GB2127636A

GB2127636A - Voltage controlled oscillator circuit

Info

Publication number: GB2127636A
Application number: GB08319600A
Authority: GB
Inventors: Masayuki Ikeda
Original assignee: Epson Corp
Current assignee: Epson Corp
Priority date: 1982-08-05
Filing date: 1983-07-20
Publication date: 1984-04-11
Also published as: JPH0566047B2; JPS5927615A; HK36988A; GB8319600D0; GB2127636B

Abstract

A voltage controlled oscillator circuit comprises: a first signal synthesising circuit (701) for combining signals applied to first and second terminals (709,710); a second signal synthesising circuit (703) having the same characteristics as the first signal synthesising means and having a first input terminal (711) for receiving a reference level signal and a second terminal (712); a first oscillator circuit (702) whose frequency of oscillation is controlled by an output signal from the first signal synthesising circuit; a second oscillator circuit (704) having the same characteristics as the first oscillator circuit and controlled by an output signal from the second signal synthesising circuit; and reference means (707) for generating a reference frequency signal, the arrangement being such that, in operation, the output from the second oscillator circuit is compared in phase with the signal generated by the reference means, the resultant signal being applied to the second terminal (712) of the second signal synthesising means to control the output frequency of the second oscillator circuit, the resultant signal being also applied to the second terminal (710) of the first signal synthesising means, the first input terminal of the first signal synthesising means acting as a control terminal of the voltage controlled oscillator circuit, the output of the first oscillator circuit acting as an output of the voltage controlled oscillator circuit. <IMAGE>

Description

SPECIFICATION Voltage controlled oscillator circuit This invention relates to voltage controlled oscillator circuits.

Although the present invention is primarily directed to any novel integer or step, or combination of integers or steps, herein disclosed and/or shown in the accompanying drawings, nevertheless, according to one particular aspect of the present invention to which, however, the invention is in no way restricted, there is provided a voltage controlled oscillator circuit comprising: a first signal synthesising means for combining signals applied to first and second terminals; a second signal synthesising means having the same characteristics as the first signal synthesising means and having a first input terminal for receiving a reference level signal and a second terminal; a first oscillator circuit whose frequency of oscillation is controlled by an output signal from the first signal synthesising means; a second oscillator circuit having the same characteristics as the first oscillator circuit and controlled by an output signal from the second signal synthesising means; and reference means for generating a reference frequency signal, the arrangement being such that, in operation, the output from the second oscillator circuit is compared in phase with the signal generated by the reference means, the resultant signal being applied to the second terminal of the second signal synthesising means to control the output frequency of the second oscillator circuit, the resultant signal being also applied to the second terminal of the first signal synthesising means, the first input terminal of the first signal synthesising means acting as a control terminal of the voltage controlled oscillator circuit, the output of the first oscillator circuit acting as an output of the voltage controlled oscillator circuit.

According to another non-restrictive aspect of the present invention there is provided a voltage controlled oscillator circuit comprising: a first signal synthesising circuit for receiving two signals and producing a signal based on those signals, one of the two signals being an external signal; a first voltage controlled oscillator connected to the output of the first signal synthesising circuit; a phase-locked loop consisting of a second signal synthesising circuit, a second voltage controlled oscillator and a phase comparator; and an oscillator for generating a signal of a stable frequency, the second signal synthesising circuit having the same characteristics as the first signal synthesising circuit, the second voltage controlled oscillator being connected to the output of the second signal synthesising circuit and having the same characteristics as the first voltage controlled oscillator, one input of the comparator being connected to the output of the second voltage controlled oscillator, the other input of the comparator being connected to the oscillator, the output of the comparator being connected to one input of the first signal synthesising circuit which does not receive the external signal and to one input of the second signal synthesising circuit.

The voltage controlled oscillator circuit may include means for supplying a reference frequency signal to the second input of the second signal synthesising circuit.

The oscillator may be a crystal oscillator.

