GB2336491A

GB2336491A - Oscillator circuit

Info

Publication number: GB2336491A
Application number: GB9808037A
Authority: GB
Inventors: Stefan Lichterfeld; Klaus Pai
Original assignee: Motorola GmbH
Current assignee: Motorola Solutions Germany GmbH
Priority date: 1998-04-15
Filing date: 1998-04-15
Publication date: 1999-10-20
Also published as: GB9808037D0

Abstract

A voltage controlled oscillator circuit (58) includes a semiconductor element (90) having a first port (92) being supplied with a bias supply, a second port (98) being operably coupled to a frequency resonator circuit and a third port (96) providing an output signal, wherein the voltage controlled oscillator circuit (58) is characterised by a processor (51) operably coupled to the first port (92) and varying the bias supply to minimise noise in the output signal. The resonant circuit may include a voltage variable impedance e.g. a varactor (74) to which a modulation signal may be applied. The processor (51) controls both the operating frequency of the oscillator and the bias current to continually minimise noise. A memory (53) in the processor may store operating characteristics of the oscillator such as bias supply current or voltage, temperature, frequency of operation, supply voltage. The bias current may be varied and the vco tuning voltage or vco carrier residual frequency modulation noise or adjacent channel phase noise may be measured in the determination of minimum phase noise.

Description

2336491 OSCILIATOR CIRCUIT AND MIETHOD OF CHARACTERISING AND OPERATING

SUCH CIRCUIT

Field of the Invention

This invention relates to frequency-generating oscillator circuits. The invention is applicable to, but not limited to, voltage controlled oscillator circuits for radio communication units and method of characterising and operating such voltage controlled oscillator circuits.

Background of the Invention

Signal generation circuitry of mobile radios is typically complex with many performance trade-offs being critical to the designer with component cost and component count issues being a high priority. One typical approach used to generate frequencies in mobile radio units is to use a voltage controlled oscillator (VCO).

As there are a number of complicated effects that have an impact on the modulation deviation performance, a trade-ofT between expected stability and expensive specific components has to be found. This is particularly the case with bipolar transistor oscillators where such tradeoffs limit the performance in wideband W0 designs, principally due to temperature variation.

A typical prior art frequency generator circuit 10, often termed a frequency resonator tank circuitry is shown in FIG. 1. The frequency resonation is formed by an inductance 26, a voltage variable capacitance 24 and further inductive and capacitive elements. A bipolar transistor 40 is coupled to the frequency resonator tank circuit via a base capacitance 28. A capacitive Collpits divider 32, 34 between the bias-emitter port and the emitter-ground port of the bipolar transistor 40 - "CgJ and "Csg" adds positive feedback for oscillation generation. The bipolar transistor 40 has a DC supply input to the collector port 42. An inductive choke 38 isolates the RF signal at emitter in the DC path from ground. The RF output signal is available at the output port 50.

A bipolar transistor's radio frequency (RF) performance is significantly afTected by the optimisation of feedback impedance and transistor DC bias adjustment. In known WO designs, a laser trim 2 process of the DC bias resistor 41 is necessary to adjust the transistor DC bias for a useful sideband noise performance. The laser trim process is iteratively performed on the DC bias resistor 41 until the minimum sideband noise is generated. The peak performance range is fairly narrow, approx. 10%, and therefore the laser trimming process needs to be very carefully controlled. Furthermore, as the laser trimming process is effectively a one-way, ever-decreasing resistance process, with each iterative cut of the resistor decreasing its resistance, there is no easy way of recovering &om an over-trimmed resistor. Over-trimming results in increasing noise and may require a replacement of the laser trim resistor and consequently re-tuning of the WO. Additionally, laser-trimming stations have a major impact on manufacturing costs and the laser trimming process, as it has to be very carefully controlled, has a great impact on the time taken to manufacture each radio. This is obviously highly undesirable when manufacturing high volume, low cost radios.

