Detented UHF television tuner with independent inductive fine tuning
BACKGROUND OF THE INVENTION 
     This invention relates generally to tuners, and more particularly, to UHF television tuners of the detented type. 
     Detented UHF television tuners are known. Such detented tuners usually employ a UHF tuner having a continuously variable main tuning shaft and a detented channel selector shaft. A gear mechanism is generally used for mechanically coupling the selector shaft to the main tuning shaft. In addition, a fine tuning shaft is usually mechanically coupled to the main tuning shaft through a gear mechanism or the like, and serves to rotate the main tuning shaft over a predetermined range of rotation about each detent position established by the channel selector shaft. A clutch is generally interposed between the channel selector shaft and the main tuning shaft to permit the main tuning shaft to be rotated by the fine tuning shaft while the channel selector shaft is constrained by the detenting mechanism. Three such prior art tuners are described in U.S. Pat. Nos. 3,774,459; 3,842,683; and 3,916,820, incorporated herein by reference. 
     While these systems provide a UHF television tuner having a detented selector shaft and a fine tuning shaft for fine tuning the tuner at each of the detent positions of the selector shaft, such tuners suffer from a phenomenon known as inertial creep. The inertial creep phenomenon is caused by a rapid acceleration and deceleration of the selector shaft as it is rotated from one detent position to another, causing gradual slippage of the clutch coupling the main tuning shaft to the selector shaft, thereby resulting in a gradual detuning of the tuner after repeated channel changes. 
     One way to eliminate the inertial creep problem is to provide a fine tuning mechanism that operates independently of the main tuning shaft. This permits the main tuning shaft to be coupled to the channel selector shaft directly without the use of a clutch and eliminates the clutch slippage problem. A circuit for electrically achieving the independent fine tuning utilizing a voltage variable capacitor to adjust the tuning of the local oscillator of the UHF tuner is described in the above-referenced U.S. Pat. No. 3,842,683. Another system utilizing a mechanically operated piston capacitor to tune the local oscillator of the UHF tuner is described in U.S. Pat. No. 3,972,240 also issued to the same inventor as the inventor of the present invention and assigned to the present assignee. 
     While the above-described systems do provide a way to eliminate the inertial creep problem, the electrical fine tuning system described in U.S. Pat. No. 3,842,683 has a tuning range that is limited by the capacitance range of the voltage variable capacitor, and which is highly dependent on the physical layout of the circuit. The mechanical system described in U.S. Pat. No. 3,972,240 is somewhat cumbersome and therefore not suited to portable applications. Furthermore, the use of a piston capacitor increases the cost of the tuner, thus making it impractical for use in low cost television receivers. Also, the mechanism for adjusting the piston capacitor tends to have some degree of backlash, thus affecting the resettability of the tuner. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a detented UHF television tuner that overcomes many of the disadvantages of the prior art systems. 
     It is another object of the present invention to provide a detented type UHF television tuner utilizing an improved mechanical fine tuning system that operates independently of the main tuning shaft. 
     It is yet another object of the present invention to provide a detented UHF television tuner having a compact, low cost mechanical fine tuning system that operates independently of the main tuning shaft. 
     It is still another object of the present invention to provide a minimum backlash fine tuning system for a detented UHF television tuner that operates independently of the main tuning shaft. 
