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Wideband Synthesizer Integrates Clean VCO

This highly integrated and versatile frequency synthesizer includes a low-noise VCO, simple control interface, and necessary circuitry to generate 57 MHz to 14.6 GHz.

Frequency-synthesizer integrated circuits (ICs) are often designed to work with an external voltage-controlled oscillator (VCO) to create a low-noise signal source. The ICs may contain much of the phase-locked-loop (PLL) circuitry needed to stabilize a VCO, but the VCO is usually extra.

Such is not the case for the ADF5610 frequency-synthesizer IC from Analog Devices: It has an on-chip VCO that would be impressive as a standalone product, with low phase noise and wide frequency range. Capable of generating signals to 14.6 GHz, the frequency synthesizer provides the VCO for use in fractional-N or integer-N frequency synthesis modes. When teamed with external PLL filters and a high-quality reference oscillator, and using on-chip frequency dividers and multipliers, the IC can produce signals over a total range of 57 MHz to 14.6 GHz, with healthy (+5 dBm) output-power levels.

The ADF5610 and its clever signal routing comes in a compact 48-terminal 7- × 7-mm LGA package (see figure). It contains multiple frequency multipliers, frequency dividers, phase/frequency detector, wideband amplifier, VCO tuning circuitry, control circuitry, and the VCO in that tiny package.

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The ADF5610, a wideband frequency-synthesizer IC in an LGA package with integrated VCO, can generate frequencies to 14.6 GHz.

The VCO has a fundamental-frequency tuning range of 3650 to 7300 MHz, which is internally doubled to reach a total VCO frequency range of 3650 MHz to 14.6 GHz at single-ended and differential output ports. The differential output allows doubled VCO signals to be divided by as much as 128 to achieve the low-frequency limit of 57 MHz (7300 MHz/128). When lower-frequency signals are not needed, this divider can be disabled to save power. The synthesizer IC is controlled by a simple three-wire serial port interface (SPI)

Tuning the VCO

The VCO runs on +3.3- and +5-V dc supplies and has a tuning range of −0 to +3.6 V dc. Whether for its fundamental (to 7.3 GHz) or doubled (to 14.6 GHz) frequencies, it exhibits outstanding single-sideband (SSB) phase noise as a starting point for any frequency synthesizer. When measured 100 kHz from the carrier, the VCO’s phase noise is typically −115 dBc/Hz from a 7.3-GHz carrier, −114 dBc/Hz from a 10-GHz carrier, and −109 dBc/Hz from a 14.6-GHz carrier. It has good spectral purity, with typical second and third harmonics of −30 dBc over its fundamental range of 3650 to 7300 MHz.

Third harmonics for divided VCO signals are typically −30 dBc for VCO divided-by-1 outputs and typically −16 dBc for VCO divided-by-2 outputs. The VCO’s power-supply pushing is typically 67 MHz/V when measured at +1.65 V dc.

The ADF5610’s VCO is a step-tuned oscillator that combines varactor diodes and digital techniques for tuning. It’s based on two VCO cores for consistent tuning across the wide frequency range. Typical tuning sensitivity is 101 MHz/V at 7.3 GHz, 128 MHz/V at 11.0 GHz, and 96 MHz/V at 14.6 GHz. The temperature sensitivity is 0.5 MHz/°C at 7.3 GHz and 1.02 MHz/°C at 14.6 GHz.

Not to be forgotten, the VCO is part of a wideband PLL frequency-synthesizer IC that operates with a reference source of typically 50 MHz (accepting sources to 350 MHz). The synthesizer IC features a wide phase-detector bandwidth of dc to 100 MHz in both fractional and integer synthesize modes. It can achieve better than 40-µs frequency-hopping speed with autocalibration enabled and, depending on the PLL bandwidth, boasts typical synthesizer frequency lock time of 100 µs.

The frequency-synthesizer IC is a low-power device, with typical power consumption of 815 mW and maintaining several power-down modes for the VCO and the PLL circuitry, including a low-power mode with less than 700-mW power consumption. The ADF5610 is designed for operating temperatures from −40 to +85°C and is well-suited for commercial and military applications.

Analog Devices Inc., One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106; (781) 329-4700.

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