This invention seeks to provide a voltage controlled oscillator circuit, and method of characterising and optimising such a voltage controlled oscillator circuit, to alleviate at least some of the problems highlighted above.

Summary of the Invention

In a first aspect of the present invention, a voltage controlled oscillator circuit is provided including a semiconductor element having a first port being supplied with a bias supply, a second port being operably coupled to a frequency resonator circuit and a third port providing an output signal. The voltage controlled oscillator circuit is characterised by a processor operably coupled to the first port and varying the bias supply to minimise noise in the output signal.

In this manner, the bias supply is continuously controlled in order to maintain a minimum level of phase noise output from the voltage controlled oscillator circuit, irrespective of varying operating conditions.

Preferably, the frequency resonator circuit includes voltage varying impedance means operably coupled to the processor, the processor respectively varying the bias supply when varying an impedance of the 3 voltage varying impedance means to minimise noise in the output signal at a particular operating frequency.

In this manner, the processor controls both the operating frequency of the voltage controlled oscillator circuit and the bias current supplied to the semiconductor element, preferably a bipolar transistor, to continuously minimise noise in the output signal. In the preferred embodiment of the invention, the processor is operably coupled to a memory element for storing operating characteristics of the voltage controlled oscillator circuit.

The operating characteristics preferably include at least one of the following: bias supply current, bias supply voltage, temperature, voltage applied to the voltage varying impedance means, frequency of operation, battery supply voltage. Temperature sensing means are provided, operably coupled to the processor for providing an operating temperature indication of the voltage controlled oscillator circuit. In the preferred embodiment of the invention, the voltage controlled oscillator is used in a radio communication unit.

In a second aspect of the preferred embodiment of the invention, a method of characterising a performance of a voltage controlled oscillator circuit is provided. The method includes varying a bias supply to the voltage controlled oscillator circuit, monitoring an operating parameter of the voltage controlled oscillator circuit to indicate a level of noise in the voltage controlled oscillator circuit and determining at least one optimal operating condition of the voltage controlled oscillator circuit in response to the bias supply variation and the operating parameter monitoring.

In this manner, the voltage controlled circuit can be characterised for a variety of its operating conditions in order to continuously optimise its performance when in use.

Preferably, the operating parameter includes at least one of the following: temperature of operation of the voltage controlled oscillator circuit, steering line voltage, adjacent channel signal level, residual FM modulation deviation, and the steps are repeated for each particular operating frequency of the voltage controlled oscillator circuit. The monitored data and associated bias supply value are preferably stored in a memory element of the voltage controlled oscillator circuit, for later use.

In a third aspect of the present invention, a method of operating a voltage controlled oscillator circuit is provided. The voltage controlled 4 is oscillator circuit includes a ftequency resonator circuit, in turn coupled to a semiconductor element, having a first port being supplied with a bias supply, a second port being operably coupled to the frequency resonator circuit and a third port providing an output signal. The voltage controlled oscillator circuit further includes a processor operably coupled to the first port and the frequency resonator circuit. The method includes receiving a modulation input signal at the modulation input port, providing a modulation dependent impedance, based on the modulation input signal at the second port of the semiconductor element and varying the bias supply provided to the first port, by the processor, to minimise noise in the output signal.

In this manner, the operation of the voltage controlled oscillator circuit is continuously optimised for minimum phase noise.

Preferably, the frequency resonator circuit includes voltage varying impedance means operably coupled to the processor and an impedance of the voltage varying impedance means is respectively varied when varying the bias supply to minimise noise in the output signal at a particular operating frequency.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the drawings.

Brief Description of the Drawinga

FIG. 1 is a prior art voltage controlled oscillator circuit.

FIG. 2 is a voltage controlled oscillator circuit according to a preferred embodiment of the invention.

FIG. 3 is a flow chart showing a method for characterising the voltage controlled oscillator circuit according to a preferred embodiment of the invention.

FIG. 4 shows a theoretical graph of a bipolar transistor bias current versus steering line voltage and sideband noise according to the preferred embodiment of the invention.

