The Si5351B have an interesting feature in that it can be FM modulated via the VCXO pin. After some discussions with Jason NT7S about the VCXO feature in his last revisions of the Si5351 library we decided I had to try that.

The VCXO pin is DC coupled and swings the crystal frequency a programmed amount to either side of the center frequency. As such, the input should be biased to a given point (VCC/2) and the input AC coupled.

Points to the first one that can tell what side the sine of the modulation signal are clipping  in the above plot (I should get around to fixing that signal generator).

By modulating with 1KHz and 40PPM pull range, I obtained a deviation of 2.5KHz with 2Vp-p  modulating signal. 5KHz was obtained with 4.3Vp-p. It is quite important to avoid overdriving the input, there will be excessive distortion if the input amplitude is larger than 3Vp-p. In order to obtain 5KHz deviation, a 60PPM pull range should put the needed modulation at less than 3Vp-p.

In addition, the input signal should be filtered, limited and added pre-emphasis.

(This article was written in january 2017, but newer published for some reason.)

Note 28/10-19: If you need a Break out board for the Si5351B try this link.

For the C version. Yes you would have to build it your self.