ICs help implement a trim-Free VCO (Part 2)

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 (MAX2605-MAX2609) can simplify the task.

In a traditional IF VCO design, the oscillator core and output buffer stage are formed by discrete transistors, resistors, capacitors, and inductors.

The tank is built from a network consisting 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.

To ensure a successful design, the component values must not only establish a desired nominal oscillation frequency, they must also guarantee an adequate tuning range, proper biasing, oscillator start-up under all conditions, and proper output-stage performance.

Problems can occur even with a good first-order design because of the trade-offs that exist among current consumption, start-up margin, frequency tuning range, and phase noise.

A major disadvantage of discrete IF VCO designs is the amount of PCB area needed. Much effort must be expended in optimising the layout to below 6 x 10 mm.

Furthermore, the PCB 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 cause an undesired shift in the nominal oscillation frequency, which causes greater design-centring 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 low power, fixed and single-frequency portable wireless applications with IF frequencies in the 45 to 650 MHz 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 (+0.4 VDC to +2.4 VDC) will tune in the corresponding frequency.

The IC's tuning-voltage 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 +2.7 to +5.5 VDC range, and the supply voltage connection does not require special regulation for proper operation.

Each IC comes in a 6-pin plastic SOT23 package.

The 2605 tunes from 45 to 70 MHz, with -117 dBc/Hz phase noise at 100 kHz from the carrier.

For the other devices, these parameters are: 70 to 150 MHz tuning with -112 dBc/Hz phase noise at 100 kHz from the carrier (MAX2606), 150 to 300 MHz with -107 dBc/Hz (MAX2607), 300 to 500 MHz with -100 dBc/Hz (MAX2608), and 500 to 650 MHz with -93 dBc/Hz (MAX2609).

The frequency tuning range, biasing, start-up, and other oscillator 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 further simplifies the task of choosing an external inductor.

The 2605 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 start-up, 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. On-chip 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 behaviour can be guaranteed.

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 on a silicon BiCMOS process developed specifically for RFICs that include monolithic oscillator structures.

This process features PNP, NMOS, and PMOS devices, NPN transistors with transition frequencies (fT) of 25 GHz, low-series-resistance varactor diodes with better than 2:1 capacitance ratio (for tuning voltages from 0.4 to 2.4), very high-Q metal-insulator-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 guarantee a set of frequency limits.

These limits provide MAX2605-MAX2609 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 0.4 V, and fOSC ³ fMAX at VTUNE = 2.4 V.

Assuming an external inductor with ±2% tolerance, including temperature drift, and a small design centring error (<0.5%), this testing guarantees that the VCO always tunes to the operating frequency selected by the inductor, without adjustment of the external inductance value.

The result is a trimless VCO design.

MAX2605-MAX2609 applications are highly simplified and easy to understand.

Two simple steps are involved:

1. Select and implement an external inductance to set the desired oscillation frequency;
2. Resistively or reactively match the output stage to the load.

The nominal operating frequency (fNOM) desired for the VCO is determined solely by the effective external inductance value at IND (pin 1), as determined by a curve.

The inductance value (LF) required for a desired operating frequency will not necessarily coincide with any of the standard values for surface mount technology (SMT) inductors, which typically increase in steps that differ by a factor of about 1.2.

To achieve the desired value in such cases, the inductance must be constructed from two inductors, LF1 and LF2.

LF1 should be chosen as the nearest standard value below the desired value. Then, choose LF2 as the nearest standard value just less than LF - LF1. LF1 should adhere to the minimum Q requirements, but LF2 can be implemented as a lower cost thin film SMT type.

Because its value is less than 20% of the total, its lower Q has only a small effect on the overall Q.

It is also permissible to adjust the total inductance value by implementing small amounts of inductance with PCB traces. For MAX2608/MAX2609 circuits, the inductance value for LF2 is sometimes more precisely implemented as a PCB trace shorted to ground than as an SMT inductor.

Once the required inductance value is established at pin IND, the VCO is guaranteed to tune to this oscillation frequency over all component variations, operating temperatures, and supply voltages.

MAX2605-MAX2609 VCOs include a differential output amplifier after the oscillator core. The amplifier stage provides valuable isolation and offers a flexible interface to IF functions such as a mixer and/or a PLL prescaler.

The output can be taken single ended or differentially, but the maximum output power and lowest harmonic output is achieved in the differential-output mode.

Both open-collector outputs (OUT- and OUT+) require a pullup element to the collector voltage (VCC).

The output stage may be applied with a pullup resistor or inductor.

A pullup resistor is the most straightforward method of forming an interface to the output and works well in applications that operate at lower frequencies or require only a modest voltage swing.

A reactive power match is required for operating frequencies above the 3 dB bandwidth of the load-resistance/capacitance network and/or when a greater voltage swing or output power is desired.

The matching network is a simple circuit with a shunt inductor and series capacitor.

To provide DC bias for the output stage, the inductors are connected from OUT- and OUT+ to VCC, and the series capacitors are connected from OUT- and OUT+ to the load.

Values for the inductor and capacitor are chosen according to the operating frequency and load impedance.

The output is applied like any conventional differential output. The only constraints are the need for a pullup to VCC and a limit to the voltage swing at OUT- and OUT+.

A comparison of the design time needed to apply each approach reveals a dramatic difference.

The classical/discrete approach shown is very design intensive, and the successful development of a discrete IF VCO may require many weeks.

Several iterations are likely before reaching a robust, manufacturable design.

On the other hand, the MAX2605-MAX2609 let you design the VCO in minutes and then verify and test it in an afternoon!

Because the MAX2605-MAX2609 solve the problems of frequency tuning range, biasing, and startup, they eliminate the difficult tasks typically associated with a VCO design.

You need only select an external inductance value based on the desired oscillation frequency, and the output load.

This task is easily accomplished by reading the desired inductance value from a graph supplied on the MAX2605-MAX2609 data sheet.

In bill-of-materials cost, the MAX2605-MAX2609 are comparable to the traditional discrete IF VCOs.

(Reprinted from 'Maxim Engineering Journal' Volume 39)