The voltage controlled oscillator circuit, in one embodiment, includes a frequency divider or frequency converter connected between the second voltage controlled oscillator and the comparator.

The voltage controlled oscillator circuit, in another embodiment, includes a frequency divider or frequency converter connected between the comparator and the oscillator.

In a still further embodiment the voltage controlled oscillator circuit includes a frequency divider or frequency converter connected between the second voltage controlled oscillator and the comparator and a further frequency divider or frequency converter connected between the comparator and the oscillator.

In yet another embodiment of the present invention the voltage controlled oscillator circuit includes a frequency divider connected to the output of the first voltage controlled oscillator circuit.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which:~ Figure 1 is a block diagram of one embodiment of a voltage controlled oscillator circuit according to the present invention; Figure 2 is a circuit diagram of another embodiment of a voltage controlled oscillator circuit according to the present invention; Figure 3 is a diagram of a conventional voltage controlled oscillator circuit; and Figure 4 is a diagram of another conventional voltage controlled oscillator circuit.

Conventionally, a quite large number of voltage-controlled oscillator (VCO) circuits have been suggested. As an example, a VCO circuit for use in a complementary metal-oxide integrated circuit on one chip used for a phase-locked loop is shown in Figure 3, in which the electric current flowing into a capacitor C1 is controlled by the control voltage applied to the gate of an Nchannel transistor N1 to control the frequency of oscillation. Resistors R1, R2 determine the control voltage sensitivity coefficient and the free-running frequency, respectively.

Another VCO circuit as disclosed in Japanese Patent Publication No. 86509/81 is shown in Figure 4, in which current flowing into ring oscillators is controlled by the gate voltage to transistors T41-T46 connected to the sources to control the frequency of oscillation. This VCO circuit has advantages over the circuit of Figure 3 in dispensing with externally attached parts having a lower current consumption and requiring less space to mount components, but it is difficult to obtain good accuracy and stability. Also, it cannot be said that the VCO circuit of Figure 3 has sufficient stability.

Generally, what must be regulated in a VCO circuit are the free-running frequency and the voltage control sensitivity coefficient. The free running frequency is defined as the frequency of oscillation when the control voltage applied to the control terminal of a VCO circuit as at a reference voltage level, which is usually set at a central value of the controllable range of input voltage.

For instance, in the case of a CMOS IC, the reference level is set to one half that of the power supply voltage. If it is assumed that the difference between the control voltage Vc and the reference voltage level, is AVc that AVc=vc~Vs (1) Thus, it may also be said that the free-running frequency is the frequency of oscillation when AVC=O. The voltage control sensitivity coefficient Kv is defined by fo=fc+Kv AV (2) where fc is the free-running frequency and f, is the frequency of oscillation of the VCO circuit.

Drift in the free-running frequency in a phaselocked loop has an adverse effect on the loop because it introduces drift in the capture range of the system. Also, dispersion in the frequency fc due to dispersion in constants of circuit components is so large that it cannot be neglected. Therefore, high-precision components have been conventionally used at the sacrifice of curtailment of cost, or otherwise adjustments or alignments using semi-fixed resistors or semifixed capacitors have been required after assembly. Also, drift in voltage in the voltage control sensitivity coefficient Kv in a phase-locked loop has an adverse effect on the loop because it results in drift in the response velocity.

These drafts are attributed to change in the ambient temperature, fluctuation in the power supply voltage, ageing of component constants, etc. Especially, the free-running frequency fc is affected greatly by these factors. On the other hand, variation in the coefficient Kv can be reduced by providing sufficient relative tracking characteristics employing semiconductor and integrated circuit techniques such that the value Kv depends on relative accuracy among components of the circuit, although absolute accuracy of the constants of elements is not obtained.

Referring to Figure 1, there is shown one embodiment of a voltage controlled oscillator (VCO) circuit according to the present invention.

