AU701261B2

AU701261B2 - Apparatus and tuning method for a voltage controlled oscillator

Info

Publication number: AU701261B2
Application number: AU68093/98A
Authority: AU
Inventors: Branko Avanic; Tan V. Dinh; John K. Mckinney; Peter J. Yeh
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1997-06-03
Filing date: 1998-05-26
Publication date: 1999-01-21
Anticipated expiration: 2018-05-26
Also published as: CA2238741A1; JPH1131966A; AU6809398A; BR9802995A; KR19990006583A

Description

APPARATUS AND TUNING METHOD FOR A VOLTAGE CONTROLLED

OSCILLATOR

This invention relates in general to voltage controlled oscillators and more specifically to the tuning of voltage controlled oscillators.

io Voltage controlled oscillator (VCO) circuits are used in communication devices as the means of generating the desired frequency of operation. It is necessary to tune the VCO, usually as part of a phase lock loop (PLL) circuit, so that the communication device will lock on the desired frequency. This tuning process typically requires manual tuning or laser trimming of a tuning capacitor in the VCO circuit. FIG. 1 shows a prior art Colpitts

VCO

circuit 100 including a trim capacitor 102. Trim capacitor 102 controls the tuning of tank circuitry 104 while active circuitry 106 generates the radio frequency (RF) output.

Several problems can arise during the manufacturing process of VCOs which utilize trim capacitors, particularly in a radio manufacturing environment. For example, laser trimming or manual tuning of the trim capacitor 102 is required in order to lock the VCO to the desired frequency.

Trimming or tuning incurs a great deal of expense in terms of production time and cost as well as equipment, test software, and manpower. Repairs and reworks of incorrectly laser-trimmed parts as well as the maintenance of the laser equipment itself can incur substantial expense.

In the past, one method of addressing the tuning problem of a VCO in a radio environment was to design the :VCO frequency to a much wider bandwidth than required thus eliminating the frequency tuning. However, the disadvantage to increasing the VCO bandwidth is that this makes the Ko (VCO gain parameter measured in megahertz/volt--MHz/V) much larger than it needs to be.

i -2- This larger Ko compromises the electrical performance of the VCO, and as a result, the performance of the radio is degraded as well. For example, the PLL loop filter response and ability to balance modulation may be adversely affected. The resulting VCO has poorer sideband noise and hum-and-noise performance than a VCO that has been frequency tuned. These degraded VCO parameters translate to reduced adjacent channel and hum-and-noise performance of the radio in both transmit and receive modes.

Additionally, many of today's factory tuning operations still require a direct frequency measurement out of the radio during the tuning process. This often necessitates the use of additional equipment, such as a spectrum analyzer or equivalent, in the manufacturing production environment.

Accordingly, there is a need for an improved VCO and VCO tuning technique which eliminates manual or laser trim tuning of the VCO.

According to one aspect of the present invention there is provided a method of tuning optimization for a voltage controlled oscillator (VCO) circuit in a phase lock loop, including the steps of: initializing a programmable voltage; applying the programmable voltage to the VCO to tune the VCO; applying a control voltage to the VCO to lock the VCO; 20 determining if the VCO is in a locked condition; measuring the control voltage if the VCO is in a locked condition; comparing the control voltage to a predetermined range; adjusting the programmable voltage if the VCO is in an unlocked condition or if the control voltage does not fall within the predetermined range; repeating the steps of determining, measuring, comparing, and adjusting until the VCO is locked and the control voltage falls within the predetermined range; and storing the programmable voltage at which the VCO locked as an optimal programmed voltage setting to be used in the step of initializing in future tuning.

MJP C:\WINWORD\MRIE\GABNODEL6893C.DOC

I

-2a According to a further aspect of the present invention there is provided a method of optimizing tuning in a VCO circuit in a phase lock loop, including the steps of: selecting a control voltage range for the VCO; initialising a programmable voltage corresponding to a predetermined tuning frequency; applying a control voltage to the VCO to attempt to lock the VCO on the predetermined tuning frequency; adjusting the tuning frequency through the programmable voltage until the VCO locks on frequency at a control voltage that falls within the selected control voltage range; and storing the programmable voltage as an optimal programmed voltage setting to be used in the step of initializing in future tunings.

