International Telemetering Conference Proceedings / October 26-29, 1998 / Town & Country Resort Hotel and Convention Center, San Diego, California / A Phase Modulator combining digital techniques with non-traditional analog circuitry can minimize the shortcomings of a traditional (purely analog) Phase Modulator. These shortcomings are: nonlinear response from input modulating signal to output modulated signal; parameters (frequency and modulation index) that are difficult to set; and the need for complex filters. The design approach discussed in this paper uses a combination of Direct Digital Synthesis (DDS) and analog devices operating in their linear range to generate a Phase Modulated RF (140 MHz) signal. A Numerically Controlled Oscillator (NCO) digitally generates the first IF yielding a very accurate, repeatable and linear signal with easily adjustable parameters such as frequency and modulation index. Linear multipliers (instead of saturated diode mixers or step recovery diodes) are used for up-conversion to RF. Using linear multipliers eases the filtering requirements due to the significantly reduced harmonics and IM (Inter-Modulation) terms. The resulting RF signal is easily translated to higher frequency bands such as L, S, C, X or K.

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Today&amp / #8217 / s analog modulators and upconverters are inadequate to synthesize and modulate signals with high &amp / #8216 / Spurious Free Dynamic Range&amp / #8217 / (SFDR). Thus, the main objective of this thesis is to design and implement a &amp / #8216 / Digital Signal Synthesizer&amp / #8217 / (DSS) that is capable of synthesizing signals between 50-100 MHz with 60dB SFDR and to modulate them variable symbol rates and modulation techniques with very high phase/frequency resolution and switching speed while keeping the amplitude modulation occurring during a modulated symbol duration as small as possible. In this thesis, digital words of the desired signals are first synthesized in a &amp / #8216 / Field Programmable Gate Array&amp / #8217 / (FPGA) using &amp / #8216 / Direct Digital Synthesizer&amp / #8217 / (DDS) fundamentals and then converted to analog signals with a high speed &amp / #8216 / Digital to Analog Converter&amp / #8217 / (DAC). In order to attain the analog requirements, the system variables such as DAC analog performance, nonlinearities, sample and hold affects, DDS parameters, system clock, bandwidth requirements of analog filters and how they effect the output performance are studied. FPGA blocks that are capable of modulating and synthesizing desired signals are designed and programmed on a FPGA. Finally, single tone and modulated signals are synthesized with this DSS implementation and measured in order to verify this system&amp / #8217 / s performance and capabilities.

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