FM in matlab/simulink
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?