MAX2605EUT-T

Maxim > Design Support > Technical Documents > Application Notes > Wireless and RF > APP 688 Keywords: rf, if, rf design, vco, rfic, voltage controlled oscillator, rf ics APPLICATION NOTE 688 Trimless IF VCO: Part 2: New ICs Simplify Implementation Sep 02, 2002 Abstract: Presents a family of IF voltage-controlled oscillators (VCOs) to cover the frequency range from 45MHz to 650MHz. The ICs are in 6-pin SOT23 packages. Phase noise is -100dBc/Hz for the MAX2608 at 300MHz to 500MHz. An off-chip inductor sets the operating frequency. The output stage can be matched with resistors or lossless techniques. Additional Information: Trimless IF VCO: Part 1: Design Considerations A new family of integrated circuits can ease the task of developing compact, fixed-frequency voltage-controlled oscillators (VCOs) for IF applications. Designing a VCO for use with a fixed intermediate frequency (IF) can be daunting. Fortunately, VCO ICs from Maxim (MAX2605MAX2609) can simplify the task. Compared to conventional discrete-device VCOs, the Maxim parts cost less and require less PC board space. Click here for an overview of the wireless components used in a typical radio transceiver. In a traditional IF VCO design, the oscillator core and the output buffer stage are formed by discrete transistors, resistors,capacitors, and inductors (Figure 1). The tank is built from a networkconsisting of the frequency-setting inductor, varactors, coupling capacitors, and feedback capacitors. The output stage uses reactive elements to match the output impedance to a particular load impedance. Page 1 of 7 Figure 1. This schematic shows an IF VCO implemented with discrete circuit elements. To ensure a successful design, the component values not only must establish a desired nominal oscillation frequency, but they must also guarantee an adequate tuning range, proper biasing, oscillator startup under all conditions, and proper output-stage performance. Problems can occur even with a good first-order design because of the trade-off that exists among current consumption, startup margin, frequency tuning range, and phase noise. A major disadvantage of discrete IF VCO designs is the amount of PC board area required. Much effort must be expended in optimizing the layout to below 6mm x 10mm. Furthermore, the PC board layout has a critical effect on the VCO's performance and design accuracy. The layout contains parasitic capacitances and inductances that affect the oscillation frequency and must therefore be taken into account to implement the oscillator properly. Parasitic elements often bring about an undesired shift in the nominal oscillation frequency, which causes greater design-centering errors and ultimately forces a need for greater tuning range to account for those errors. The MAX2605-MAX2609 IF VCO family offers a better alternative. These five ICs are designed for lowpower, fixed- and single-frequency, portable wireless applications with IF frequencies in the 45MHz to 650MHz range. Much of the required circuitry is included on-chip; only the tank inductor (which establishes the oscillation frequency) is external. Once you choose the correct external inductance value, the IC guarantees that some level within the tuning-voltage range (+0.4VDC to +2.4VDC) will tune in the corresponding frequency. The IC's tuningvoltage input can be driven directly from the loop-filter output following a phase-locked loop (PLL). MAX2605-MAX2609 ICs are designed for supply voltages in the +2.7VDC to +5.5VDC range, and the supply voltage connection does not require special regulation for proper operation. Each IC comes in a tiny, 6-pin, plastic SOT23 package (Figure 2). Page 2 of 7 Figure 2. The MAX2605-MAX2609 IF VCO ICs come in a 6-pin surface-mount SOT23 package designed to occupy minimum PC board space. The MAX2605 tunes from 45MHz to 70MHz, with -117dBc/Hz phase noise at 100kHz from the carrier. For the other devices, these parameters are 70MHz to 150MHz tuning with -112dBc/Hz phase noise at 100kHz from the carrier (MAX2606), 150MHz to 300MHz with -107dBc/Hz (MAX2607), 300MHz to 500MHz with -100dBc/Hz (MAX2608), and 500MHz to 650MHz with -93dBc/Hz (MAX2609). The frequency tuning range, biasing, startup, and otheroscillator characteristics are all managed within the IC, eliminating the design headaches typically associated with VCO design. An on-chip varactor and capacitors simplify IF VCO design by eliminating the need for external tuning elements. A graph of inductance versus oscillation frequency (see the MAX2605-MAX2609 data sheet) further simplifies the task of choosing an external inductor. The MAX2605 family provides several important new benefits for RF designers. The ICs are designed to create VCOs that are trimless and do not need external adjustments. To accommodate the anticipated range of system IFs found in dual-conversion systems, they are designed to cover a wide range of application frequencies. In addition, they have a flexible output interface to help reduce the cost of IF VCOs and shrink the size of the final design. Because the MAX2605-MAX2609 represent a new concept in VCOs, they required a fundamentally new circuit approach to achieve the product objectives. Maxim devised an oscillator scheme based on the reliable and flexible Colpitts oscillator structure. This topology was adapted so that all the oscillator circuit elements (except the inductor) could be integrated within the IC. Integrating nearly the entire oscillator on-chip provides all the desired operating objectives of a good VCO: proper oscillator startup, wide frequency range, required tuning characteristics for trimless operation, controlled current consumption, and biasing that was independent of temperature and the power-supply voltages. An off-chip inductor allows the VCO to be applied over a very wide range of operating frequencies. Onchip capacitance remains the same, but changing external inductance values modifies the resonant frequency of the oscillator tank circuit. If the inductor has a minimum quality factor (Q), the phase-noise and startup behavior can be guaranteed (Figure 3). Page 3 of 7 Figure 3. This simplified circuit diagram of the MAX2605-MAX2609 VCO ICs shows that only an external inductor is necessary to complete the resonant circuit that sets the oscillation frequency. To implement this new approach, the IC technology needed a full complement of active and passive elements to support construction of the oscillator circuit shown. Specifically, the process technology had to provide high-frequency transistors, high-Q capacitors, high-Q varactor diodes with high capacitance ratios, and PNP or PMOS devices. The MAX2605-MAX2609 are fabricated by means of a silicon BiCMOS process developed specifically for RF ICs that include monolithic oscillator structures. This process features PNP, NMOS, and PMOS devices, NPN transistors with transition frequencies (fT) of 25GHz, low-series-resistance varactor diodes with better than a 2:1 capacitance ratio (for tuning voltages from 0.4V to 2.4V), very-high-Q metalinsulator-metal (MIM) RF capacitors, precision-thin film resistors, and three layers of metal. This full complement of devices allowed implementation of the complete IC. The VCO design required careful and extensive computer simulations, including multiple design iterations between various aspects of performance to ensure that all specifications and requirements could be guaranteed over all operating conditions. Finally, to guarantee that the oscillator possessed a sufficient frequency-tuning range to account for the shift in operating frequency caused by component tolerances, Maxim elected to perform production testing on the devices and to guarantee a set of frequency limits. These limits provide MAX2605MAX2609 users with a guaranteed set of high- and low-frequency tuning limits (fMAX and fMIN ), in which passing ICs have a frequency of oscillation (fOSC) < fMIN at a tuning voltage (VTUNE) of 0.4V, and fOSC < fMAX at VTUNE = 2.4V. Assuming an external inductor with ±2% tolerance, including temperature drift, and a small design centering error (