The VCO circuit includes signal synthesising circuits 701, 703 designed so as to have the same characteristics. Although voltage is treated in this circuit, other physical quantities such as current or electric charge can be similarly treated.

The circuit 701 has a characteristic expressed by Vol=f (V11, V21) (3) where fis an arbitrary function when voltages V and V21 are applied to terminals 709, 710, respectively, and a voltage V01 appears at output terminal 714 of the signal synthesising circuit 701. For simplicity, the equation above is expanded as follows: V01=aV11+bV21+c (4) where a, b and c are constants.

Similarly, when the voltages V12 and V22 are applied to terminals 711, 712, respectively, and a voltage V02 appears at output terminal 715 of the signal synthesising circuit 703, it is assumed that the following relation holds: V02=aV12+bV22+c (5) The present circuit also includes a pair of voltagecontrolled oscillators 702. 704 having the same characteristics. It is now assumed that the frequencies Ft and F2 of the output signals from their respective VCOs 702, 704 can be expressed by F1=Kv V01+d (6) F2=Kv ~ V02+d (7) where d is a constant.

Also provided are a phase comparator circuit 706 and an oscillator circuit 707, which generates a signal of a stable frequency, such as a crystal oscillator circuit. The comparator circuit 706 acts to compare the output from the VCO 704 in phase with the output signal from the oscillator circuit 707 and to deliver a signal whose frequency is proportional to the phase difference between the two signals. A low-pass filter 705 is inserted into the circuit to derive only a desired signal component from the output from the phase comparator circuit in the ordinary way.

The output from the filter 705 is inversely fed back to the input terminal 712 of the signal synthesising circuit 703. That is, the VCO 704, comparator circuit 704, filter 75 and the signal synthesising circuit 703 form a phase-locked loop and the oscillation frequency of the second VCO is made equal to the oscillation frequency f,ef of the oscillator circuit 707. It is possible to cause the frequency of the output from the VCO 704 to agree entirely even in phase with the frequency fref by virtue of the characteristics of the comparator circuit 706 and the filter 705.

Alternatively, that frequency may be caused not to respond to a sudden change in the frequency fref. Since a circuit oscillating sufficiently stably is used for the oscillator circuit 707 in the ordinary manner, making the response of the system more rapid will introduce no problem. Also, because the output frequency of the VCO 704 can be in step with the frequency #rnf only in frequency and presence of error in phase is allowed, a considerable degree of freedom is given to the circuit configuration. If the oscillator circuit 707 does not operate stably and jitter, for example, takes place, the system can be so designed that it is less affected thereby.

When the free-running frequency of the VCO 704 varies due to variation in the power supply voltage, temperature characteristics, ageing or the like, the system automatically increases or decreases the voltage applied to the terminal 712 to maintain the oscillation frequency of the VCO 704 at freP Also, in the case where an arbitrary value of voltage is applied to the terminal 711 of the signal synthesising circuit 703, the voltage applied to the terminal 712 is automatically controlled so that the oscillation frequency of the VCO 704 may be kept at fed irrespective of the value of the applied voltage.

Thus, when a reference voltage V5 is applied to the terminal 711 of the signal synthesising circuit 703, the level of voltage applied to the terminal 712 is automatically adjusted such that the oscillation frequency of the VCO 704 is made equal to free. In a situation where the voltage applied to the terminal 712 of the second signal synthesising circuit 703 is also fed to the terminal 710 of the signal synthesising circuit 701 as shown in Figure 1 , the frequency of oscillation of the VCO 702 is set to fre, when the voltage applied to the terminal 709 of the signal synthesising circuit 701 is Vs, because the signal synthesising circuit 701 and the VCO 702 have the same characteristics as the signal synthesising circuit 702 and the VCO 704, respectively.When the voltage is applied to the terminal 709 is Vs, the frequency of oscillation of the VCO 702 assumes a value fc if the frequency f,,, is set to a desired free-running frequency fce Therefore, a voltage controlled oscillator circuit whose free-running frequency is fc is provided by using the terminals 702,713 as the control terminal and the output terminal respectively, of the whole circuit of Figure 1.

