run an adjacent channel power ratio simulation
in matlab simulink but have been running into problems.

I'll just outline the parameters im using...

modulation signal: 1kHz sine wave @ 57600Hz sample rate

FM modulator passband block: 30kHz carrier @ 230400Hz sample rate
(so I can see more of the spectrum).

When modulating with a sine wave like this I expect the spectrum to
have alot of power around the carrier frequency and close to nothing
elsewhere. However I find at 90kHz there is a sizable 'bump' around
60 dB below the peaks at my carrier.

I don't know what causes this...it is a multiple of my carrier so I
guess it could be some sort of harmonic, but I don't think it's
meant to be there ?

The same thing occurs if I shift my carrier to 1MHz, there is a band
of energy at 1.06MHz.

This is giving me incorrect values on my adjacent channel power
ratio calculation as when I add non-sine wave modulation, power from
this second band is included in my calculation.

I know something is wrong as when I look at a similar situation on a
real spectrum analyzer there is only power centered around the

Could anyone please provide some insight on this? If it's a problem
with matlab is there a way to fix it?