    
    
     These and other objects and advantages of the present invention will be readily apparent from the following specification and attached drawings wherein: 
     FIG. 1 is a side view of a detented UHF television tuner utilizing the fine tuning system according to the present invention; 
     FIG. 2 is a partial top view of the UHF tuner illustrated in FIG. 1 taken along line 2--2 of FIG. 1; 
     FIG. 3 is a front sectional view of the UHF tuner taken along line 3--3 of FIG. 1; 
     FIG. 4 is a partial side sectional view of the UHF television tuner illustrating the fine tuning mechanism in detail taken along line 4--4 of FIG. 3; 
     FIG. 5 is a front sectional view of the UHF television tuner taken along line 5--5 of FIG. 4; 
     FIG. 6 is a detailed sectional view of a portion of the fine tuning mechanism taken along line 6--6 of FIG. 4; 
     FIG. 7 is another detailed sectional view of the fine tuning mechanism taken along line 7--7 of FIG. 4; 
     FIG. 8 is an exploded perspective view of a portion of the fine tuning mechanism according to the invention; 
     FIG. 9 is a detailed partial sectional view of the fine tuning mechanism; 
     FIG. 10 is a sectional view similar to FIG. 6 taken along line 10--10 of FIG. 9 and showing the fine tuning mechanism in an alternate position from that of FIG. 6; 
     FIG. 11 is a schematic diagram of the local oscillator of the UHF television tuner; and 
     FIG. 12 is a partial sectional view of an alternative embodiment of the fine tuning mechanism according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, with particular attention to FIG. 1, there is shown a detented UHF tuner according to the invention. The detented UHF tuner according to the invention utilizes a continuously variable UHF tuner generally designated by the reference numeral 10 having a continuously variable main tuning shaft 12 (FIGS. 3 and 4) extending therefrom. A detenting mechanism, generally designated by the reference numeral 14 utilizes a pair of gears 16 and 18 to couple the main tuning shaft to a channel selector shaft 20 (FIGS. 1 and 4). A detent wheel 22 and a detenting spring 24 provide a plurality of individual stops for the selector shaft 20, each stop being associated with a unique position of the main tuning shaft corresponding to one of the television channels in the UHF band. A torsion spring 26 applies a preloading force to the gears 16 and 18 to minimize backlash. The detenting mechanism 14 is similar to the one described in the referenced U.S. Pat. Nos. 3,774,459; 3,916,820; and 3,972,240 and will not be explained in great detail in the present specification. 
     A channel selector knob 28 is affixed to the channel selector shaft 20 and rotates the main tuning shaft 12 via the gears 16 and 18. In addition, the channel selector knob 28 operates a channel indicator dial 30 via a gear 32 affixed to the channel selector shaft 20. The channel indicator 30 may be similar to the channel indicator described in the referenced U.S. Pat. No. 3,916,820 or any other suitable indicator that provides an indication of the channel to which the UHF tuner 10 is tuned. In the present embodiment, the indicator 20 is advanced one increment each time the channel selector shaft is advanced by one detent position, and provides a unique and unambiguous indication of the number of the channel being received. 
     Rotation of the channel selector knob 28 imparts a rotational motion to the main tuning shaft 12 via the channel selector shaft 20 and the gears 16 and 18. This causes a pair of rotor plates 34 and 36 of a variable capacitor 38 to be rotated with respect to a stationary stator plate 40, thereby adjusting the operating frequency of the local oscillator of the tuner 10. In addition, second and third variable capacitors (not shown), each similar to the capacitor 38, are also adjusted by the main tuning shaft 12 for tuning the antenna and mixing circuits of the tuner 10 in a conventional manner. 
     The local oscillator circuit (shown best physically in FIG. 4 and schematically in FIG. 11) utilizes an oscillator transistor 42 having an emitter resistor 44, a collector resistor 46 and a pair of base biasing resistors 48 and 50. A base bypass capacitor 52 serves to bypass the base of the transistor 42 to ground at radio frequencies, and a pair of capacitors 54 and 56 serve as feedback capacitors. In the present embodiment, the collector-to-emitter capacitance of the transistor 42 serves as the feedback capacitor 54, however, an external capacitor may be used to increase the value of the feedback capacitance. Similarly, the capacitance between the emitter lead of the transistor 42 and the case of the tuner 10 serves as the capacitor 56, but a separate capacitor may be used. A variable inductor 58 having a self-supporting coil 59 is connected to the variable capacitor 38 and to a fixed capacitor 60. The capacitors 38 and 60 and the variable inductor 58 form a tuned circuit for determining the operating frequency of the oscillator. 