FIG. 5 shows a measured graph of a bipolar transistor bias current versus steering line voltage versus WO sideband (phase) noise according to the preferred embodiment of the invention.

FIG. 6 shows a measured graph of a bipolar transistor bias current versus temperature versus WO sideband (phase) noise according to the preferred embodiment of the invention.

Detailed Description of the D. -

Referring first to FIG. 2, a block diagram of a voltage controlled oscillator circuit 58 is shown, according to a preferred embodiment of the invention. The voltage controlled oscillator circuit 58 includes a frequency resonation circuit formed by an inductance 76, a voltage variable capacitance 74 and further inductive and capacitive elements. A bipolar transistor 90 is coupled to the frequency resonator tank circuit via a base capacitance 28. A capacitive Collpits divider 82, 84 is positioned between the base port 98 and the emitter port 96 of the bipolar transistor 90 - "CgJ and "Csg" - adds positive feedback for oscillation generation. An inductive choke 88 isolates the RF signal at emitter in the DC path from ground. The RF output signal is available at the output port 100.

The processor 51 contains a memory element 53 and has a first port 57, operably coupled to the bias resistor 41 for controlling a bias current supplied to the bipolar transistor 90, and a second port 55.

In operation, the WO circuit performance is characterised, to determine the optimal control of oscillator circuit components for minimum phase noise of the WO. It is preferred that the W0 characterisation, tuning and aligrunent procedures are performed during the factory set up phase, however, it is within the contemplation of the invention that such procedures can be carried out in the field, remotely or as part of standard operations of the radio unit.

The characterisation phase is performed by varying the bias current supplied to the bipolar transistor and monitoring/measuring one or more characteristics of the VCO circuit whilst varying the bias current, for example the WO free running frequency. Such monitoring of the VCO free running frequency can be performed by automatic DC-probing the synthesiser steering voltage line while tuning. On an assembled and working radio this measurement can be readily performed. There is a defined relation between this steering line (U-Control) voltage, Iopt. and the W0s sideband noise performance. This monitoring requires a WO 6 design that peaks reliably at Ucontrol-max. for best phase noise. The peak may be flat-ish and will require a fine resolution of a DC measurement meter, for example 1%.

Dependent upon the WO design the best noise rejection will occur at 5 the highest steering line voltage with a narrow peak, as indicated in FIG. 3, FIG. 4 and Table 1.

An alternative characterising approach is to measure the WO carrier residual frequency modulation noise. If a WO is tuned to its best phase noise value, then usually its residual FM modulation deviation is at its minimum, e.g. 5 Hz for a 200 MHz W0. This noise increases proportionally to the phase noise degradation. The required measurement equipment, when using this determination is more complex and time consuming than with the steering line voltage, but the accuracy may be better.

is A yet further alternative approach within the contemplation of the invention is to measure the adjacent channel phase noise of the VCO. With high performance phase noise measurement equipment one can directly measure the phase noise. However, the measurement time is somewhat longer than the previous two methods but the accuracy will be better.

Whichever monitoring/measuring approach is selected, the relationship between bias current at a particular tuning temperature, a certain phase noise performance is obtained. A temperature sensor (not shown) can be operably coupl;d to the processor to further characterise the operation of the VCO circuit across a temperature range, or interpolation can be used to assess optimal operating conditions between sets of values. The determined relationship is preferably stored in a memory element 53 coupled to, or contained within, the processor 51.

In this manner, once the VCO circuit has been fully characterised for phase noise versus bias current, the processor 51 can control the operation of the VCO circuit according to the operating conditions of the WO circuit, for example temperature variations, supply voltage/current variations of say, a battery, and the frequency of operation.

In the preferred embodiment of the invention, the voltage controlled oscillator circuit is described with reference to a bipolar transistor.

