A digital ionosonde of the type known as the Canadian Advanced Digital Ionosonde (CADI) is operational at S¿o Jos¿ dos Campos (23.2¿S, 45.9¿W), Brazil. This ionospheric sounding station is located under the southern crest of the ionospheric equatorial anomaly. The F layer electron density profile presents considerable day-to-day variability, even during undisturbed conditions, and this variability is still one of the less understood aspects of the physics of the ionosphere. The propagation of waves into the ionosphere may play an important role in this day-to-day ionospheric variability. In this paper we present a new technique that uses multifrequency virtual height variations, from ionospheric sounding observations at this low-latitude station, to investigate how the F layer is modulated by planetary wave type oscillations. We have also considered the possible influence of oscillations due to solar origin (solar rotation variation). In the present study, observations during the months of June to September 2003 (season of low-range type equatorial spread F occurrence at this location) have been used. The present study indicates the presence of 2-day (the observed 3-day periods are possibly associated with the quasi 2-day oscillations), 5-day, 10-day, and 16-day periods related to planetary wave type oscillations due to tropospheric sources during all the day. Also, it appears that during the day and dusk times, the longer periods 24--30 days are related to solar rotation (27 days), whereas the shorter periods 9--17 days have possibly some influence from half solar rotation (13.5 days). During the nighttime, possibly the short period oscillations 3--9 days may have some influence from the geomagnetic disturbances.