SFFM: Frequency Modulation time dependent value

## 4.11 `SFFM`: Frequency Modulation time dependent value

### 4.11.1 Syntax

`SFFM` args
`SFFM` offset amplitude carrier modindex signal

### 4.11.2 Purpose

The component value is a sinusoid, frequency modulated by another sinusoid.

### 4.11.3 Comments

For voltage and current sources, this is the same as the Spice `SFFM` function, with some extensions.
The shape of the waveform is described by the following equations:

    mod = (_modindex * sin(2*PI*_signal*time));
    ev = _offset + _amplitude
            * sin(2*PI*_carrier*time + mod);

### 4.11.4 Parameters

`Offset` = x
  Output offset. (Default = 0.)
`Amplitude` = x
  Amplitude. (Default = 1.)
`Carrier` = x
  Carrier frequency, Hz. (required)
`Modindex` = x
  Modulation index. (required)
`Signal` = x
  Signal frequency. (required)