GB2189113A - A voltage-controlled oscillator - Google Patents

Abstract

A V-to-I converter 1 converts input voltage Vin to a control current Ic, which is fed to a capacitor C. A detection logic circuit 2 senses voltage Vc across capacitor C and applies it to two comparators A2 and A3 having a reference voltage of -Vx and OV respectively. Comparator A3 is biased to apply a "clear" to a D-type flip-flop 5, whenever voltage Vc exceeds OV, and comparator A2 is biased to clock a "1" whenever voltage Vc exceeds -Vx in the positive-going direction. The circuit 2 generates a frequency f, indicative of voltage Vc, which is fed to a charge pump 3, which discharges capacitor C by a predetermined amount at a rate dependent on the frequency f. Control current Ic then recharges capacitor C and the cycle is repeated. <IMAGE>

Description

SPECIFICATION A voltage-controlled oscillator This invention relates to a voltage-controlled oscillator and in particular, though not exclusively, to such an oscillatorfor use in applications of frequency modulation, such as in data recording.

Known oscillators used heretofore forthese a pal ications include a conventional astable multivibrator, the frequency of which is controlled by modulation of the currentfor charging the capacitors of the multivibrator.

However, such oscillators are generally only cap- able of operating within a bandwidth of up to 500KHz, whereas it is now considered desirable to increase the available bandwidth to 1MHz Ifthe operating frequency ofthese known oscillators is increased, it is usually necessary to protect the transistors against possible saturation, which may reduce circuit activity to a point where oscillation may not be capableoftaking place.

Furthermore, the effect of anytime delays in the circuit components ofthe oscillator, caused for example by propagation of switching, tends to become more significant as the operating frequency is increased, and unacceptable non-linearities can re suit, even if linearity correction techniques are employed.

It is therefore an object of the present invention to provide a voltage-controlled oscillator, which is capable of being used forfrequency modulation with a bandwidth of at least 1 without incurring the afore-mentioned problems associated with known oscillators.

In accordance with the present invention there is provided a voltage-controlled oscillator comprising means for converting an inputvoltageto a control current, charge storage means arranged to be charged by said control current, means for comparing the voltage of said charge storage means with a reference voltage and for directly controlling a logic circuit capable of generating an output of frequency dependent on said comparison, and means for discharging said charge storage means buy a predetermined amount at a rate dependent on said outputfrequency.

The invention will now be further described by way of example onlywith reference to the accompanying drawings, wherein: Figure 1 shows a block circuit diagram of a preferred embodiment of the invention, Figures 2 to 5 show circuit component diagrams for each of the block circuits to Figure 1, and Figure 6 shows a waveform diagram forthe circuit.

In Figure ia Vto I converter 1 converts input voltage Vjn to a control current lc, which is fed to a capacitor C. A detection logic circuit 2 senses voltage Vc acrnssthe capacitor C and generates a frequency f, indicative of voltage Vo at its output Q. The frequencyf isalsQfedfte a charge pump 3, which discharges capacitor/: Wa a predetermined amountata rate dependent on the frequency f. Control current it then charges capacitor C again and the cycle is repeated. A starter circuit4 is also provided to ensure that the circuit oscillates on each cycle.The ciruit, as briefly described, can thus be seen to operate in accordance with the known "charge-balance" principle.

The V and I converter 1, as shown in more detail in Figure 2, includes two equal resistors R1 and R2 connected respectively to transistors T, and T2. An amplifier A1 ensures voltage V1 is equal to the input voltage Vin, and therefore input voltage Vjn is also equal to IR3, where Ii is the current passing through transistorT1 and resistor R3. Since resistors R1 and R2 are equal, the output current lc ofthe converter 1 is equal to current 1, and is therefore also equal to Vjn/R3.

The outputcurrent Ic is therefore proportional to input voltage Vjn.

The current lc is fed to the capacitor C1 as shown in Figure 1, and the voltage Vc across capacitor C is sensed by detection logic circuit 2, which is shown in more detail in Figure 2.

The sensed voltage Vc is applied to the noninverting inputs of two high-speed, low-bias comparators A2 and A2, comparator A2 having a reference voltage of-Vx and comparator A3 having a reference voltage of 0V.

Referring now also to Figure 6, which shows a waveform of voltage Vc, comparator A3 is biased to apply a "clear" to a D-typeflip-flop 5, whenever voltage Vc exceeds 0V, and comparator A2 is biased to clock a "1 " flip-flop 5 whenever voltage Vc crosses reference -Vx in the positive-going direction, Output Q of the logic circuit 2 isthen passed to charge pump 3.