However, it is within the contemplation of the invention that the 7 compensating effect also works with any driver element, for example a field effect transistor (FET) device or indeed any semiconductor element which has a non-constant input impedance versus oscillator amplitude and temperature performance. In addition, it is within the contemplation of the invention, that alternative oscillator design topologies would benefit from the inventive concept of the present invention. Furthermore, even higher frequency semiconductor devices, for example silicon devices, operating in the gigahertz (GHz) frequency range will benefit from the inventive concept described herein. The voltage controlled oscillator circuit in the preferred embodiment of the invention is described with particular reference to its operation in a radio communication unit.

Referring now to FIG. 3, a flowchart showing a method of characterising the frequency performance of the oscillator circuit is provided. The method includes the steps of starting a bias tuning routine applied to the biasing port which is preferably the base of the bipolar transistor for a particular oscillator frequency, as shown in step 100. The bias current supplied to the-bipolar transistor is then increased, as in step 102, and the steering line voltage measured, as shown in step 104. If the steering line voltage Wcont) is increasing I step 106, the bias current supplied to the bipolar transistor is accordingly increased, as shown in steps 102, 104 and 106. When the steering line voltage is determined as no longer increasing in step 106, the bias current value is then stored in a memory element in the radio, as shown in step 108. once this bias current value has been stored, the oscillator frequency is changed to a new frequency and the characterising routine commenced again, as in step 110. This process is repeated until the performance of the oscillator circuit, and in particular variations in the steering line voltage, has been characterised across the desired frequency range of the oscillator circuit.

According to the previously mentioned alternative approaches for monitoring/measuring the performance of the WO circuit, other parameters would be used instead of the voltage steering line measurement in steps 104 and 106.

This concept of the characterisation routine as provided for in FIG. 3, is more clearly shown in FIG. 4. FIG. 4 shows a theoretical graph 130 of a bipolar transistor bias current 132 versus sideband noise 134 for a , 1 1 ' i; 1 1 1 8 varying steering line voltage, according to the preferred embodiment of the invention. As previously described in the flowchart of FIG. 3, the bias current supplied to the bipolar transistor is steadily increased whilst the steering line voltage is measured. Men the steering line voltage reaches a maximum value 138 in the graph, once the optimal Ibi,'. current is applied the associated sideband noise of the oscillator circuit reaches a minimum 136. This value of bias current, for the particular frequency, temperature etc. of operation, is then stored in a memory element, to be looked up and applied to the bipolar transistor when in use.

FIG. 5 shows a graph 150 of the measured relationship between a bipolar transistor bias current 152 versus steering line voltage 155 versus sideband noise 154, based on the measured results listed in Table 1, according to the preferred embodiment of the invention. When the steering line voltage reaches a maximum value 158 in the graph, once the optimal Ibi.. current is applied the associated sideband noise of the oscillator circuit reaches a minimum 156. This value of bias current is then stored in the memory element 53.

1 9 Table 1: Sideband Noise Performance and VCO Control Voltage as a function of Transistor Bias Current, with phase noise being measured in dBc/Hz on the adjacent channel and the steering line voltage being is measured on a particular frequency of interest.

I-bias 1 mA Phasenoise U-contro dBe/Hz 11 v 1 -100 2 1,5 -102 2,015 2 -104 2,03 2,5 -106 2,045 3 -108 2,06 3,5 -110 2,075 4 -112 2,09 4,5 -114 2,105 -116 2,119 5,5 -118 2,2 6 -118 2,205 6,5 -117 2,18 7 -116 2,137 7,5 -112 2,105 8 -109 2,08 8,5 -106 2,05 9 -103 2,02 9,5 -100 1,99 -97 1,96 As previously mentioned, the performance of the VCO circuit also varies as a function of temperature. FIG. 6 shows a graph 170 of the measured relationship between the sideband (phase) noise of a VCO circuit with bias current compensation over temperature 172 and without bias current compensation over temperature 174, based on the measured results listed in Table 2.

