Frequency synthesizers are one of the main parts of transceivers. The quality of the transmitted or received signal directly depends on the quality of the frequency synthesizer output spectrum used as the transceiver local oscillator. Among the three frequency synthesizing methods; direct analog, indirect digital using PLLs and direct digital, the indirect digital frequency synthesizing using PLLs is widely used in low-cost portable multiband transceivers. The integer-N type of PLL-based frequency synthesizers was firstly used in wireless telecommunication. Their structure is fairly simple but they can synthesize just integer multiples of the reference frequency which is equal to the channel spacing. In the designing of integer-N frequency synthesizers, there is a severe trade-off among frequency resolution, spectral purity, and PLL dynamic behavior. Fractional-N frequency synthesizers which can synthesize fractional multiples of reference frequency are a solution for the mentioned trade-off. They eliminate the need for the reference frequency to be equal to channel spacing. Fractional-N synthesizers use dual (or multi) modulus prescaler to generate fractional multiple of the reference frequency. Main problem of the fractional-N synthesizers is spurious tones caused by repeated operation of prescaler. y randomizing the input control of the prescaler in such a way that its division factor is equal to the digital control word, the systematic fractional sidebands could be converted to random white noise. High pass noise shaping also can be used to improve signal to noise ratio. Delta-sigma modulators which have been using in data converters are known as a tool which can do the randomization and noise shaping together. The delta-sigma modulators in fractional-N frequency synthesizers act like digital to analog data converters without the decimation filter. The delta-sigma modulators for this purpose should have the following characteristics: low complexity to reduce power consumption, long repetition duration and high dynamic range. Delta-sigma structures for frequency synthesizers are just reported as a part of a whole synthesizer and there are few works concentrated on this subject distinctively. Two mostly-used structures of delta-sigma modulators are MASH and single loop that the second one introduces better noise characteristics. This work introduces a new delta-sigma structure consisted of two stages based on previous works. The first stage is a second order error feedback modulator which decreases the effective number of input control word bits and the second stage is a Ritchie based third order single loop modulator that acts as a concentrator to Key Words RF Transceiver, Local Oscillator, Frequency Synthesizers, PLL, Fractional-N, Spurious Tones, Phase Noise, Delta Sigma Modulation
