Tuning system
BACKGROUND OF THE INVENTION 
     The present invention relates to tuning systems for radio frequency devices such as radio transmitters and receivers and, more particularly, to improvements in such systems which are particularly adapted for use in radio transceiver. 
     In conventional radio receivers, the tuning is often accomplished by a superheterodyne circuit; that is, one in which a mixer combines or &#34;beats&#34; an incoming radio-frequency signal with an internally-generated (&#34;local oscillator&#34;) frequency to obtain a difference signal which is applied to the intermediate frequency (IF) circuitry of the receiver in a well-known manner. In some receivers, more than one mixer-local oscillator stage is employed and two intermediate frequencies are thus produced (e.g., 10.7 MHz and 455 KHz). The frequency of the local oscillator is typically made variable by a user-operable control to thus permit tuning of the receiver to various desired radio frequencies. 
     One particular type of local oscillator tuning system employs the well-known phase-locked-loop (PLL) technique in a tuning system which has sometimes been referred to as the &#34;homodyne&#34; or &#34;synchrodyne&#34; circuit. Although these circuits have been known and used for at least 35 years, the relatively high cost thereof had made them unfeasible for most commercial radio devices. More recently, however, integrated circuit (IC) technology has revitalized and refined the phase-locke-loop technique to the point where it has become quite attractive for use in modern radio frequency devices, particularly where it is desired to tune to a plurality of frequencies with a relatively high degree of precision. 
     In general, the phase locked loop is a feedback system composed of four basic elements: a phase detector or comparator, a low-pass filter, a voltage-controlled oscillator, and a feedback loop containing a frequency divider. In operation, a reference signal having a fixed frequency (often crystal-controlled for stability) is applied to one input of a phase detector and a second signal having a frequency relatively close to that of the reference signal is applied to the second input of the phase comparator. The phase comparator produces an output signal having a voltage that is proportional to the phase difference between the frequency of the reference signal and the frequency of the other signal. The output signal of the phase detector is filtered by a low-pass filter to obtain a DC error signal which is applied to the voltage-controlled oscillator to adjust its frequency. This frequency is applied, by means of the frequency divider, back to the second input of the phase comparator. 
     Thus, the frequency of the voltage controlled oscillator is changed until it reaches that frequency which, when divided by the divider in the feedback loop, equals the frequency of the reference signal, at which point it is locked in. In so-called frequency synthesizer circuits employing the PLL technique, the divider is made variable or programmable in a predetermined fashion so that a number of predetermined frequencies can be generated with considerable accuracy. 
     Such circuitry is well-known and reference may be had, for example, to the Apr. 11, 1968 issue of Electronic Design (pages 76-80); pages 52 and 53 of Floyd M. Gardner&#39;s book entitled &#34;Phase Lock Techniques&#34;, John Wiley &amp; Sons Inc., 1966; and U.S. Pat. No. 4,013,957 to Tojo. As shown in the U.S. Pat. No. 4,013,957 patent, for example, similar techniques also may be utilized in radio transceiver circuitry to produce the carrier frequency which is modulated by the information signal. 
     Although the recent developments in integrated circuit technology have greatly reduced the cost of phase-locked-loop circuits and have therefore made them quite popular in tuning systems for multi-channel radio receivers, transmitters, and transceivers, it is still desirable to simplify and further reduce the cost of such tuning systems. In conventional transceivers such as that in the aforesaid U.S. Pat. No. 4,013,957 patent, for example, two or more crystal-controlled reference frequency sources are required. 
     It is therefore an object of the invention to provide a new and improved tuning system for radio frequency devices. 
     It is another object of the invention to provide such a new and improved system which is highly adaptable to radio frequency receivers, transmitters, and transceivers to simplify the circuitry and reduce the costs thereof. 
     It is yet another object of the invention to provide such a tuning system which requires only a single reference frequency source. 
     SUMMARY OF THE INVENTION 
     A system for generating a plurality of preselected frequencies in a radio frequency device comprises means for generating a first reference signal having a predetermined frequency and means, including at least two controllable variable-frequency oscillators, a mixer, a programmable frequency divider, and a switch, for generating a second reference signal, with the mixer being coupled to the first reference signal generating means and being selectively coupled to one of the variable-frequency oscillators by the switch to generate an output signal having a frequency corresponding to the difference between the frequency of the first reference signal and the frequency of the selected variable-frequency oscillator. Means are coupled to the first reference signal generating means and to the second reference signal generating means for comparing the first and second reference signals and developing a control signal which varies systematically in accordance with the frequency difference between the first and second reference signals, with the control signal being applied to both of the controllable variable-frequency oscillators to vary the oscillation frequencies thereof until the frequency of the second reference signal is substantially equal to the predetermined frequency of the first reference signal, whereby the output signal of the selected controllable variable-frequency oscillator has a frequency which corresponds to one of the plurality of predetermined receiver frequencies as selected by both the programmable frequency divider and the controllable variable-frequency oscillator selected by the switch. 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The organization and manner of operation of the invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing in which the drawing is a block diagram, partially in schematic diagram form, of a preferred embodiment of a tuning system constructed in accordance with the principles of the present invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With respect to the FIGURE, there is shown a system for generating a plurality of pre-selected frequencies in a radio frequency device. Means in the form of a crystal-controlled oscillator 1 are provided for generating a first reference signal having a predetermined frequency. For example, the frequency of oscillator 1 could be selected to be 10.24 megahertz when the invention is utilized in a citizen&#39;s band transceiver embodiment. It is understood, of course, that while the invention is described particularly with reference to a citizen&#39;s band transceiver, the principles involved may be applied to various types of radio frequency devices such as multi-band receivers or transmitters, for example. In the particular embodiment of the invention illustrated, a fixed frequency divider 2 is provided to divide the frequency of the reference signal down to a frequency such as 5 KHz, for example, which is better suited for application to a phase comparator 11. 
