the mini-QSOs [@2003ApJ...596...34B; @2006ApJ...648L...1C; @2007MNRAS.375.1269Z; @thomas08; @2009ApJ...701L.133A; @2012MNRAS.425.2974T] and the HMXBs . Besides these, the metal free PopIII stars are believed to be the another common source during the cosmic dawn. It is however expected that the individual PopIII stars may not be detected in the observations of the redshifted 21-cm signal even with advanced telescopes like the SKA1-low because of the very small region of influence (see Paper I). Thus we have not considered them in this study. While we consider the PopII stars combined with the mini-QSOs in galaxies as our fiducial model source, we will briefly discuss the detectability of other sources too. In the following, we summarize the main properties of the sources used in this paper and refer the reader to Paper I for the details of their spectral energy distribution (SED).

We assume that the first galaxies form in a low-metal region with metallicity $10^{-3} ~ Z_\odot$ [@Lai07; @Finkelstein09b; @2010MNRAS.407.1003M] and consist of PopII stars. We assume that the stars follow a Salpeter IMF with masses between 1 to 100 ${{\rm M}_{\odot}}$. The SED of the stellar component of the model galaxy is generated using the stellar synthesis code [pegase2]{}[^8] [@Fioc97]. We assume that 