Self-tuning and temperature compensated voltage controlled oscillator

A self-tuning VCO (116) receives a control voltage input (Vcont) (114) and an adjustable programmable voltage (Vadj) (122) and provides optimized locked conditions even under variations in temperature. A radio temperature is measured and stored (204) while Vadj (122) is initialized and stepped and Vcont (114) attempts to lock the VCO on frequency. Once a locked condition is achieved, the Vcont (114) is monitored to determine if it falls within a predetermined voltage range. If a non-optimized condition is detected, then the Vadj (122) is automatically adjusted until the VCO (116) becomes locked with a Vcont which falls within the predetermined voltage range. When a locked condition is achieved, the radio temperature is monitored and compared (212) to the original stored temperature (204). If a temperature threshold limit is reached (216) then the VCO is re-checks itself for a locked condition and re-optimizes itself to accommodate for the variations in temperature.

TECHNICAL FIELD 
This invention relates in general to voltage controlled oscillators and 
more specifically to the tuning of voltage controlled oscillators. 
BACKGROUND 
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 locked loop (PLL) 
circuit, so that the communication device will lock on the desired 
frequency. 
The bandwidth of a VCO is one of the most important parameters in VCO 
designs, however many types of compromises are often incurred due to the 
lack of bandwidth which generally relates back to lack of control voltage 
range. In many 3 volt radio designs, for example, the control voltage 
available to steer the VCO can be as little as 1 volt. This reduction in 
available control voltage is due to noise restrictions, tolerance 
restrictions, as well as temperature restrictions. Both noise and 
tolerance restrictions are fairly well understood in the art, and VCO 
designs are optimized with respect to these parameters. Temperature 
tolerance restrictions, on the other hand, are generally compensated for 
by narrowing the control voltage margins at both the low end and high end 
of the control voltage range which again limits the VCO's performance. 
In many VCO designs wider bandwidth is considered desirable, however the 
disadvantage to increasing the bandwidth is that it makes the Ko(VCO gain 
parameter in MHz/V) much larger than it needs to be. The larger Ko tends 
to compromise the electrical performance of the VCO in terms of sideband 
noise and hum-and-noise. In the past, many VCOs were frequency-tuned which 
required some form of manual or factory level adjustment. One method used 
to eliminate frequency-tuning is to design the VCO frequency to a much 
wider bandwidth than necessary, but again, this has the disadvantage of 
making the Ko larger and degrading circuit performance. Voltage multiplier 
circuits can also be used to widen the VCO bandwidth by way of increasing 
the control voltage range and without necessarily increasing the Ko, but 
these circuits add unwanted complexity and cost to the circuit design. 
Accordingly, there is a need for an improved VCO tuning apparatus and 
technique which provides for a wider control voltage range to be used 
which will enable improved performance in terms of noise, bandwidth, and 
other VCO parameters, without severely impacting the Ko.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 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 a 
circuit which locks on frequency within a phase locked loop (PLL) at a 
predetermined desired control voltage. This circuit then maintains the 
locked condition over temperature in a manner which ensures that the 
control voltage stays within optimized limits. This VCO and tuning 
technique provide the advantages of self-tuning capability. 
Referring now to FIG. 1, there is shown a block diagram of a phase locked 
loop (PLL) 100 such as would be used in a radio communications product in 
accordance with the present invention. The PLL 100 receives reference 
input signal 102 which is typically derived from a crystal controlled 
reference oscillator 104. A phase detector 106 compares the phase 
difference between the reference input 102 and a divided radio frequency 
(RF) 108 in order to produce an error voltage 110. The error voltage 110 
is filtered at a loop filter 112 to produce a DC signal which will be 
referred to as a voltage control signal (Vcont) 114. The control voltage 
signal 114 is fed to a VCO 116 to steer the frequency of the VCO. The 
output of the VCO (RF out) 126 is divided by integer N through loop 
divider 128 for phase locking. 
In accordance with the present invention, the control voltage signal 114 is 
monitored by a microprocessor 118. In accordance with the present 
invention, the tuning of the VCO 116 is performed by checking for a locked 
VCO condition by monitoring the control voltage 114 through a lock detect 
circuit 120 while an adjustable programmable voltage (Vadj) 122 is stepped 
in value via internal programming of the microprocessor 118. The 
programmable voltage 122 is preferably generated from a digital-to-analog 
(D/A) converter 124 under microprocessor control 118. 
In accordance with the present invention, the programmable voltage (Vadj) 
122 is initialized and stepped in value to tune the VCO 116 to a 
predetermined desired frequency of operation while the control voltage 114 
attempts to lock the VCO on frequency. Once the VCO is in a locked 
condition as determined by lock detect circuit 120, the control voltage 
114 is then measured by the microprocessor 118 and compared to a 
predetermined voltage range stored in the microprocessor's internal memory 
130. In accordance with the present invention, the D/A 124 will continue 
to adjust the programmable voltage 122 until the VCO locks on frequency 
with a control voltage that falls within the stored predetermined limits. 
Control voltage value 114 is generated independently of external or manual 
control but is dependent on the D-to-A value that is set via internal 
programming. The microprocessor 118 adjusts the programmable voltage 122 
via D-to-A 124 in order to optimize control voltage 114. The programmable 
voltage 122 is then preferably written to the radio programming code. This 
stored programming voltage can be used as the initialization value with 
which to attempt to lock the VCO 116 on the next power up sequence or 
channel change of the PLL 100. Alternatively, the initialization value may 
start at a minimum (or maximum or some other starting point) and step up 
(or down) from that point. 
