For assembling a fractional-N frequency synthesizer for use from 1.8 to 2.4 GHz, a model ROS-2432-119+ VCO IC from Mini-Circuits was one of the starting points for the design layout. It operates in the frequency band from 1.6 to 2.5 GHz with low phase noise of -100 dBc/Hz offset 10 kHz from the carrier. Also, a model ADF4118 integer-N frequency synthesizer IC from Analog Devices Co. capable of operating to 3 GHz was used in the synthesizer for division and phase-frequency detection (PFD). For the reference oscillator, a model TXO200U temperature-compensated crystal oscillator (TCXO) from Rakon Ltd. was used. It operates at 10 MHz with typical phase noise of -150 dBc/Hz offset 10 kHz from the carrier.
5. This layout represents the 1.8-to-2.4-GHz frequency synthesizer.
The primary spurious frequencies in this design are from reference spurious signals. The synthesizer step size and loop bandwidth relationship dictates the attenuation level of these spurious products. The spacing between the first three spurious products is usually equal to the step size or one-half the channel step size. For optimum reduction of spurious levels, a step size of 200 kHz and loop bandwidth of 100 Hz were established for the fractional-N frequency-synthesizer design. Widening the loop bandwidth would increase the locking speed and step size, but would also increase the number and levels of spurious products.
6. This photograph shows the fabricated 1.8-to-2.4-GHz frequency synthesizer.
7. This screen shows a spectrum view of the synthesizer at 1.80 GHz.
After validating the results from computer simulations (Fig. 4, Table 1), a printed-circuit-board (PCB) layout of the fractional-N frequency synthesizer was created with the help of Protel software (Fig. 5). This PCB design/layout software, which was originally developed by Altium (formerly Protel), is available for free download from a number of different websites. Altium also offers higher-level software tools, including Altium Design. The synthesizer was fabricated as a PCB (Fig. 6) and evaluated at center frequencies of 1.8 and 2.1 GHz (Figs. 7 and 8) with the help of a model HP8563A spectrum analyzer from Agilent Technologies.
8. This screen shows a spectrum view of the synthesizer at 2.10 GHz.
• Synthesizer Consumes 400 μW To Support ISM
• Small Synthesizer Spans 1.1 To 2.5 GHz
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Spurious performance is less impressive at 2.1 GHz than at 1.8 GHz. The bandwidth for the 1.8-GHz measurements was narrower, meaning that the phase-noise performance will be superior at the lower band frequencies. The spectrum analyzer’s span was set to 50 kHz, with a resolution-bandwidth (RBW) filter at 1 kHz and a video-bandwidth (VBW) filter set at 10 Hz. The 55.67-dB power difference between the carrier and the phase noise, offset 10 kHz from the carrier, indicates that the phase noise level will be:
Phase noise (at 2.1 GHz) = -68.33 – 10log(RBW) = -98.33 dBc/Hz.
The fractional-N frequency synthesizer was evaluated for phase noise at carrier frequencies from 1.8 to 2.4 GHz at 100-MHz intervals and for offset frequencies of 1 kHz, 10 kHz, 100 kHz, and 1 MHz. The results are compiled in Table 2. The synthesizer achieved respectable phase-noise performance, with a level of -98 dBc/Hz offset 10 kHz from the carrier. The design is applicable to a number of wireless systems, including for Bluetooth, DCS, GSM, and wireless-local-area-network (WLAN) systems.
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