  We report on the frequency performance of a low cost (~500$) radio-frequency
sine wave generator, using direct digital synthesis (DDS) and a
field-programmable gate array (FPGA). The output frequency of the device may be
changed dynamically to any arbitrary value ranging from DC to 10 MHz without
any phase slip. Sampling effects are substantially reduced by a high sample
rate, up to 1 MHz, and by a large memory length, more than 2.10^5 samples. By
using a low noise external oscillator to clock the DDS, we demonstrate a phase
noise as low as that of the master clock, that is at the level of -113
dB.rad^2/Hz at 1 Hz from the carrier for an output frequency of 3.75 MHz. The
device is successfully used to confine an ultracold atomic cloud of rubidium 87
in a RF-based trap, and there is no extra heating from the RF source.
