# Structure of All-Digital Frequency Synthesiser for IoT and IoV Applications
## 3 Requirements for Blocks of All-Digital Multiband Frequency Synthesiser  
In order to evaluate the impact of main blocks on noise performance of the frequency synthesiser, the total phase noise of the synthesiser will be calculated by using (4) equation. Calculation results of phase noise and its components of type-II frequency synthesiser are shown in Figure 2. Total phase noise of frequency synthesiser is marked as \(\mathcal{L}_{\text{FS}}(f)\), phase noise induced by reference signal is marked as \(\mathcal{L}_{\text{REF}}(f)\), phase noise induced by TDC is marked as \(\mathcal{L}_{\text{TDC}}(f)\), the oscillator's component of DCO's phase noise is marked as \(\mathcal{L}_{\text{DCO}}(f)\), DCO's quantization and \(\Delta\Sigma\) shaping noise is marked as \(\mathcal{L}_{\text{DCOAZ}}(f)\). Parameters provided in references [16; 17] were used to carry out these calculations: the operating frequency of the DCO is equal to 5.38 GHz, the frequency of the reference signal is 48 MHz, the frequency tuning step is 26 kHz, the \(\Delta\Sigma\) modulator controls 9 bits. The \(\Delta\Sigma\) dithering frequency and order of \(\Delta\Sigma\) modulator are not provided, therefore we assume that the \(\Delta\Sigma\) dithering frequency is equal to the DCO's frequency divided by 4, and the order of the \(\Delta\Sigma\) modulator is 2 nd. Phase noise of the reference signal and oscillator's component of DCO's noise are approximated respectively by Equations (9) and (10). \[\mathcal{L}_{\text{Osc,REF}}(f)=\frac{10^{-4.8}}{f^{3}}+\frac{10^{-8.3}}{f^{2 }}+\frac{10^{-10.3}}{f}+10^{-14.7}, \tag{9}\]  
\[\mathcal{L}_{\text{Osc,DCO}}(f)=\frac{10^{-0.2}}{f^{2}}+10^{-13.8}. \tag{10}\]  
Configuration of the filter is not provided in reference [16], therefore two configurations of the filter were considered. First configuration is made of only proportional and integral parts, with proportional and integral coefficients respectively set to \(\alpha=2^{-6}\) and \(\rho=2^{-12}\). This corresponds to the filter's configuration for a type-II frequency synthesiser. Results of calculations for this configuration are shown in Figure 2a. For the second configuration it is assumed that the filter is composed of proportional and integral gain paths and four first-order stages of IIR filter. Such configuration corresponds to filter's structure for the type-II higher order frequency synthesiser. Coefficients of proportional and integral paths are set respectively to \(\alpha=2^{-6}\) and \(\rho=2^{-10}\). Transfer coefficients of all stages of IIR filters are equal and set to \(\lambda_{1}=\lambda_{2}=\lambda_{3}=\lambda_{4}=2^{-5}\). Results of calculations for the second configuration of the filter is shown in Figure 2b. It is seen from both cases shown in Figure 2 that noise induced by TDC dominates at low frequency offsets from the carrier (in-band) and noise induced by DCO dominates at high frequency offsets from the carrier (out-of-band). Also, it is seen that the higher order synthesiser's filter enables more precise control of the synthesiser's passband and has better noise-filtering capabilities: in the case of the first configuration (Figure 2a), noise induced by TDC has a high impact on total phase noise up to 10 MHz frequency offset, while in the second configuration (Figure 2b), noise of reference signal and noise of TDC is highly attenuated outside of the synthesiser's passband and DCO's noise clearly dominates out-of-band. Therefore, it is important to have a reprogrammable filter, to control the synthesiser's frequency response and noise filtering capabilities. Since noise induced by TDC and DCO's noise have separate dominance ranges, the requirements of phase noise performance for the frequency synthesiser can be transformed to the requirements for TDC and DCO. As mentioned in the first chapter, reports and analysis of current and developing standards, which are used for IoT, IoV and other wireless applications, use frequency bands from hundreds of megahertz up to 6 GHz.