     Means including two controllable variable frequency oscillators 6 and 7, a mixer 10, a variable or programmable frequency divider 3, and a switch are provided for generating a second reference signal. In the illustrated embodiment, the switch can be four separate switches or a single switch with four poles, S 1 , S 2 , S 3  and S 4  and oscillators 6 and 7 may take the form of conventional voltage-controlled oscillators (VCO). Poles S 2  and S 4  are used to selectively couple oscillators 6 and 7 to mixer 10 and poles S1 and S3 are used to selectively couple the outputs of oscillators 6 and 7 via buffer amplifiers 14 and 15, respectively, to utilization circuitry (not shown) such as the remaining portion of the radio&#39;s tuning and/or modulating circuitry. An input of mixer 10 is also coupled to the crystal oscillator 1 so that mixer 10 generates an output signal having a frequency which corresponds to the difference between the frequency of the output signal of crystal oscillator 1 and the frequency of the controllable oscillator 6 or 7, as selected by switch poles S 2 , S 4 . 
     Phase comparator 4 is coupled to crystal oscillator 1, by means of fixed frequency divider 2, and to the output of programmable frequency divider 3 for comparing the reference signals from crystal oscillator 1 and programmable frequency divider 3 and developing a control signal which varies systematically in accordance with the frequency difference between the two reference signals. The control signal developed by phase comparator 4 is typically a DC voltage which is proportional to the frequency difference although, of course, any other systematic relationship (i.e., inversely proportional, an exponential function, or the like) may be employed without departing from the principles of the present invention. The control signal is applied to both controllable oscillators 6 and 7 by means of a low-pass filter 5 which removes any spurious signals from the control signal. One of oscillators 6 and 7 is thus responsive to the control signal to generate various oscillation frequencies until the frequency of the output signal of the programmable frequency divider 3 is equal to that of crystal oscillator 1, as divided by fixed frequency divider 2. In other words, until both input frequencies to phase comparator 11 are 5 KHz in the illustrated embodiment of the invention. 
     In the illustrated embodiment of the invention, the output signals of controlled oscillators 6 and 7 are applied to mixer 10 by means of buffer amplifiers 8 and 9, respectively. Buffer amplifiers 8 and 9 are provided to isolate the output of oscillators 6 and 7 from the input of mixer 10. A second pair of buffer amplifiers 14 and 15 may be employed as shown in the figure to isolate the output of oscillators 6 and 7 from the ultimate utilization circuits (not shown) which are connected at circuit points designated O 1  and O 2 . Such utilization circuits may be the so-called &#34;front-end&#34; or tuning circuitry of a radio receiver or transceiver or the modulation circuitry of a radio transmitter or transceiver. The particular utilization circuits do not form an important part of the present invention and therefore are not described in greater detail. Thus, the output frequency of the illustrated system, taken in the illustrated embodiment at the outputs of the oscillators 6 and 7 through buffer amplifiers 14 and 15, respectively, has a frequency which corresponds to one of the plurality of frequencies as selected by both programmable frequency divider 3 and the particular controllable variable frequency oscillators 6 and 7 selected by the switch poles S 2  and S 4 . 
     The output of the mixer is applied to programmable frequency divider 3 by means of a buffer amplifier 13 and, as described above, is used to isolate the output of mixer 10 from the input of divider 3. Also connected to the output of mixer 10 is a selector circuit in the form of a tuned LC network 12 which acts as a filter to pass only the desired mixer output frequencies (e.g. 6.03 MHz and 6.485 MHz) to buffer amplifier 13. 
     In accordance with another aspect of the present invention, a frequency multiplier circuit in the form of a frequency doubler 11 is coupled between the crystal oscillator 1 and the mixer and is responsive to the output of crystal oscillator 1 for applying a signal to mixer 10 which has a frequency that is a corresponding preset multiple of the frequency of the crystal oscillator 1. With the frequency multiplier circuit taking the form of a frequency doubler 11, of course, the corresponding preset multiple is twice the frequency of crystal oscillator 1. Thus, the mixer 10 may respond to four different signals to obtain the desired beat signal: 10.24 MHz, 20.48 MHz, and the two VCO frequencies in the illustrated embodiment of the invention. 
