VCO oscillators

INTRODUCTION TO VCO

A VCO (Voltage Controlled Oscillator) is a free oscillator whose output frequency is variable by means of a DC tuning voltage supplied externally to the VCO, the varicap diode contained within the VCO is the component responsible for making the frequency variation. The frequency variation is possible because the varicap diode acts as a variable capacitor on the LC circuit which determines the oscillation frequency of the VCO itself, we can therefore say that the VCO was born with the appearance of varicap diodes (Yig and Gunn apart).

With modern so-called hyperabrupt varicap diodes it is possible to obtain VCOs with large bandwidths, in fact these varicaps have a very large capacitance range Δc > 15 (Δc = ratio of variation between min. and max. capacitance), there are also diodes optimised for low tuning voltages of 1 to 4 V for modern wireless applications.

FM audio (or FM video) modulation can also be applied on top of the tuning voltage; other VCOs have a separate modulation pin simply as a matter of convenience or to have a tuning sensitivity (MHz / V) different from the odulation sensitivity. The effect of modulation in a VCO is perfectly similar to the effect of frequency variation via the dc applied to the varicap, in fact, if the control dc voltage were just a little dirty with mains residual, there would be residual background modulation. The modulation bandwidth depends mainly on the time constant between the modulation input and the varicap.

As a rule of thumb, a VCO with a wide bandwidth is mainly used for instrumentation, whereas a VCO with a narrow bandwidth is used for FM modulations, VCOs with narrow bandwidths have better phase noise. VCOs with a DR ceramic resonator have even more limited frequency excursions but excellent phase noise and better intrinsic stability due to the higher Q of the DR resonator compared to a classic LC circuit. The DR resonator behaves exactly like a λ/4 cavity by oscillating in TEM mode, thanks to ceramics with a high εr value of 20 to 90 it is possible to achieve true ultra-miniature cavities (see DR resonators with square cross-section).

The lowest frequency in a VCO is obtained with zero tuning voltage, i.e. maximum varicap capacitance, and the highest frequency with high tuning voltage, i.e. minimum varicap capacitance; as an empirical fact, it follows that the lowest frequency cannot be lowered any further, while a slight overrange is possible for the highest frequency.

Linear tuning VCOs (MHz response / constant V over the entire band) are more complicated to obtain, they are mainly used for instrumentation and with generally very wide bands, in a PLL chain they are used with phase comparators where it is necessary to have a constant loop gain, linear response VCOs are also used where it is essential to have constant modulation over the entire operating band.

VCOs constitute a delicate and important circuit in the design of synthesisers and equipment in general for RF, their behaviour together with their performance (phase noise) often determine the performance of the entire design, and only by means of complicated synthesis networks is it possible to clean up the noise of the VCO itself.

VCOs find applications in synthesisers for generating PLL-linked frequencies, in the OL circuit of receivers and spectrum analysers, but also in bugs, small video transmitters, etc.

To make the best use of VCOs and obtain the best performance, 2 very important rules must be observed separate the VCO's RF output very well from the "outside world" by means of a suitable buffer stage with high inverse isolation (see parameters S12 or reverse isolation in MMICs), filter the power supply and modulation with the utmost care; it would be better to be able to power it by means of a battery with shielded and very short wires filtering by means of appropriate EMI filters is very important, for example let's say we have a VCO with a tuning sensitivity of 10MHz / V, this means that a ripple on the tuning voltage of only 1 mV is enough to have a residual modulation of as much as 10 KHz.

The most important specification in a VCO is low phase noise it depends mainly on 3 factors:

• Use of low flicker noise transistors or Fets (1/f), this is mainly caused by the phase noise near the carrier
• Use of low-noise transistors or Fets (high S/N ratio of the output signal)
• Q under load of the resonant circuit, as resonant circuit means the LC or cavity circuit or DR resonator etc...

In addition, in order to guarantee low phase noise, the capacitors that act as a divider and the coupling with the active element also play a very important role. In fact, it is not enough to have a resonator with a high no-load Q, the real trick is to find the right coupling with the circuit.

Phase noise is expressed in dBc / Hz ( dB carrier per Hz ) which means how much noise is contained at a certain distance from the carrier considering a unit bandwidth of 1 Hz and related to the carrier level.

It is therefore to be considered as a relative number that in a nutshell tells how many dB the phase noise is attenuated with respect to the carrier level as if it were measured by a receiver with 1 Hz bandwidth.

Many special components are available for the construction of VCOs up to microwaves, for example:

• varicap tuning diodes in ceramic cases, in SMD, low cost or hyperabrupt, also suitable for direct microwave VCOs see MA46H037
• high Q inductors, in SMD and ultraminiature, with a wide range available
• high Q chip capacitors see ATC100A series in porcelain high stability
• ceramic SAR, or DR dielectric resonators in both coaxial and pillar design
• various transistors and GaAs-FET oscillators, especially bipolar with low flicker noise at Ft 45 GHz see BFP405 - 520 and BFT92, both PNP and NPN