Further in accordance with the present invention, a temperature detect 
circuit 132 measures the radio temperature upon power up or channel change 
and stores this value in memory 130. Once the PLL 100 has reached a locked 
condition within the desired control voltage limits, the microprocessor 
118 monitors the temperature detect circuit 132. As long as the VCO 
remains locked within a given temperature range the optimized locking 
parameters will remain substantially unchanged. However, if the radio 
temperature varies from the original stored temperature by some 
predetermined threshold, then the locked condition will be verified and 
the control voltage parameter will be rechecked to make sure it still 
falls within the desired limits. In accordance with the invention, the 
programmable voltage 122 is readjusted, if need be, to maintain an 
optimized locked condition for the new temperature environment. Thus the 
PLL 100 is able to maintain, not just a locked connection, but an 
optimized locked condition even over variations in temperature. 
The PLL 100 of the present invention is completely self-tuning thereby 
eliminating the need for any test points or external programming. The VCO 
116 locks on frequency without manual tuning of trim capacitors, without 
external adjustment of the programmable voltage (Vadj), and without 
external monitoring of the control voltage (Vcont). 
Referring now to FIG. 2, there is shown a flowchart 200 of a VCO tuning 
technique 200 in accordance with the present invention. At step 202, the 
radio is powered up or has incurred a channel change. At step 204 the 
radio temperature is measured through the temperature detect circuit 132 
and stored in memory 130. The programmable voltage is initialized to a 
value corresponding to the desired tuning frequency at step 206. This 
initialization step 206 is preferably performed by the D/A 124. The VCO is 
then checked to determine if it is in a locked condition via lock detect 
120 at step 208. If the VCO is locked, then the VCO control voltage 
(Vcont) is measured internally to the radio by the microprocessor and 
compared to a predetermined voltage range, stored in internal memory 130, 
having upper and lower voltage limits at step 210. If the VCO is either 
unlocked at step 208 or if the control voltage value in a locked condition 
is not within the predetermined control voltage limits of step 210, then 
the programmable voltage is automatically adjusted by a predetermined 
amount in step 214. The step of adjusting the programmable voltage is 
performed by incrementing or decrementing the D/A through internal radio 
programming controlled by microprocessor 118. The steps of determining, 
measuring, comparing, and adjusting are repeated until the VCO has locked 
and the control voltage falls within the predetermined control voltage 
range stored in internal memory 130. 
When the control voltage value does fall within the predetermined limits at 
step 210 the programmable voltage (Vadj) is preferably stored to the 
radio's programming code at step 211. In accordance with the present 
invention, the tuning procedure then proceeds to step 212 where the radio 
temperature is monitored and compared to the stored temperature. If the 
temperature delta between the two temperatures does not reach a 
predetermined threshold at step 216, the radio continues to monitor the 
current temperature and compare it to the initial stored temperature while 
maintaining the presently optimized locked condition in accordance with 
the present invention. However, in further accordance of the present 
invention, if the temperature delta does reach the predetermined threshold 
at step 216, then the tuning procedure returns to step 208 to ensure that 
a locked condition within optimized limits is maintained. 
Upon the next power up or channel change, the initialization step 206 
preferably uses the previously stored programmable voltage value from step 
211. Alternatively, the initialization value at step 206 may start at a 
minimum (or maximum or some other starting point) and step up (or down) 
from that point. 
As an example of the PLL 100 and tuning technique 200, consider the 
operation of a radio that remains on the same channel as its temperature 
environment changes. With the changes in temperature the control voltage 
may start to drift. If the temperature change gets extreme enough, the 
control voltage could shift out of the optimized region and conceivably 
even drift enough so that the PLL loses its locked condition. The PLL 100 
and tuning technique 200 described by the invention will adjust the 
programming voltage so that the control voltage remains within the 
optimized limits. By maintaining the control voltage within optimized 
limits, the Ko value, which is determined by the VCO locking on frequency 
with a control voltage that falls within the optimized limits, will be 
maintained even over variations in temperature. The control voltage limits 
used in the VCO apparatus 100 and tuning technique 200 of the present 
invention are selected so as to provide optimal Ko conditions which in 
turn provide improved VCO performance (e.g. linear). The lower limit for 
Ko is preferably selected based on system transceiver frequency bandwidth 
requirements while the upper limit is preferably set where Ko is still 
within a linear region of VCO operation. Generally speaking for most VCOs 
the higher the control voltage the lower the Ko. 
In accordance with the present invention, there are several programmable 
voltage settings (Vadj) where the VCO will be locked for a given PLL. By 
monitoring the control voltage (Vcont) of the PLL 100 as well as the 
temperature internally to the radio under locked conditions, the final 
programmable voltage (Vadj) setting 122 will correspond to a control 
voltage value 114 that falls within the upper and lower predetermined 
limits. Thus an optimized locked condition is maintained even over 
variations in temperature. 
The PLL 100 and VCO tuning technique 200 described by the invention extend 
to a variety of oscillator configurations including but not limited to 
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 
even as this radio incurs extremes in environmental changes out in the 
field. 
Accordingly, there has been provided a self-tuning VCO apparatus and tuning 
technique which allows a VCO to be frequency tuned internally to the radio 
without outside intervention and without additional hardware. The 
self-tuning VCO described by the invention minimizes manufacturing 
problems associated with the tuning and trimming of VCOs. External 
monitoring and external programming have been eliminated thereby 
eliminating the need for any test points thereby further simplifying the 
manufacturing testing and tuning of the radio. The self-tuning VCO 
described by the invention provides the benefit of tuning the VCO in a 
closed-loop system and compensating for variations in temperature within 
that system to maintain a VCO that is both locked and operating in an 
optimal region of control voltage values. Furthermore, the self-tuning VCO 
apparatus and the tuning technique described by the invention are achieved 
without having to increase the control voltage range and thus does not 
require the use of voltage multiplier circuits. 
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.