A New Wideband Family of Oscillators

Product Feature

A New Wideband Family of Oscillators

Vari-L Co. Inc.
Denver, CO

Modern communication systems are designed to handle ever increasing amounts of data by utilizing more bandwidth and more sophisticated modulation schemes. Voltage controlled oscillators (VCO) are used in almost all radio and RF applications including cell phones, cable modems, satellite terminals, basestations, radar, missile guidance systems, radios, military communication equipment, optical multiplexers and transmitters. In general, today's systems demand the largest possible frequency range, the lowest possible phase noise, a linear tuning curve and insensitivity to load conditions. A new line of VCOs has been introduced that meets the needs of today's broadband systems. The family of products and a performance summary is shown in Table 1 .

VCO Performance Parameters

Phase noise is usually the most critical parameter of a VCO. This is the "fuzz" on a signal. It has two important impacts on system performance. The first is channel utilization or what is known as desensitization. This is one factor that limits how closely signals can be placed. If two signals are present in adjacent channels, the phase noise from one signal can spill over into the adjacent channel and jam that channel. This is one factor in dropped calls. A graphical depiction of this condition is shown in Figure 1 . There is some dead space allocated between channels to minimize this affect. The more closely channels can be spaced, the more bandwidth can be utilized and the more data a system can handle without costly additional spectrum, infrastructure or new cables.

Fig. 1 Typical channel-to-channel spacing.

Phase noise also affects the achievable data rates of a system by contributing to the noise in the channel. Phase noise is integrated over a bandwidth to produce an integrated RMS phase error. In digital modulation schemes such as quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM), the lower the integrated phase, the higher the data rate. This drives circuit and system designers to attempt to maintain the lowest possible phase noise.

Leeson described phase noise mathematically using the well known equation

where

f₀ = carrier center frequency
f_m = frequency offset from the carrier center frequency
f_c = flicker corner frequency of device used as amplifying element in the oscillator
Q = loaded Q of the tuned circuit
F = noise factor of the active device
k = Boltzmann's constant (4.1 x 1021)
T = temperature in Kelvins
P_s = average power at the oscillator input
R = equivalent noise resistance of the tuning diode
K_o = oscillator voltage tuning gain

Fig. 2 Typical measured phase noise.

This equation clearly shows that the circuit designer should attempt to keep the carrier frequency as low as possible and the loaded Q of the tuned circuit as high as possible to achieve the best phase noise. These circuit design issues conflict with the desire to achieve the largest possible bandwidth in the system at the lowest possible noise level. Leeson's equation offers some clues as to how to overcome this conflict. It is easily seen that increasing the signal-to-noise ratio in the amplifying element of the oscillator can offset the lower Q and the higher carrier center frequency. This characteristic is represented by the (FkT/Ps) term in the equation. The circuit designer must devise a way to provide enough bandwidth by lowering the Q and increasing the power to offset this loss of Q. Achieving this condition results in a broad bandwidth and low phase noise.

Fig. 3 Typical measured tuning sensitivity and voltage-tuning characteristics.

This family of VCO products has attempted to accomplish this increase in power and wider bandwidth by incorporating multiple tuned circuits in the oscillator design. The VCO is a common base circuit with a tuned circuit that is incorporated on the base of the transistor to maximize the gain and output power over the frequency range of the oscillator. A second tuned circuit is used as the tank circuit to achieve the desired bandwidth. These two circuits are both attached to the tuning port of the VCO so the system designer has a simple interface. This approach has led to excellent phase noise performance and wide frequency ranges. VCOs such as the model VCO793-1550T achieve more than an octave of tuning range. The phase noise, frequency range and tuning sensitivity were measured with the Agilent 4352 VCO/PLL tester, and the results are shown in Figures 2 and 3 , respectively. The complete performance summary is shown in Table 2 . The company has developed an efficient design technique that allows these models to be customized to meet the specific needs of each system. Additional information may be obtained by e-mail at sales@vari-L.com.

Vari-L Co. Inc., Denver, CO (303) 371-1560, www.vari-L.com.
Circle No. 304