Charge pump 3, as shown in detail in Figure 4, includes an amplifierA4which switches the input voltage between 0 and +Vrvolts atfrequencyf. On positive transitions of the inputvoltagefrom 0 to +Vr volts, a charging current for capacitor C1 flows through diode D1 and no current flows through transistorT3. On negativetransistionsoftheinput voltage from +Vr to 0 volts, the charging current for capacitor C1 flows through transistorT3 and dis charges capacitorC, shown in Figure 1, bya predeter mined amount, so that the voltage Vc across capacitor C drops to approximately - -2Vx, thereby making the waveform of output 0 substantially symmetrical.

Resistor R4 in charge pum p 3 limits the current throughthecircuitto preventtransistorT3from saturating. Capacitor C1 preferably has a temperature coefficient, which cancels any temperature variations in the voltages of diode D1 and transistorT3, so that the charge on capacitor C1 remains substantially constant with temperature. Voltage Vr is constant regulated voltage and amplifier A4 is preferably a CMOS inverter, which has no inherent voltage drop and only an output resistance which can be added to that of resistor R4. Time constant C1 R4 is chosen to be sufficiently short to enable capacitor C1 to charge with sufficient accuracy when the frequency f is relatively high.

Once the charge pump 3 has discharged capacitor C sothatthevoltagethereacross has dropped to approximately 2Vx, the control current IC charges capacitor Cto a voltage Vc of 0V again and then the cycle is repeated.

In normal operation, the voltage Vctherefore oscillates between 2Vx and 0V. If it has, however, an instantaneous value less than 2Vx, then it will ramp in the positive direction and carry on oscillating. If, on the other hand, it has an instantaneous value greater than 0V, it could ramp in the positive direction until the voltage is limited bysaturation ofthe current source and no oscillation occurs. This condition can be detected and corrected by the starter circuit 4 shown in more detail in Figure 3.

Normally, the capacitor C has an average voltage of approximately -Vx and amplifierA4saturates a positive-holding diode D2. When the voltage Vc goes above 0V, capacitor C charges through resistor R5 until the output of amplifierA4 goes negative. Due to resistor R5 and capacitor C1, amplifier A4 is unstable and its output makes diode D2 and the voltage V, go negative. When the voltage Vc goes below -2V,, capacitor C discharges through resistor Rg until the output of amplifier A4 goes positive. Diode D2 is thus made non-conducting and voltage Vc ramps positively into a normal oscillation cycle.

By use, of, inter alia, a logic circuit such as D-type flip-flop 5, and direct inputs into this circuitfrom comparators A2 and A3, the present circuit is capable of achieving high speed and accurate operation, which is required for an operating frequency having a bandwidth of at least up to 1 MHz.

Furthermore, by making the outputQofthe logic circuit5 approximately symmetrical, sufficient time is allowed for both charging and discharging of capacitor C, during each cycle of the voltage waveform.

It will be appreciated by persons skilled in the art that alternative circuit a rrang ments to these described may be employed for various parts, such as Vto I converter, ofthe presentvoltage-controlled oscillator.

Claims (4)

1. Avoltage-controlled oscillator comprising means for converting an inputvoltageto a control current, charge storage means arranged to be charged by said control current, means for comparing the voltage of said charge storage means with a reference voltage andfor directly controlling a logic circuit capable of generating an output offrequency dependent on said comparison, and means for discharging said charge storage means by a predetermined amount at a rate dependent on said outputfrequency.

2. An oscillator as claimed in Claim 1 and including means for ensuring that oscillation still occurs wheneverthevoltageofsaid storage means exceeds a predetermined maximum value orfalls below a predetermined minimum value.

3. An oscillator as claimed in Claims 1 or 2 wherein said logiccircuit comprises a D-typeflip-flop.

4. Avoltage-controlled oscillator substantially as herein described with reference to the accompanying drawings.

4. An oscillator as claimed in any preceding claim wherein said comparing means comprises a first comparator having a first reference voltage and a second comparator having a second reference vol tage, said comparators each having respective out puts directly into said logic circuit.

5. A voltage-controlled oscillator substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed, and havethefollowing effect: Claims 1-5 above have been deleted ortextually amended.

New ortextually amended claims have been filed as follows:

1. A voltage-controlled oscillator comprising means for converting an inputvoltageto a control current, charge storage means arranged to be charged by said control current to produce a voltage across said charge storage means varying from a first voltage level to a second voltage level at a rate dependent on said control current, bistable switching means for generating a signal offrequency dependenton said voltage across said charge storage means, said bistable switching means being settoafirststate by first comparator means whenever said voltage across said charge storage means exceeds a third voltage level intermediate said first and second levels and being setto a second state by a second comparator whenever said voltage across said charge storage means exceeds said second level, and means for discharging said charge storage means to said first level in response to said signal.

2. An oscillator as claimed in claim 1 and including meansfor ensuring that oscillation still occurs whenever the voltage across said charge storage meansfalis belowsaidfirst level or exceeds said second level.

3. An oscillator as claimed in claim 1 or 2 wherein said bistable switching means comprises a D-type flip-flop.