     The ouput of the oscillator is coupled to a mixer diode 62 which also receives the television signal applied to a pair of antenna terminals 64 and 66 of the tuner 10. The signals from the antenna terminals 64 and 66 and from the oscillator are combined in the mixer diode 62 and generate an intermediate frequency signal at an output 68 of the tuner 10 for application to the intermediate frequency circuits of the television receiver (not shown). In the present embodiment, the signal from the oscillator is coupled to the mixer diode 62 by means of radiation between the oscillator and the mixer diode 62, but a direct or other electrical connection may also be used. 
     In operation, the rotation of the main tuning shaft 12 effected by the channel selector shaft 20 and the channel selector knob 28 adjusts the variable capacitor 38 a sufficient amount to tune the oscillator over the entire range of frequencies necessary to cover the UHF television band, thus providing coarse tuning for each channel in the UHF television band. Fine tuning is effected by the variable inductor 58 which tunes the oscillator over a more limited range of frequencies, generally less than the frequency separation between adjacent channels in the UHF band, and typically approximately ±3mHz. Since the antenna and mixer circuits (not shown) are generally broader than ±3mHz, and since the tuner is aligned well enough that the antenna and mixer circuits accurately track the oscillator circuit, only the oscillator circuit need be adjusted to provide fine tuning. The adjustment of the oscillator within the limited tuning range provided by the variable inductor 58 will not bring the received signal out of the pass band of the antenna and mixer circuits. 
     In accordance with an important aspect of the present invention, the variable inductor 58 is controlled by a fine tuning mechanism, generally designated by the reference numeral 70 (FIG. 4) located within a fine tuning shaft 72. The fine tuning mechanism 70 includes an axially movable inductor tuning shaft 74. An inductor tuning sleeve 76 is affixed to the axially movable tuning shaft 74 and serves to tune the inductor 58 when axially moved within the coil 59 by the fine tuning mechanism 70. In the present embodiment, the tuning sleeve 76 is fabricated from brass and serves to decrease the inductance of the inductor 58 when it is inserted into the coil 59; however, it may be fabricated from other materials, such as, for example, a ferromagnetic material such as iron that serves to increase the inductance of the inductor 58 as the sleeve 76 is inserted into the coil 59. 
     A biasing spring 78 serves to bias the inductor tuning shaft 74 in a rearward direction, and maintains a conically shaped portion 80 of the inductor tuning shaft 74 in contact with a ball 82. The inductor tuning shaft 74 and the biasing spring 80 are retained within the channel selector shaft 20 which, in the present embodiment, is formed in two shaft sections 20a and 20b to permit the inductor tuning shaft 74 and the biasing spring 78 to be inserted into the hollow shaft section 20a. 
     The fine tuning shaft 72 is rotatably supported by the shaft section 20b of the channel selector shaft 20, and retained in place by a C-ring 83. The fine tuning shaft 72 includes an enlarged section 84 having a counter bore 86 formed therein. The counter bore 86 has a central axis that is radially displaced from the axis of the fine tuning shaft 72, and causes the ball 82 to be radially displaced with respect to the inductor tuning shaft 74 as the fine tuning shaft 72 is rotated by a fine tuning knob 88. An aperture 90 formed in the section 20a of the channel selector shaft 20 permits the ball 82 to move axially with respect to the channel selector shaft 20, but inhibits circumferential movement. 
     As previously mentioned, rotation of the fine tuning shaft 72 imparted to the shaft 72 by the fine tuning knob 88 causes the ball 82 to be displaced radially from the axis of the channel selector shaft 20 and the inductor tuning shaft 74. This radial displacement causes an axial displacement of the inductor tuning shaft 74 as a result of the contact between the ball 82 and the conical surface 80, and changes in degree of insertion of the sleeve 76 into the coil 59. For example, when the ball 82 is displaced the maximum distance from the axis of the inductor tuning shaft 74 (FIGS. 4 and 6), the ball 82 contacts the conical surface 80 at a point near the largest diameter of the conically shaped surface 80 and permits the inductor tuning shaft 74 to be biased by the spring 78 the maximum amount in the rearward direction. This causes the maximum degree of insertion of the sleeve 76 into the coil 59. When the fine tuning shaft 72 is rotated 180° so as to cause the ball 82 to be displaced to a position nearest the axis of the inductor tuning shaft 74 (FIGS. 9 and 10), the point of contact between the ball 82 and the conical surface 80 is moved to a point nearer the axis of the shaft 74 and results in a forward displacement of the shaft 74 against the force of the biasing spring 78. This forward motion of the shaft 74 results in a partial withdrawal of the sleeve 76 from the coil 59, and changes the inductance value of the variable inductor 58. 
