High frequency voltage controlled oscillator IC using diffusion capacitance modulation

A variable high frequency voltage controlled oscillator (VCO) which is fully integrated. By controlling, with a control voltage, the collector current flowing through one transistor of the integrated oscillator circuit, the diffusion capacitance (also known as base-charging capacitance) of that transistor is varied. Since, in conjunction with an inductance, this capacitance to a large degree determines the frequency of oscillation of the integrated circuit, a fully integrated VCO is made possible.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of integrated circuits and, more 
particularly, to a high frequency, voltage controlled oscillator that can 
be fully integrated. 
2. Description of the Prior Art 
Oscillators are astable circuits which find many uses in electronic 
equipment, both stationary and portable. Oscillators frequently consist of 
one or two transistors, an inductor (L), and a capacitor (C) in an LC tank 
circuit, followed by a buffering amplifier. Oscillators have followed the 
trend toward circuit integration, but external devices are still required 
for many reasons. Reasons are stability of oscillation requiring special 
components such as crystal oscillators, or the need of an inductor as part 
of the LC tank circuit, or the use of a voltage variable capacitor 
(varactor) also known as variable-reactance diode. 
An example of the prior art is disclosed in U.S. Pat. No. 5,055,889 
(Beall), which provides a lateral varactor that can be implemented in a 
MESFET integrated circuit. The fabrication methods for MESFET's are not 
suitable for bipolar circuits. U.S. Pat. No. 5,187,450 (Wagner et al.) 
describes a high-frequency voltage controlled oscillator (VCO) that can be 
implemented on a single integrated circuit (IC), without a varactor, but 
an external LC network is still required for frequency control. A highly 
stable, high frequency VCO for phase locked loops is disclosed in U.S. 
Pat. No. 5,191,301 (Mullgrav, Jr.). Here, the design may be integrated on 
a single chip; however, voltage control of the frequency is achieved by 
d.c. differential amplifiers coupled to a series of logic blocks formed in 
a ring. 
In the Oct. 3, 1994 issue of Electronic Design an article was published by 
Michael A. Wyatt and Larry A. Geis, titled "Varactorless HF Modulator". In 
it a circuit is described which utilizes the base-charging capacitance of 
a transistor in conjunction with a high-quality, low-loss, ceramic coaxial 
shorted quarter-wave transmission line. Sustained oscillations are 
produced when a negative resistance "seen" at the base of the transistor 
reacts with the transmission line. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide circuits and a method 
for a variable frequency, voltage controlled oscillator which is fully 
integrated. 
Another object of the present invention is to provide a circuit for a 
variable frequency, voltage controlled oscillator where the control 
voltage is within the range of the supply voltage. 
These objects have been achieved by varying, with a control voltage, the 
diffusion capacitance of one transistor of a high frequency, integrated 
oscillator circuit. A change in the diffusion capacitance affects the 
capacitance of the oscillator's LC network and, thereby, changes the 
frequency of oscillation of the circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, we show a high level block diagram according to a 
preferred embodiment of this invention. Oscillator Circuit 100 has output 
101 and output 102 (V.sub.OUT). Current Source Circuit 200 has input 201 
(V.sub.CONTROL) and input 202. Input 201 controls current I.sub.CONTROL 
flowing from output 101 to input 202. I.sub.CONTROL can be implemented 
many ways, such as using a simple resistor or any of the existing current 
source circuits. A preferred embodiment of such a current source circuit 
will be shown later. 
FIG. 2 is a graph, where Curve 1 relates the frequency of oscillation 
f.sub.OSC on the y-axis to the current I.sub.CONTROL on the x-axis. It is 
apparent that for most of the range of I.sub.CONTROL the frequency 
decreases monotonically. I.sub.CONTROL, therefore, is well suited to 
controlling the oscillator frequency. 
Referring now to FIG. 3, we show a circuit diagram according to an 
embodiment of the invention. Transistor pair 110 and 120 form the 
oscillator. The inductor 111 together with the total capacitance from Node 
N1 to ground, namely C1, determines the frequency of oscillation. A major 
part of C1 is formed by the diffusion capacitance (also known as 
base-charging capacitance) of transistor 120. The diffusion capacitance 
C.sub.D of a bipolar transistor is given by: 
##EQU1## 
where .tau..sub.F is the base transit time, I.sub.C is the collector 
current, q is the electron charge, k is Boltzmann's constant and T is the 
temperature in Kelvin. 
For this circuit, I.sub.CONTROL is given by the sum of the collector 
currents of transistors 110 and 120. Thus, by changing I.sub.CONTROL the 
frequency of oscillation can be varied. 
Still referring to FIG. 3, the voltage reference V.sub.CC is connected to 
terminal 103, and the reference potential is connected to terminal 203. 
