Oscillators – Solid state active element oscillator – Transistors
Reexamination Certificate
1998-10-23
2001-04-17
Kinkead, Arnold (Department: 2817)
Oscillators
Solid state active element oscillator
Transistors
C331S17700V, C331S03600C, C331S03600C, C331S179000, C455S191200
Reexamination Certificate
active
06218909
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to radio frequency (RF) oscillators and more particularly relates to RF oscillators able to generate RF carriers or modulated signals over a wide range of frequencies.
BACKGROUND OF THE INVENTION
Achieving wide band frequency coverage in conventional voltage controlled oscillators (VCOs) requires that the VCO have a wide tuning voltage range. This is often difficult to implement and also creates modulation-related problems in VCOs that are intended to be used for frequency modulation (either digital or analog). The implementation difficulties arise from voltage supply limitations and the modulation related problems arise due to the fact that a single tank circuit is used to determine the center frequency of the oscillator. In order to achieve a wide frequency range while maintaining a reasonable level of modulation sensitivity, i.e., tuning factor, a consequently wide tuning voltage range is required. VCOs having very wide tuning ranges are, however, generally difficult to construct. Usually, in order to achieve a wide frequency tuning range with a reasonable control voltage range, a VCO having a high tuning/modulation factor (in units of MHz/V) must be used.
A consequence of using a high tuning factor, i.e., a limited tuning voltage range, is that the tuning input of the VCO becomes highly sensitive to additive noise and more prone to extraneous signal pick up. This causes the phase noise level of the oscillator to increase resulting in the degradation of the performance of the system that incorporates the VCO. In addition, costly shielding may be required to prevent pick up of such noise.
When a modulating baseband signal is to be summed directly with the tuning voltage signal, or indirectly through a separate port affecting the center frequency of the oscillator's tank circuit, the resultant frequency deviation may vary as a function of the oscillator center frequency. The oscillator's center frequency is determined by the average, i.e., DC value, of the control voltage applied to the modulation/tuning input of the VCO. Note that both analog frequency modulation and digital Frequency Shift Keying (FSK) can be achieved by summing the tuning voltage signal and the baseband signal representing the input data signal.
In the majority of applications it is undesirable to have different tuning sensitivities at different points in the tuning range of the oscillator. Thus, the use of simple wide band oscillators is precluded as such devices typically exhibit this undesirable characteristic. Avoiding an oscillator having different tuning sensitivities within the tuning range typically requires the use of complicated circuitry or the addition of linearization circuits to alleviate the linearity problems associated with tuning and modulating a conventional oscillator over a wide frequency range.
One solution to this problem is to construct a VCO to have a narrow range of frequency coverage with a limited tuning voltage range, but which is capable of switching between frequency bands. The ability to switch between frequency bands permits the VCO to maintain a reasonable tuning/modulation factor within each of the frequency bands. This is typically realized by switching various components such as capacitors, inductors or variable-capacitors that are used in the tank circuit of the oscillator in and out of the circuit. The switching of the components causes the VCO to shift from one band to another. Thus, the frequency tuning range of the VCO is extended without extending the actual tuning voltage range (on a single variable capacitance diode for example) and without imposing a high tuning factor. Note that in addition to the switching elements themselves, e.g., RF PIN diodes, additional capacitors, inductors or variable capacitance diodes are typically necessary.
Oscillators with the capability of switching resonant frequencies are known in the art. U.S. Pat. No. 4,694,262, issued to Inoue et al., discloses an oscillation circuit consisting of a resonator and having a frequency switching means for switching the oscillation frequency of the resonator. The switching element used is a diode.
U.S. Pat. No. 4,536,724, issued to Hasegawa et al., discloses a VCO having an LC resonant circuit which includes a varactor circuit configured so as to control the resonant frequency by means of a DC bias control voltage applied to the varactor circuit.
Various types of well known oscillator circuits can be modified to achieve oscillation and frequency range shifts. Presented below are examples of prior art oscillators adapted to provide frequency range shifting. More detailed descriptions on the various prior art oscillators discussed below can be found in Chapter 5 of H. L. Krauss, C. W. Bostian and F. H. Raab, Solid State Radio Engineering, John Wiley, 1980.
A schematic diagram illustrating a prior art grounded base Colpitts oscillator adapted to provide frequency range switching is shown in FIG.
1
. The basic oscillator circuit, generally referenced
10
, comprises a transistor
32
and resonant circuit that consists of capacitors
24
,
26
,
30
and inductor
28
. Applying a tuning voltage via resistor
14
to varactor
18
in series with capacitor
16
varies the frequency of oscillation. The frequency of oscillation can be modulated via a modulation input signal by varying the capacitance coupled to the emitter. A modulation input is applied to a varactor
36
in series with capacitor
34
and the emitter of transistor
32
. The RF output of the circuit is the emitter voltage.
Note that the circuit described above is one representative possibility. In practice, the modulator circuit comprising capacitor
34
, varactor
36
and resistor
38
can be connected across any frequency determining impedance, e.g., inductor
28
and capacitors
30
,
34
,
24
or
26
.
The operating point of the transistor
32
is defined by a voltage divider (not shown) that defines the bias voltage of the transistor base. With this bias system and supply of collector voltage, the transistor is placed in an operating state in which it can provide amplification. Feedback capacitors connected from collector to emitter and from emitter to ground create a state in which connection of a parallel tuned circuit from collector to ground will give rise to electrical oscillations and the circuit becomes an oscillator. Since the feedback capacitors are effectively in parallel with the tuned circuit, the resultant capacitive loading greatly restricts the available tuning range available from this type of oscillator.
The shift in the frequency range is accomplished by switching a range shift capacitor
20
in and out of the circuit. The switching action is performed by diode
22
, which may be a PIN, or regular diode. A range control signal is applied via resistor
12
. When the range control signal is high, the diode is forward biased and functions to couple the capacitor
20
to ground. Adding the capacitance
20
to the circuit causes a shift in the resonant frequency. To remove the capacitor, a high impedance is applied to the range control input. During the positive half cycle of the RF, the capacitor
20
charges providing a current through forward biased diode
22
. During the negative half cycle of the RF, the capacitor cannot discharge as the diode is reverse biased. After a few cycles of RF, the capacitor is charged and the current is reduced. Eventually, the current stops and the diode is no longer forward biased. At this point, the diode and the capacitor are effectively out of the circuit.
This may also be accomplished by applying a negative voltage at the range control input. It is important to note that the value of capacitor
20
is critical to the oscillation frequency. Capacitor
20
, together with the other resonant components, determines the frequency of oscillation. Thus, to achieve accurate frequencies, a precision capacitor must be used which increases the cost of the circuit.
Note also that the schematic diagrams shown in
FIGS. 1 thro
Kinkead Arnold
Texas Insturments Israel Ltd.
Zaretsky Howard
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