STEL-1175+80/CM

The STEL-1175+80/CM is one member of the STEL-1175 series.The STEL-1175 modulated numerically controlled oscillator(MNCO)uses digital techniques to provide a cost-effective solution for low noise signal sources.The device can operate at clock frequency up to 125 MHz and provides high frequency resolution and high spectral purity of outputs up to 50 MHz.

Features of the STEL-1175 are:(1)sine or cosine signal generation-12-bit outputs;(2)microprocessor compatible inputs.The STEL-1175 also features phase modulation at rates up to 25%of the clock frequency.The 12-bit output can be selected to be a sine or cosine function,so that two STEL-1175 NCOs can be used to generate quadrature signals with independent phase modulation.Two independent frequency control registers are provided,allowing high speed frequency hopping or binary FSK at rates to 25%of the clock frequency.The device combines low power 0.5CMOS technology with a unique architectural design resulting in a power efficient,high-speed sinusoidal waveform generator able to achieve fine tuning resolution and high spectral purity at clock frequencies up to 125 MHz.The NCO is designed to provide a simple interface to an 8-bit microprocessor bus.

Care must be taken in interfacing the STEL-1175 with the DAC.The clock to output delay in the STEL-1175 is typically 4 nS,but does change with processing variations, voltage,temperature,and loading.The circuit design must take this delay into account so that the input to the DAC is stable at the instant it is clocked into the DAC.The hold time of the DAC must be less than the minimum clock to output delay.The set-up time of the DAC must be less than the clock period minus the maximum clock to output delay.The NCO generates a sampled sine wave where the sampling function is the clock.The practical upper limit of the NCO output frequency is about 40%of the clock frequency due to spurious components that are created by sampling.Those components are at frequencies greater than half the clock frequency and can be removed by filtering.