I understand that a DDS generates a sine wave. It seems logical that modulating the sine wave should follow easily.
Modulation is, in fact, a logical extension of DDS.
Because DDS provides digital tuning of the output frequency, the most obvious modulation format for DDS is FSK (frequency shift keying). Especially those DDS products that employ profiles (programming registers activated via one or more external pins). Users can program two or more profiles, each with a different frequency tuning word. Then use the profile pins to jump from one frequency to another at will. Generic FM is possible, too, but requires a controller capable of programming DDS frequency changes quickly enough to yield an FM signal.
Most DDSs provide for programmable phase offsets. This feature opens the door for direct phase modulation. PSK (phase shift keying), like FSK, is readily implemented via profiles assuming the DDS includes phase offset as a profile parameter. Other phase modulation formats are possible, as well, but require a controller capable of programming phase changes quickly enough to yield the desire phase modulated signal.
Some DDSs include a digital multiplier for scaling the DDS data just before delivering it to the DAC. Like the frequency and phase modulation examples above, AM (amplitude modulation) is possible by appropriately changing the digital scale factor over time.
One DDS product in particular, the AD9910, provides the user with a 16-bit parallel port for real-time control of any one of the three DDS parameters (frequency, phase, amplitude). It even supports a polar modulation mode for simultaneously controlling both the amplitude and phase parameters.
ADI also has a line of TxDACs (transmit DACs) and QDUCs (quadrature digital up-converters) that support a broad range of modulation types.
Iam new to DDS Design.Iam making DDS based synthesizer for 88-108MHz band.I can able to get this full spectrum with 30.720MHz reference.But iam confused with Modulation frequency.Our requirement for modulation freq is: 70kHz.Pls suggest me how to use AD9954 DDS IC for this requirement.Pls guide me to meet this requirement.If possible ,pls share ckt diagram for this with me.Pls help me...
The AD9954 is capable of operating at sample rates as high as 400MHz implying a maximum Nyquist frequency of 200MHz, which is well above your 108MHz requirement. So, the AD9954 is certainly fast enough to cover the 88-108MHz frequency band.
The AD9954 also has an integrated PLL frequency multiplier that will allow you to scale up your 30.72MHz reference clock by a factor of 4 to 20 to provide a suitable system clock frequency. A minimum system clock rate of 270MHz would be suitable in your application. So a PLL multiplication factor of at least 9 is necessary yielding a 276.48MHz system clock, but your application will support a multiplication factor as high as 13 yielding a system clock of 399.36MHz.
The AD9954 has a 32-bit DDS, which allows you to program a carrier frequency anywhere from dc up to 50% of the system clock frequency with a tuning resolution of less than 1Hz. So you will have no problem covering the 88-108MHz band.
I can only guess at your 70kHz modulation frequency requirement. I assume that it is an IF frequency. That is, your baseband (music, voice, etc.) is first modulated onto a 70kHz subcarrier. Then, the 70kHz carrier (along with its modulated baseband signal) is modulated onto the final carrier frequency placing the signal in the 88-108MHz range. It is this second step for which the DDS is especially well suited.
I am not aware of a reference design for your application, but perhaps there are others on the forum who can help with that.