Transistors 110 and 120 have their emitters connected to output 101. The 
base of transistor 110 is connected to terminal 103 and the collector is 
connected to inductor 111. The other side of inductor 111 is connected to 
terminal 103, as is the collector of transistor 120. The base of 
transistor 120 is connected to both the collector of transistor 110 and 
the base of transistor 130. This node is labeled node N1. Output stage 150 
buffers the oscillator from output 102 (V.sub.OUT) by providing signal 
amplification. Still referring to FIG. 3 an embodiment of the output stage 
150 is shown using npn transistors. Transistor 130 with its emitter 
resistor 131, and transistor 140 with its emitter resistor 141 are emitter 
follower circuits and provide current amplification for output 102 
(V.sub.OUT), with a low impedance output of typically 50 ohms. The low 
output impedance of an emitter follower matches the impedance of external 
test equipment and is therefore preferred when making measurements on the 
oscillator. 
FIGS. 4a, and 4b, show two other embodiments of the output stage 150. In 
the embodiment of FIG. 4a, transistor 130 is implemented as a n-channel 
metal oxide semiconductor field effect transistor (MOSFET). Since a MOSFET 
provides a much higher input impedance than a bipolar transistor there is 
less loading on the tank circuit (comprising transistors 110 and 120, 
inductor 111, and node capacitance N1), resulting in better phase-noise 
performance. In the embodiment shown in FIG. 4b, resistor 141 is placed 
between terminal 103 (V.sub.CC) and the collector of transistor 140, 
changing the output stage from an emitter follower to a common emitter 
configuration. In contrast to an emitter follower stage, the output 
impedance of a common emitter stage can be nicely matched to the typical 
impedance of an integrated circuit of about 300 ohms, and is therefore 
preferred when the oscillator is used in an integrated circuit. Other 
variations of transistor type and choice of circuit for the output stage 
are possible and known to those skilled in the art. 
The preferred embodiment of the Current Source Circuit 200 consists of 
transistors 210 and 220. The collector of transistor 210 is connected 
through resistor 211 to input 201 (V.sub.CONTROL). Both bases are 
connected to each other, while both emitters are connected to the 
reference potential, terminal 203. Transistor 210 is a current source by 
having its collector connected to its base. The collector of transistor 
220 is connected to input 202, which is connected to output 101 of the 
Oscillator Circuit 100. When V.sub.CONTROL is increased, the current 
flowing into the base of transistor 220 is increased, thus increasing the 
current flowing into the collector of transistor 220. A voltage change at 
input 201, which can vary between approximately 1 volt and V.sub.CC, 
provides thus a convenient means of current control for transistor 120, 
and causes the oscillating voltage at output 102 to be varied in 
frequency. Other embodiments of the current source circuit are also 
possible. In addition, I.sub.CONTROL may be implemented using a simple 
resistor. 
One other embodiment of current source 200 is illustrated in FIG. 5. Both 
transistors 210 and 220 are implemented as n-channel MOSFET transistors. 
Other embodiments, such as using p-channel MOSFET transistors, are 
possible and known to those skilled in the art. 
Referring now to FIG. 6, we show a block diagram of the method of the 
present invention. Block 601 shows providing an oscillator with an LC 
network, which furnishes the time constant for the frequency of 
oscillation. In Block 602, a transistor for the oscillator is provided 
having a diffusion capacitance C.sub.D. This diffusion capacitance is a 
significant part of the capacitance C of the LC network. Block 603 shows 
varying the collector current of the transistor. By varying the collector 
current, the diffusion capacitance is varied as indicated in Block 604, 
thus changing the capacitance C of the LC time constant and, thereby, the 
frequency of oscillation. Varying the collector current can be achieved by 
the use of a current source circuit. 
One advantages of the described circuits and method of this invention is 
that implementing the modulation of the diffusion capacitance is 
inexpensive, thus minimizing cost. A second advantage of this invention is 
that now a variable frequency, voltage controlled oscillator can be fully 
integrated into an IC, avoiding any or all external devices. A third 
advantage is that the voltage required to vary I.sub.CONTROL is within the 
range of the supply voltage V.sub.CC. This is of great importance in 
portable systems with limited, single voltage supplies. 
The disadvantage of the prior art, using a variable capacitance diode 
(varactor), is that a varactor is an external device and is difficult to 
integrate, and that it requires a large voltage swing (typically 6 volt) 
to achieve a reasonable capacitance change. This can be a problem in many 
portable systems where the battery voltages are limited. Also varactors do 
not permit linear control of frequency in contrast to this present 
invention and as discussed earlier and shown in the graph of FIG. 2. 
The present invention also differs from the circuit described in the Oct. 
3, 1997 article in `Electronic Design`, and referenced in `Description of 
Prior Art` earlier in this document by not requiring an external 
transmission line. A high-quality, low-loss, ceramic coaxial shorted 
quarter-wave transmission line adds extra expense to a product. 
Additionally, the change in frequency is very small. In contrast, in the 
present invention the control current changes a lot and the frequency of 
operation varies over several hundred MegaHertz. For example, FIG. 2 shows 
the control current ranging from 4 mA to 12 mA and the frequency ranging 
from nearly 1100 MHz to almost 1500 MHz. 
While the invention has been particularly shown and described with 
reference to the preferred embodiments thereof, it will be understood by 
those skilled in the art that various changes in form and details may be 
made without departing from the spirit and scope of the invention.