According to a still further aspect of the present invention there is provided a programmable VCO for a phase lock loop, including a tank circuit, the tank circuit receiving a programmable voltage for tuning the frequency of the VCO, and the tank circuit receiving a control voltage for locking the VCO onto the tuned frequency, the VCO being characterized by a Ko value, the Ko value being determined by the VCO locking on frequency with a control voltage 20 which falls within predetermined limits, the programmable voltage at which the VCO locks on frequency being stored as an optimal programmed voltage setting to be used in future tunings of the VCO; and an active circuit coupled to the tank circuit, the active circuit generating an RF output.

According to a still further aspect of the present invention there is provided a phase lock loop, including a VCO generating a radio frequency (RF) output, the VCO characterized by a Ko value; a phase detector for comparing a reference frequency to a divided VCO frequency and producing an error signal; a loop filter for filtering the error signal and providing a control voltage for steering the VCO; MJP C:\WINWORD\MARIE\GABNODEL68093C.DOC -2b the VCO also receiving an adjustable programmable voltage for tuning the VCO to a desired frequency of operation, the VCO being characterized by a Ko value, the Ko value being determined by the VCO locking on frequency with a control voltage which falls within predetermined limits, the programmable voltage at which the VCO locks on frequency being stored as an optimal programmed voltage setting to be used in future tunings of the VCO; and a divider for dividing the RF output into the divided VCO frequency.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings wherein:- Fig. 1 is a prior art voltage controlled oscillator.

Fig. 2 is a block diagram of a PLL in accordance with the present invention.

Fig. 3 is a block diagram of the VCO of Fig. 2 in accordance with the present invention.

Fig. 4 is a circuit diagram of a VCO in accordance with the preferred embodiment of the invention.

Fig. 5 is a flow chart of a VCO tuning technique in accordance with the present invention.

Fig. 6 is a graph showing an example of a response showing Ko versus 20 control voltage for a VCO in accordance with the present invention.

While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood MJP C:\WINWOR\MARIE\GABNODEL68093C.DOc CM01263L 3 from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.

A VCO apparatus and tuning technique, to be described herein, provides for a narrower bandwidth VCO to achieve improved communication performance in a radio product without the drawbacks of the tuning techniques previously described.

Referring now to FIG. 2, there is shown a block diagram of a phase lock loop (PLL) 200 such as would be used in a radio communications product in accordance with the present invention. The PLL 200 receives reference input signal 202 which is typically derived from a crystal *controlled oscillator (not shown). A phase detector 204 o* 15 compares the phase difference between the reference input 202 and a divided VCO frequency 206 in order to produce an :.:.error voltage 208. The error voltage 208 is filtered at a loop filter 210 to produce a DC signal which will be referred to as a voltage control signal (Vcont) 212. The S 20 control voltage signal 212 is fed to a VCO 214 to steer the frequency of the VCO. In accordance with the present invention, the tuning of the VCO 214 is performed by checking for a locked VCO condition, such as through a lock detect circuit (not shown), and monitoring the control 25 voltage 212 while an adjustable programmable voltage (Vadj) 216 is stepped in value via external programming. The programmable voltage 216 is preferably generated from a digital-to-analog converter 218 under microprocessor control 220. The output of the VCO (RF out) is divided by integer N through loop divider 222 for phase locking.

In accordance with the present invention, the programmable voltage (Vadj) 216 is initialized to tune the VCO 214 to a predetermined desired frequency of operation while the control voltage 212 attempts to lock the VCO on frequency. Once the VCO locks on frequency, the control voltage 212 is measured, preferably through a test point 224, and compared to a predetermined voltage range, CM01263L 4 preferably stored in a memory portion of the microprocessor 220. If the control voltage (Vcont) 212 falls within the predetermined range, then the tuning of the VCO is complete. If the VCO fails to lock, the programmable voltage (Vadj) 216 is adjusted in value by a predetermined amount and the VCO will attempt to lock on frequency again.

The D/A will continue to adjust the programmable voltage 216 until the VCO locks on frequency or until a predetermined programming voltage limit is reached.

FIG. 3 shows a simplified block diagram of the VCO 214 of FIG. 2 in accordance with the present invention. VCO 214 includes a tank circuit 302 and an active circuit 304.