The VCO circuit of Figure 1 has been described thus far on the assumption that the signal synthesising circuit 701 and the VCO 702 have the same characteristics as the signal synthesising circuit 703 and the VCO 704, respectively. This assumption is quite reasonable.

Particularly when the VCO circuit is formed into a monolithic integrated circuit, each pair of elements can be accurately symmetrically fabricated on a chip, a few millimetres in each dimension. Since the components are fabricated at the same time, even if they experience ageing, their characteristics change in the same manner with time and so it is unlikely that the characteristics come to differ from each other widely. Further, because each pair of components is supplied with electric power from the same power supply and because they are disposed in close proximity, variation in temperature from component to component is a minimum and the possibility that their characteristics come to differ from each other widely is also small.In making designs for the integrated circuit, if the symmetry of each pair of components is considered fully so that variations in the characteristics may cancel each other, then a stable voltage controlled oscillator circuit can be obtained which is not accompanied by large drift.

Referring next to Figure 2 there is shown another embodiment of a voltage controlled oscillator (VCO) circuit according to the present invention in the form of a semiconductor integrated circuit fabricated on the concept described above. A signal synthesising circuit 801 has transistors T1, T2 whose drain currents are changed by changing the gate voltages of the transistors. The drain current of the transistors T1, T2 are combined into one and caused to flow into a transistor Ta3, in which it is converted into a voltage. This voltage is applied to a first voltagecontrolled oscillator (VCO) 802 consisting of a MOS transistor as its control voltage. This VCO includes an odd number of inverter stages made up of transistors T7, T10.Ts, T11 . .. Tg, T12 to form ring oscillators. Other transistors T4,T5, ..... . T1s, T,6, T17 are connected in series with the sources of the former transistors, and the gate voltage of the latter transistors are connected to control the current supplied to the ring oscillators from the power supply for exerting control over the frequency of oscillation. In the example, as the voltages at terminals 812, 814 decrease, the drain voltage of transistor T,3, that is, the control voltage of the VCO 802, increses, resulting in increase in the frequency of oscillation. This is the case where a and b are negative in equation (4) and Kv is positive in equation (6).When the voltage levels at the terminals 812, 814 are high, if a high frequency of oscillation is desired, then the polarities of the transistors in the VCOs 801, 802 should all be reversed, that is, a P-channel transistor is changed to an N-channel transistor and vice versa, such that a, b and Kv assume negative values. The VCO circuit of Figure 8 also includes buffer circuits 805, 806 to deliver an output.

A signal synthesising circuit 803 and a voltage controlled oscillator 804 have the same circuit configurations as the circuits 801, 802, respectively. Having the same internal construction, the construction of each of the circuits 803, 804 is not shown. The output from the VCO 804 is applied to a phase comparator circuit 806 through a buffer 807. A crystal oscillator circuit 811 generates a frequency #ref(=fc) that is used as a standard for the freerunning frequency. Usually, this signal is applied to the phase comparator circuit 808, in which it is compared in phase with the VCO 804. This signal generated by the oscillator 811 is used together with timing clock signals for other circuits, the clock signal for the system or the like. If the frequency of clock signals necessary for another circuit differs from fcf then frequency divider circuits or frequency converter circuits can be connected to both or one of nodes 818,819 to set the frequency fc to a frequency an integer times the frequency of oscillation of the oscillator circuit 811, the frequency of oscillation divided by an integer, differences between those frequencies or the frequency of oscillation multiplied by a rational fraction. Any frequency divider circuit or frequency converter circuit can be comprised of a digital circuit and no difficulty is introduced in forming it into a semiconductor integrated circuit.

