Synthesizer ICs Integrate VCOs

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Model LTC6946 integer-N frequency synthesizers feature an on-board VCO and can be specified for three frequency ranges from 2.24 to 5.79 GHz.

Phase-lock-loop (PLL) circuits are usually added to a voltage-controlled oscillator (VCO) for improved frequency stability. But with the three integer-N frequency synthesizers in the LTC6946 series from Linear Technology, the VCO is integrated along with a low-noise charge pump and other associated circuits. The synthesizers are available in three versions: Model LTC6946-1 tunes from 2.24 to 3.74 GHz, model LTC6946-2 operates from 3.08 to 4.91 GHz, and model LTC6946-3 covers 3.84 to 5.79 GHz. All three are optimized for outstanding spectral purity, with low phase noise and minimal spurious products. Each frequency synthesizer incorporates an on-chip buffered output divider that is programmable in division ratios from 1 through 6 to achieve frequency coverage as low as 373 MHz.

The frequency synthesizer ICs feature a low-noise 5.7-GHz PLL, which includes a reference divider, phase-frequency detector (PFD) with phase-locked indicator, low-noise charge pump, and integer feedback divider. The PLL circuitry is tightly coupled to a low-noise VCO and self-calibration circuitry for optimal phase-noise performance. An on-chip SPI-compatible bidirectional serial port allows frequency tuning and control, as well as read back of register and loop status information.

The on-board VCO and frequency synthesizer are designed for reference inputs from 10 to 250 MHz at single-ended input levels from 0.5 to 3.3 V peak to peak. The VCO provides as much as +2.3 dBm output power into a 50-Ω matched load at 900 MHz. The oscillator includes an output mute control which reduces VCO output level to -60 dBm. This is a high-speed muting function, with enable time of 110 ns or less and disable time of 170 ns or less. The on-board charge pump allows current to be adjusted from 0.25 to 11.2 mA in 12 settings. The charge pump contains selectable high- and low-voltage clamps that can be used for VCO monitoring purposes. The VCO requires no external components and is internally calibrated to the correct output frequency with no external system support.

The closed-loop phase-noise performance for these synthesizers is impressive. For the lowest-frequency unit, model LTC6946-1, operating with a VCO frequency of 3 GHz, the phase noise is no worse than -80 dBc/Hz offset 10 kHz from the carrier, -130 dBc/Hz offset 1 MHz from the carrier, and -157 dBc/Hz offset 40 MHz from the carrier. For the model LTC6946-2 running at a VCO frequency of 4 GHz, the phase noise is no worse than -77 dBc/Hz offset 10 kHz from the carrier, -127 dBc/Hz offset 1 MHz from the carrier, and -156 dBc/Hz offset 40 MHz from the carrier. For the highest-frequency model LTC6946-3 operating at a VCO frequency of 5 GHz, the phase noise is no worse than -75 dBc/Hz offset 10 kHz from the carrier, -126 dBc/Hz offset 1 MHz from the carrier, and -155 dBc/Hz offset 40 MHz from the carrier. The spurious noise performance is as impressive, with typical performance of -103 dBc.

The LTC6946 line can be supplied as part of an evaluation board, and the firm offers the PLLWizard software to assist in controlling and optimizing the sources for different conditions. The software can be used to perform simulations and optimization, allowing designers to verify the performance of their designs on an actual demonstration circuit. The PLLWizard design tool software is available for download free of charge from the Linear Technology website (visit www.linear.com/synthesizers).

The frequency synthesizers are ideal for multiband cellular base stations supporting LTE, W-CDMA, UMTS, CDMA, GSM, and WiMAX standards. They are also suitable for applications in military radios, broadband wireless access systems, and test equipment. The LTC6946 integer-N frequency synthesizers are supplied in 4 x 5 mm, 28-lead plastic QFN packages (see figure). All versions are specified over a full case temperature range from -40 to +105C. P&A: $5.75 (1000 qty.); stock.

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Linear Technology Corp.
1630 McCarthy Blvd.
Milpitas, CA 95035-7414
(408) 432-1900
FAX: (408) 434-0507