In accordance with the present invention, the tank circuit 302 receives the programmable voltage (Vadj)216 for tuning 15 the frequency of the VCO while the control voltage (Vcont) 212 attempts to lock the VCO.

.FIG. 4 shows the preferred embodiment of the VCO 214 in a Colpitts oscillator configuration such as would be .*.used in a radio. The programmable voltage 216 tunes the 20 tank circuit 302, preferably through biasing circuitry comprising a choke 402, a varactor 404, and a capacitor 406. Other biasing circuit arrangements could also be implemented to tune the VCO through the tank 302.

Referring now to FIG. 5, there is shown a flowchart 25 500 of a VCO tuning technique 500 in accordance with a preferred embodiment of the present invention. At step 502, the programmable voltage is initialized to a value corresponding to the desired tuning frequency. This initialization step is preferably performed by the D/A 218.

The VCO is then checked to determine if it is in a locked condition (via lock detect or other means) at step 504. If the VCO is locked, then the VCO control voltage (Vcont) is measured at step 506. The control voltage measured at step 506 under locked conditions is then compared at step 508 to a predetermined voltage range having upper and lower voltage limits to see if the measured reading falls within this desired window. If the control voltage value does CM01263L fall between the predetermined limits at step 508, then the tuning procedure is complete, step 510. If the VCO is either unlocked at step 504 or if the control voltage value in a locked condition is not within the predetermined voltage limits of step 508, then the programmable voltage is adjusted by a predetermined amount in step 512. The step of adjusting the programmable voltage is preferably performed by incrementing or decrementing the D/A. The steps of determining, measuring, comparing, and adjusting are repeated until the VCO has locked and the control voltage falls within predetermined voltage range. Prior to adjusting the programmable voltage at step 512, the programmable voltage is preferably compared to a maximum voltage setting at step 514 above which the VCO is 15 considered to have failed the tuning procedure at step 516.

In accordance with the present invention, there are several programmable voltage settings where the VCO will be locked in the PLL. By monitoring the control voltage (Vcont)of the VCO under locked conditions for these different values of D/A programmable voltages (Vadj), the optimal programmed voltage setting (or programming word) can be written to the radio's programming code. The final programmable voltage adjust(Vadj) setting will correspond to a control voltage value that falls within the upper and lower predetermined limits.

While shown and described in terms of a Colpitts oscillator, one skilled in the art recognizes that the VCO 214 and VCO tuning technique 500 described by the invention extend to other oscillator configurations, such as Hartley, Clapp, and Pierce oscillator configurations.

The upper and lower control voltage limits are selected such that the final control voltage value will result in predictable electrical performance for every radio that goes through the tuning procedure 500 of the present invention. This is accomplished by setting these control voltage limits so that the final value is within a range of settings where Ko is substantially fixed, CM01263L 6 consistent, and its response known. An example of such a response is shown in FIG. 6. Graph 600 depicts Ko (MHz/V) versus the VCO control voltage For the curve shown in graph 600, the acceptable control voltage limit could be selected to be around 1.1 volts (designator 602) while the upper control voltage limit could be selected to be about 2.3 volts (designator 604) in order to provide a range of acceptable values guaranteeing a locked condition of the VCO with a desired Ko. Maintaining Ko within a reasonable range of predetermined values, helps to maintain most other radio circuit parameters and prevents the radio from deviating from predicted design responses.

Accordingly, there has been provided a VCO apparatus and tuning technique which allows a VCO to be frequency 15 tuned electronically during the production phase of a radio or the production phase of a transceiver product. The electronically tunable VCO described by the invention reduces the manufacturing problems associated with the tuning and trimming of VCOs. The elimination of the trim 20 capacitor eliminates the need for manual tuning or laser trimming providing both a savings in time and expense.

Repairs and reworks are thus minimized and all manual tuning is essentially eliminated. The electronically tunable VCO described by the present invention also S 25 provides the added benefit of tuning the VCO in a closedloop system, eliminating the need of a direct frequency measurement out of the radio during the tuning operation.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

-7- THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS: 1. A method of tuning optimization for a voltage controlled oscillator (VCO) circuit in a phase lock loop, including the steps of: initializing a programmable voltage; applying the programmable voltage to the VCO to tune the VCO; applying a control voltage to the VCO to lock the VCO; determining if the VCO is in a locked condition; measuring the control voltage if the VCO is in a locked condition; comparing the control voltage to a predetermined range; adjusting the programmable voltage if the VCO is in an unlocked condition or if the control voltage does not fall within the predetermined range; repeating the steps of determining, measuring, comparing, and adjusting until the VCO is locked and the control voltage falls within the predetermined range; and storing the programmable voltage at which the VCO locked as an optimal programmed voltage setting to be used in the step of initializing in future tuning.