     In accordance with another aspect of the invention, a fixed frequency divider circuit 2 is coupled between the crystal oscillator 1 and phase comparator 11 for applying to phase comparator 11 the signal which has a frequency that is a pre-selected ratio of the frequency of the crystal oscillator. Using such a circuit enables the system to apply a relatively high frequency signal to the mixer 10 and a relatively low frequency to the phase comparator 4, both of which facilitate the operation of the embodiment of the invention shown in the FIGURE. In other words, a phse comparator such as that used for phase comparator 4 generally tends to operate better at lower frequencies and a crystal-controlled oscillator and a mixer such as those employed for oscillator 1 and mixer 10 generally tend to operate better at higher frequencies. 
     In the preferred embodiment of the invention illustrated in the FIGURE, which is particularly adapted for use in a citizen&#39;s band transceiver, significant advantage may be taken by employing voltage controlled oscillators for controllable variable frequency oscillators 6 and 7, with each voltage controlled oscillator having a different oscillation frequency in response to the same control signal, and with the resonant frequency of the crystal controlled oscillator 1 being a frequency approximately equal to the difference in oscillation frequencies of the voltage controlled oscillators. For example, the oscillation frequency of crystal oscillator 1 may be selected and controlled by crystal 1a such that oscillator 1 has a fixed reference frequency of 10.24 megahertz, and voltage controlled oscillators 6 and 7 may be constructed to have oscillation frequencies, for a given channel, of 16.27 megahertz and 26.965 megahertz, respectively, in response to the same control signal from low-pass filter 5. Thus, the difference in oscillation frequencies between voltage controlled oscillators 6 and 7 is 10.695 megahertz which is approximately equal to the 10.24 megahertz frequency of crystal oscillator 1. The comparatively small remaining difference, 0.455 megahertz, corresponds to the commonly used second IF frequencies in conventional citizen&#39;s band transceivers. 
     For the frequencies discussed immediately above, mixer 10 is thus responsive to the signal having a frequency of 10.24 megahertz from crystal oscillator 1, 20.48 megahertz from the output of frequency doubler 11 and either (for a given channel) 16.27 megahertz from oscillator 6 or 26.965 megahertz from oscillator 7, depending upon which switch is closed. If switch pole S 2  is closed (and S 4  is thus open), for example, then mixer 10 will beat 16.27 megahertz with 10.24 megahertz to get a difference frequency of 6.03 megahertz, and will beat 16.27 megahertz with 20.48 megahertz to get a difference frequency of 4.91 megahertz which may be filtered out by network 12. On the other hand, if switch S 4  is closed (and switch S 2  is therefore open), mixer 10 will beat 26.965 megahertz with 10.24 megahertz to get a difference frequency of 16.725 megahertz which may be filtered out by network 12, and 26.965 megahertz with 20.48 megahertz to get a difference frequency of 6.485 megahertz. Network 12 is therefore tuned such that the unwanted difference frequencies, and other frequencies, are shunted to ground and only the desired mixer output frequencies are applied to the programmable frequency divider 3 by means of buffer amplifier 13. In this example, the 26.965 megahertz output frequency of oscillator 7 could be used for transmission; the 16.270 megahertz frequency of oscillator 6, for reception. 
     Programmable frequency divider 3 is made adjustable so that, by changing its dividing ratio, such as with a channel selector switch (not shown), the oscillation frequencies of oscillators 6 and 7 may be varied to obtain the desired output frequencies at output terminals O 1  and O 2  for the channel desired. 
     Thus there has been shown and described a new and improved system for generating a plurality of preselected frequencies in a radio frequency device. The system employs a relatively simple and inexpensive circuit to generate a number of frequencies, some of which are in widely separated frequency bands such as, for example, the different bands of a multi-band receiver or transmitter, or the local oscillator frequencies required to receive a citizen&#39;s band radio frequency signal and those required to modulate a carrier signal for transmission by a citizen&#39;s band transceiver or transmitter. Particular advantage is obtained by using a single reference frequency source and two voltage-controlled oscillators wherein the oscillation frequency of the first voltage controlled oscillator in response to a given control signal differs from the oscillation frequencies of the second voltage-controlled oscillator in response to the same control signal by an amount which is equal to the sum of the frequency (in particular applications of the invention) of the fixed reference signal from a crystal-controlled oscillator and the receiver&#39;s intermediate frequency signal. With the present invention, a plurality of frequency bands may be accommodated using a single fixed frequency reference source and a single mixer, which provides significant advantages in terms of cost and complexity. 
     While particular embodiments of the invention have been shown and described in detail, it will be obvious to those skilled in the art that changes and modifications of the present invention, in its various aspects, may be made without departing from the invention in its broader aspects, some of which changes and modifications being matters of routine engineering or design and others being apparent only after study. As such, the scope of the invention should not be limited by the particular embodiment and specific construction described herein but should be defined by the appended claims, and equivalents thereof. Accordingly, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.