Table 2: Sideband noise performance as a function of WO temperature I_bias = 5 mA 025C Temperature/C Phasenoise_1 I_bias 1 dBe/Hz _.opt 1 mA 0 -112 4,8 -116 4,9 -117 5 -118 5 -116 5,2 -114 5,3 -113 5,5 -111 5,8 -108 6 Phasenoise-2 1 dBc/Hz -117 -118 -118 -118 -117 -117 -118 -117,5 -117 -116 -116 -115,5 Thus a voltage controlled oscillator circuit and methods of characterising its performance and optimising the operating factors of the voltage control oscillator circuit in use are provided that mitigate a number of problems associated with prior art oscillator circuits.

11

Claims

Claims

1. A voltage controlled oscillator circuit comprising: a semiconductor element having a first port being supplied with a bias supply, a second port being operably coupled to a frequency resonator circuit and a third port providing an output signal, wherein the voltage controlled oscillator circuit is characterised by a processor operably coupled to the first port and varying the bias supply to minimise noise in the output signal.

2. A voltage controlled oscillator circuit according to claim 1, wherein the frequency resonator circuit includes voltage varying impedance means operably coupled to the processor, the processor respectively varying the bias supply when varying an impedance of the voltage varying impedance means to minimise noise in the output signal at a particular operating frequency.

3. A voltage controlled oscillator circuit according to claims 1 or 2, wherein the processor is operably coupled to a memory element for storing operating characteristics of the voltage controlled oscillator circuit, the operating characteristics including at least one of the following: bias supply current, bias supply voltage, temperature, voltage applied to the voltage varying impedance means, frequency of operation, battery supply voltage.

4. A voltage controlled oscillator circuit according to claims 1, 2 or 3 wherein the voltage controlled oscillator circuit includes temperature sensing means operably coupled to the processor for providing an operating temperature indication of the voltage controlled oscillator circuit.

5. A radio communication unit comprising a voltage controlled oscillator according to any of the preceding claims.

12 6. A method of characterising a performance of a voltage controlled oscillator circuit comprising the steps of. varying a bias supply to the voltage controlled oscillator circuit; monitoring an operating parameter of the voltage controlled oscillator circuit to indicate a level of noise in the voltage controlled oscillator circuit; and determining at least one optimal operating condition of the voltage controlled oscillator circuit in response to the bias supply variation and the operating parameter monitoring.

7. A method of characterising a performance of a voltage controlled oscillator circuit according to claim 6, wherein the operating parameter includes at least one of the following: temperature of operation of the voltage controlled oscillator circuit, steering line voltage, adjacent channel signal level, residual FM modulation deviation.

8. A method of characterising a performance of a voltage controlled oscillator circuit according to claim 6 or 7, wherein the steps are repeated for each particular operating frequency of the voltage controlled oscillator 20 circuit.

9. A method of characterising a performance of a voltage controlled oscillator circuit according to any one of preceding claims 6 to 8, further comprising the step of storing the monitored data and bias supply value in a memory element of the voltage controlled oscillator circuit.

13 10. A method of operating a voltage controlled oscillator circuit having a semiconductor element having a first port being supplied with a bias supply, a second port being operably coupled to the frequency resonator circuit and a third port providing an output signal, wherein the voltage controlled oscillator circuit includes a processor operably coupled to the first port and the frequency resonator circuit, the method comprising the steps of.. receiving a modulation input signal at the modulation input port; providing a modulation dependent impedance, based on the modulation input signal at the second port of the semiconductor element; and varying the bias supply provided to the first port, by the processor, to minimise noise in the output signal.

11. A method of operating a voltage controlled oscillator circuit according to claim 10, wherein the frequency resonator circuit includes voltage varying impedance means operably coupled to the processor, the method further comprising the step of respectively varying an impedance of the voltage varying impedance means when varying the bias supply to minimise noise in the output signal at a particular operating frequency.

12. A voltage controlled oscillator circuit substantially as hereinbefore described with reference to, or as illustrated by FIG. 2 of the drawings.

13. A method of characterising a voltage controlled oscillator circuit substantially as hereinbefore described with reference to, or as illustrated by FIG. 3 of the drawings.