2. The method of claim 1, wherein the programming voltage is generated by a digital-to-analog converter.

3. The method of claim 2, wherein the step of adjusting comprises the step of incrementing the D/A.

4. The method of claim 2, wherein the step of adjusting comprises :the step of decrementing the D/A.

A method of optimizing tuning in a VCO circuit in a phase lock loop, including the steps of: selecting a control voltage range for the VCO; initialising a programmable voltage corresponding to a predetermined tuning frequency; applying a control voltage to the VCO to attempt to lock the VCO on the predetermined tuning frequency; MJP CAWINWORD\MARIE\GABNODEL\68093C.DOC

Claims

6. The method of claim 5, further including the step of increasing the step of adjusting when the programming voltage exceeds a predetermined limit.
7. A programmable VCO for a phase lock loop, including: a tank circuit, the tank circuit receiving a programmable voltage for tuning the frequency of the VCO, and the tank circuit receiving a control voltage for locking the VCO onto the tuned frequency, the VCO being characterized by a Ko value, the Ko value being determined by the VCO locking on frequency with a control voltage which falls within predetermined limits, the programmable voltage at which the VCO locks on frequency being stored as an optimal programmed voltage setting to be used in future tunings of the VCO; and an active circuit coupled to the tank circuit, the active circuit generating oo an RF output. 20 8. A programmable VCO as described in claim 7, wherein the programmable voltage is generated from a digital-to-analog-converter.
9. A programmable VCO as described in claim 8, wherein the digital-to-analog converter is under microprocessor control. A phase lock loop, including: 25 a VCO generating a radio frequency (RF) output, the VCO characterized by a Ko value; a phase detector for comparing a reference frequency to a divided VCO frequency and producing an error signal; a loop filter for filtering the error signal and providing a control voltage for steering the VCO; MJP C:\WINWORD\MARIE\GABNODEL\68093C.DOC -9- the VCO also receiving an adjustable programmable voltage for tuning the VCO to a desired frequency of operation, the VCO being characterized by a Ko value, the Ko value being determined by the VCO locking on frequency with a control voltage which falls within predetermined limits, the programmable voltage at which the VCO locks on frequency being stored as an optimal programmed voltage setting to be used in future tunings of the VCO; and a divider for dividing the RF output into the divided VCO frequency.
11. A method of tuning a voltage controlled oscillator circuit substantially as herein described with reference to Figures 2-6 of the accompanying drawings.
12. A method of optimising tuning in a VCO circuit substantially as herein described with reference to Figures 2-6 of the accompanying drawings.
13. A programmable VCO substantially as herein described with reference to Figures 2-6 of the accompanying drawings.
14. A phase lock loop substantially as herein described with reference to Figures 2-6 of the accompanying drawings. DATED: 10 November, 1998 PHILLIPS ORMONDE FITZPATRICK 20 Attorneys for: MOTOROLA, INC. 9 *oo° °°8o *oo• MJP C:\WINWORDMARIE\GABNODEL\68093C DOC I CM01263L APPARATUS AND TUNING METHOD FOR A VOLTAGE CONTROLLED OSCILLATOR ABSTRACT OF THE DISCLOSURE A method (500) of tuning a voltage controlled oscillator (VCO) includes the steps of initializing a programmable voltage and applying the programmable voltage to the VCO in order to tune the VCO (502). A control voltage is applied to the VCO to lock the VCO, and the VCO is checked to determine if it is in a locked condition (504). If the VCO is in a locked condition, the control voltage is measured and compared to a predetermined voltage range (506). If an unlocked condition is detected or if the control voltage does not fall within the predetermined 15 voltage range then the programmable voltage is adjusted (512) and the steps are repeated until the VCO is locked with a control voltage which falls within a predetermined voltage range. oo. P e•* 50 *et