DDS

DDS is a rather new frequency synthesis method which appeared in early 70s, last century. All the methods described have been available to the developers for decades but only recently such a close attention has been paid to DDS. Nowadays the appearance of cheap micro circuits with DDS and convenient development tools make them attractive for various application fields.

DDS is unique for its digital definition, therefore any signal generated by it is synthesized with the precision peculiar to digital systems. The simplest DDS is fomed the following way: the binary counter forms an address for ROM where the one sin period of function table is recorded, ROM output reports goes to DAC which forms output sine signal that goes to the low-pass filter and then to the output.

Signal frequency, amplitude and phase are known at any time and stay under control. DDS is practically not affected by temperature drift and ageing. The only element which has the inherent instability of analog circuits is DAC. Because of high specifications, which are actually the reason, recently DDS starts to displace simple analog frequency synthesizers. The main DDS advantages:

• very high frequency and phase resolution which are digitally controlled;
• extremely fast switching to another frequency (or phase), frequency alteration without phase breakage, without overshoots and other anomalies associated with the setting time;
• due to very small phase alteration step the DDS-based architecture excludes the necessity in the precise reference frequency adjustment and also provides parametric temperature compensation;
• the digital interface allows the easy realization of microcontroller operation;
• for quadrature synthesizers there is DDS available with I and Q outputs consistently working.

DDS frequency resolution is the hundredth and even thousandth part of Hz when the output frequency of about decades of MHz. Such resolution is unreachable for other synthesis methods. Another unique feature of DDS is its very high transition speed to another frequency. PLL-based synthesizers use the feedback and error signal filtration that causes the slowdown of the frequency transition. For DDS the transition speed is practically limited only by the speed of the digital control interface. Moreover all frequency transitions in DDS are without output signal phase breakage. Since an output signal is digitally synthesized it’s very easy to make the modulation of different types.

Frequency synthesizer parameters are very important for communication equipment. Being the core of setting system the synthesizer basically identifies the consumer properties of a certain device. Both technically and economically DDS satisfies the majority of ideal frequency synthesizer criteria: it’s simple, highly integrated, with compact dimensions. Besides many DDS parameters are software programmable that allows adding new capabilities to the device. Modern DDS uses submicron CMOS-technology, 3 volts logic, handheld cases. Simultaneously their prices constantly decrease. Everything said above makes DDS devices very promising.