A low-pass filter 81 7 removes the high frequency components contained in the output from the phase comparator circuit 808, and the output from the filter 817 is fed back to a second input terminal 815 of the signal synthesising circuit 803. Resistors R11, R12 are connected to a first input terminal 813 to obtain a fraction of the voltage of the power supply so that an input signal level or reference level sufficient to allow an oscillation at frequency fc is provided. Although it is difficult to form resistors having accurate values in a semiconductor integrated circuit, the relative accuracy among those values can be enhanced greatly. It is also possible to connect a more accurate voltage source, such as a reference voltage source using zener diodes, to this terminal.The input terminals 814,815 of the signal synthesising circuits are connected to different points on opposite sides of a resistor R14 inside the filter 81 7. This resistor serves to stabilise the phase-locked loop in essentially the same manner as in the case of Figure 1.

The configuration of Figure 2 consists entirely of MOS transistors other than resistors R11-R15, capacitors C11-C13 and crystal oscillator C.

Absolute accuracy is not required for resistors and capacitors and so resistors can be incorporated into a semiconductor integrated circuit. Further, capacitor C11 can be often incorporated into it, although it depends on the required range of oscillation frequency. When a low frequency is needed, a frequency divider circuit is connected to an output terminal 81 6 to increase the frequency of oscillation of the phase-locked loop. Hence, only a small value is necessary for the capacitor C1 with the result that forming the circuit into an integrated circuit is rendered easier.

As described hereinbefore, in accordance with the present invention, a quite stable voltage controlled oscillator circuit can be fabricated without using accurately fabricated components, which makes it quite easy to fabricate the VCO circuit in the form of an integrated circuit. Thus, there are great advantages in the VCO circuit from the point of view of packaging of components and manufacture.

When the VCO circuits shown in Figures 1 and 2 are compared with the VCO circuit shown in Figures 3 and 4 one might suppose that they are considerably more complex. In actuality, however, when the VCO circuit of Figure 2 is fabricated in a semiconductor integrated circuit, the area occupied by the circuit on that chip is small. On the other hand, the VCO of Figure 4 requires externally attached parts, which needs a large area occupied by bonding pads on a semiconductor integrated circuit and output transistors, for example, P4, Pst N2, N3 for driving the externally attached capacitor C1 and P1, N1 for driving the externally attached resistors R1, R2, The area occupied by these parts is much greater than the area of the VCO circuit of the present invention.

The present invention necessitates a stable oscillator circuit such as a crystal oscillator, but this introduces no problem, because large-scaled integrated circuits frequently require stable reference pulse trains besides voltage-controlled oscillator circuits and these pulse trains may be also use for that purpose. In addition, in the VCO circuits according to the present invention, freerunning frequency can be set at will with the same circuit by connecting frequency divider circuits or frequency converter circuits in series with the node 71 6 or 717 of Figure 1 or the node 818 or 819 of Figure 8 and controlling the frequency division ratio or other factor using a logic circuit.

A voltage controlled oscillator circuit according to the present invention may be readily formed into an integrated circuit such as a digital integrated circuit. This achieves a reduction in cost and packaging space, thus greatly contributing to the manufacture of devices incorporating VCO circuits.

Claims

1. A voltage controlled oscillator circuit comprising: a first signal synthesising means for combining signals applied to first and second terminals; a second signal synthesising means having the same characteristics as the first signal synthesising means and having a first input terminal for receiving a reference level signal and a second terminal; a first oscillator circuit whose frequency of oscillation is controlled by an output signal from the first signal synthesising means; a second oscillator circuit having the same characteristics as the first oscillator circuit and controlled by an output signal from the second signal synthesising means; and reference means for generating a reference frequency signal, the arrangement being such that, in operation, the output from the second oscillator circuit is compared in phase with the signal generated by the reference means, the resultant signal being applied to the second terminal of the second signal synthesising means to control the output frequency of the second oscillator circuit, the resultant signal being also applied to the second terminal of the first signal synthesising means, the first input terminal of the first signal synthesising means acting as a control terminal of the voltage controlled oscillator circuit, the output of the first oscillator circuit acting as an output of the voltage controlled oscillator circuit.