     As can be seen from the drawings, the total displacement of the inductor tuning shaft 74 and the sleeve 76 is very small as the fine tuning shaft 72 is rotated between its 180° extremes, and the fine tuning system according to the present invention provides an efficient way to achieve rotary to axial motion translation while retaining a high positioning accuracy of the sleeve 76. Furthermore, the amount of axial movement of the sleeve 76 relative to the amount of rotation of the fine tuning shaft 72 may readily be altered by altering the amount of offset between the axes of the fine tuning shaft 72 and the counter bore 86. In addition, the amount of displacement of the sleeve 76 may be altered by changing the slope of the conically shaped surface 80. The amount and rate of displacement may also be altered by changing the shape of the surface 80 so that it has other than a linearly increasing diameter to achieve an axial motion of the sleeve 76 that is nonlinearly related to the position of the fine tuning shaft 72. For example, the surface 80 could be formed with an arcuately, parbolically, or otherwise nonlinearly increasing diameter. Also, the counter bore 86 could be made noncircular to achieve other linear or nonlinear effects. The position of the sleeve 76 may be altered with respect to the inductor tuning shaft 74 in the factory, or by a service man to adjust the nominal value of the inductor 58 so that the frequency of the tuner may be adjusted above and below the frequency of each UHF television channel by substantially equal amounts, thus centering the tuning range of the fine tuning system about each television channel. 
     An alternative embodiment of the invention wherein the coil 59 is not positioned coaxially with the inductor tuning shaft 74 is illustrated in FIG. 12. The embodiment illustrated in FIG. 12 utilizes a variable inductor 58&#39; having a coil 59&#39; connected to the capacitor 60&#39; and the stator plate 40&#39; of a variable capacitor 38&#39;. A sleeve 76&#39; is axially moved within the coil 59&#39; by an inductor tuning shaft 74&#39;. The components designated by primed numbers in FIG. 12 are analogous to components designated by similar unprimed numbers in FIGS. 1-11. 
     As in the previously described embodiment, the variable inductor 58&#39; and the capacitors 38&#39; and 60&#39; form a tuned circuit for determining the frequency of oscillation of the local oscillator of the tuner 10 in response to the position of the inductor tuning shaft 74&#39;. The tuning shaft 74&#39; is controlled by a fine tuning mechanism (not shown) similar to the mechanism 70. The sleeve 76&#39; is supported by a shaft 174 which is, in turn, supported by a flexible bracket 100 affixed to the case of the tuner 10 by a rivet 102 or otherwise. The bracket 100 is fabricated from a resilient material and is deflected by the axial movement of the inductor tuning shaft 74&#39;. Such deflection causes an axial displacement of the shaft 174, and varies the position of the sleeve 76&#39; within the coil 59&#39;. 
     The above-described structure permits the coil 59&#39; to be axially offset from the inductor tuning shaft 74&#39;, and increases tuner design flexibility. In addition, the degree of axial movement of the shaft 174 relative to the movement of the shaft 74&#39; may be altered by altering the relative positions of the shaft 174, the shaft 74&#39; and the rivel 102 to vary the mechanical advantage between the shaft 74&#39; and the shaft 174. This permits the shaft 174 to be moved more than or less than the shaft 74&#39; in response to a given movement of the shaft 74&#39; as required by the design of any particular tuner, and further increases design flexibility. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above.