Optical frequency combs (OFCs) are involved in a large diversity of applications such as metrology, telecommunication or spectroscopy. Different techniques have been explored during the last years for their generation. Using an electro-optical modulator (EOM), it is possible to generate a fully tunable OFC for which the optical repetition rate is set by the frequency of the applied electrical radio frequency (RF) signal. In order to realize on-chip OFC generators, silicon photonics is a well-suited technology, benefiting from large scale fabrication facilities and the possibility to integrate the electronics with the EOM. However, observing OFCs with a repetition rate lower than 10 GHz can be challenging since such spacings are smaller than the typical resolution of grating-based optical spectrum analyzers. To overcome this issue, two alternative solutions based on heterodyne detection techniques are used to image the OFC on the electrical RF domain. The first technique consists in applying two frequencies close to each other simultaneously on the modulator, and observing the beating between the resulting two combs. Another method consists in observing the beating between the OFC and the input laser, once the frequency of this input laser has been shifted from the center of the OFC by means of an acousto-optic modulator. Based on both measurement techniques, OFCs containing more than 10 lines spaced with repetition rates from 100 MHz to 15 GHz have been observed. They are generated using a 4-mm long silicon depletion-based traveling-wave Mach-Zehnder modulator (MZM) operating at a wavelength of 1550 nm.