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ADF4350 Datasheet(PDF) 2 Page - Analog Devices |
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ADF4350 Datasheet(HTML) 2 Page - Analog Devices |
2 / 5 page CN-0134 Circuit Note Rev. B | Page 2 of 5 Figure 2. Evaluation Board for CN-0134 Direct Conversion Transmitter Low noise LDOs ensure that the power management scheme has no adverse impact on phase noise and EVM. This combination of components represents industry-leading direct conversion transmitter performance over a frequency range of 500 MHz to 4.4 GHz CIRCUIT DESCRIPTION The circuit shown in Figure 1 utilizes the ADF4350, a fully integrated fractional-N PLL IC, and the ADL5375 wideband transmit modulator. The ADF4350 provides the local oscillator (LO) signal for the ADL5375 transmit quadrature modulator, which upconverts analog I/Q signals to RF. Taken together, the two devices provide a wideband baseband IQ to RF transmit solution. The ADF4350 is powered off the ultralow noise 3.3 V ADP150 regulator for optimal LO phase noise performance. The ADL5375 is powered off a 5 V ADP3334 LDO. The ADP150 LDO has an output voltage noise of only 9 µV rms and helps to optimize VCO phase noise and reduce the impact of VCO pushing (equivalent to power supply rejection). Filtering is required on the ADF4350 RF outputs to attenuate harmonic levels so as to minimize errors in the quadrature generation block of the ADL5375. From measurement and simulation, the odd order harmonics contribute more than even order harmonics to quadrature error and, if attenuated to below −30 dBc, results in sideband suppression performance of −40 dBc or better. The ADF4350’s 2nd harmonic (2H) and 3rd harmonic (3H) levels are as given in the data sheet and shown in Table 1. To get the 3rd harmonic below -30 dBc, approximately 20 dB of attenuation is required. Table 1. ADF4350 RF Output Harmonic Levels Unfiltered Harmonic Content (Second) −19 dBc Fundamental VCO output Harmonic Content (Third) −13 dBc Fundamental VCO output Harmonic Content (Second) −20 dBc Divided VCO output Harmonic Content (Third) −10 dBc Divided VCO output This circuit gives four different filter options to cover four different bands. The filters were designed for a 100 Ω differen- tial input (ADF4350 RF outputs with appropriate matching) and 50 Ω differential output (ADL5375 LOIN differential impedance). A Chebyshev response was used for optimal filter roll-off at the expense of increased pass-band ripple. The filter schematic is shown in Figure 3. This topology allows the use of either a fully differential filter to minimize component count, a single-ended filter for each output, or a combination of the two. It was determined that for higher frequencies (>2 GHz) two single-ended filters gave the best performance because the series inductor values are twice the value compared to a fully differential filter and, hence, the impact of component parasitics is reduced. For lower frequencies (<2 GHz), a fully differential filter provides adequate results. |
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