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Not sure what type of modulation you are looking for, but there are two that can be manipulated, either individually or in conjunction:

Frequency modulation index refers to the relation between the sine wave frequency (sine_freq) and the triangle (or saw-tooth) wave frequency (triang_freq).

The frequency modulation index is equal to ((triang_freq)/(sine_freq)).

Amplitude modulation index refers to the relation between the sine wave amplitude (sine_amp) and the triangle (or saw-tooth) wave amplitude (triang_amp).

The amplitude modulation index is equal to ((sine_amp)/(triang_amp)).

Varying the modulation index (normally by varying the frequency or amplitude of the triangle wave form) changes that respective modulation index.

From personal experience, an appropriate amplitude modulation index for an SPWM waveform should be around 0.8(that is, if the triangle has an amplitude of 10, the sine would have an amplitude of 8). This index should never be equal to 1 (one); it should always be less. A.K.A.: the triangle-wave amplitude should always be greater than the sine-wave.

On the other hand, a triangle-wave frequency much greaterthan the sine-wave frequency makes an SPWM that in turn generates a "cleaner" synthesized sine-wave in the H-bridge you are probably using. Try different freq. modulation indexes, but an index of at least 10 should be used (preferably somewhere around 100 if you want a good SPWM). That is, if the sine-wave frequency is 60 Hz, the triangle-wave frequency should be above 600, preferably 6,000 or more. Complications in the filter design in the "output" of the H-bridge will vary greatly when playing around with the frequency modulation index. That being said, keeping the amplitude modulation index at a static 0.8, and playing around with the triangle-wave frequency should be your best bet.

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