2. A voltage controlled oscillator circuit comprising: a first signal synthesising circuit for receiving two signals and producing a signal based on those signals, one of the two signals being an external signal; a first voltage controlled oscillator connected to the output of the first signal synthesising circuit; a phase-locked loop consisting of a second signal synthesising circuit, a second voltage controlled oscillator and a phase comparator; and an oscillator for generating a signal of a stable frequency, the second signal synthesising circuit having the same characteristics as the first signal synthesising circuit, the second voltage controlled oscillator being connected to the output of the second signal synthesising circuit and having the same characteristics as the first voltage controlled oscillator, one input of the comparator being connected to the output of the second voltage controlled oscillator, the other input of the comparator being connected to the oscillator, the output of the comparator being connected to one input of the first signal synthesising circuit which does not receive the external signal and to one input of the second signal synthesising circuit.

3. A voltage controlled oscillator circuit as claimed in claim 2 including means for supplying a reference frequency signal to the second input of the second signal synthesising circuit.

4. A voltage controlled oscillator circuit as claimed in claim 2 or 3 in which the oscillator is a crystal oscillator.

5. A voltage controlled oscillator circuit as claimed in any of claims 2 to 4 including a frequency divider or frequency converter connected between the second voltage controlled oscillator and the comparator.

6. A voltage controlled oscillator circuit as claimed in any of claims 2 to 4 including a frequency divider or frequency converter connected between the comparator and the oscillator.

7. A voltage controlled oscillator circuit as claimed in any of claims 2 to 4 including a frequency divider or frequency converter connected between the second voltage controlled oscillator and the comparator and a further frequency divider or frequency converter connected between the comparator and the oscillator.

8. A voltage controlled oscillator circuit as claimed in any of claims 2 to 4 in which a frequency divider is connected to the output of the first voltage controlled oscillator circuit.

9. A voltage controlled oscillator circuit substantially as herein described with reference to and as shown in Figure 1 or Figure 2 of the accompanying drawings.

10. A voltage controlled oscillator circuit comprising: a first signal synthesising means having a first and a second terminals receiving their respective signals, the first means combining these signals into one and delivering the same, a second signal synthesising means having the same characteristics as the first means and having a first input terminal receiving a reference level signal and a second terminal, a first oscillator circuit whose frequency of oscillation is controlled by the signal from the first means, a second oscillator circuit having the same characteristics as the first oscillator circuit and controlled by the signal from the second means, and a means for generating a reference frequency signal, the output from the second oscillator circuit being compared in phase with the signal generated by the means, the resultant signal being applied to the second terminal of the second means to control the output frequency of the second oscillator circuit, the resultant signal being also applied to the second terminal of the first means, the first input terminal of the first means acting as the control terminal of the voltage controlled oscillator circuit, the output of the first oscillator circuit acting as the output of the voltage controlled oscillator circuit.

11. A voltage controlled oscillator circuit comprising: a first signal synthesising circuit for receiving two signals and producing a signal based on these signals, one of the two signals being an external signal, a first voltage controlled oscillator connected to the output of the synthesising circuit, a phase-locked loop consisting of a second signal synthesising circuit, a second voltage controlled oscillator and a phase comparator, and an oscillator for generating a signal of a stable frequency, the second signal synthesising circuit having the same characteristics as the first circuit the second voltage controlled oscillator being connected to the output of the second synthesising circuit and having the same characteristics as the first voltage controlled oscillator, one input of the comparator being connected to the output of the second voltage controlled oscillator, the other input of the comparator being connected to the oscillator, the output of the comparator being connected to one input of the first synthesising circuit which does not receive the external signal and to one input of the second synthesising circuit.

12. Any novel integer or step, or combination of integers or steps, hereinbefore described, irrespective of whether the present claim is within the scope of, or relates to the same or a different invention from that of, the preceding claims.