Abstract:
A frequency modulation system is disclosed which includes a voltage-controlled oscillator (VCO)  43  and a phase detector  47  configured to receive an output signal from the VCO. The phase detector is arranged to output an error signal representing the phase difference between the signal from the VCO and a reference signal. The system also includes control means  62  arranged to monitor the error signal to derive an indication of the frequency deviation of the VCO, and, in accordance with this derivation, to maintain the frequency deviation of the VCO substantially constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    The present application claims priority to currently pending United Kingdom Patent Application number 0212727.2, filed on May 31, 2002.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    N/A  
         BACKGROUND OF THE INVENTION  
         [0003]    Frequency modulation (FM) and phase modulation (PM) techniques are used for transmitting information on a carrier signal, e.g. sound information on a carrier radio wave. Unlike amplitude modulation techniques (AM) where the amplitude of a carrier signal is modulated by the information signal, in FM and PM, the frequency and phase of the carrier signal, respectively, is modulated. FM and PM are usually referred to collectively as ‘angle modulation’ techniques. In this description, although the term frequency modulation, or FM, is used throughout, it should be understood that the term is intended to apply equally to PM techniques.  
           [0004]    As will be appreciated by those skilled in the art, a quality measure for FM modulation systems is the ‘modulation index’, which is defined as the ratio of the frequency deviation during modulation to the modulating frequency. Certain applications require a high modulation index, whilst others require a low modulation index. In applications where a high modulation index accuracy is desirable, such as in a radio system, a number of circuit arrangements can be used.  
           [0005]    Referring to FIG. 1, a known FM circuit employs the well known I (in-phase) and Q (quadrature) modulation scheme. First and second modulating inputs  3 ,  5  are provided for receiving an input signal in phase quadrature. The signals are applied to first and second balanced modulators  7 ,  9  that receive, on first and second lines  10 ,  11  a quadrature input from a quadrature phase shift network  13 . The quadrature input is received at the signal frequency in accordance with the operation of a carrier frequency source  15 . The outputs from the first and second modulators  7 ,  9  are passed, on respective lines  17 ,  19 , to a summing circuit  21 . The summed output is provided on output line  23 .  
           [0006]    Modulation circuits such as that shown in FIG. 1 are used widely and are able to produce almost any form of modulation, albeit with some limitations in frequency shift keying (FSK) modulation. However, the provision of the two balanced modulators  7 ,  9  with accurate balance of gain and phase, and the provision of the phase shift network  13 , results in a complex circuit that requires relatively high power.  
           [0007]    A simplified system is shown in FIG. 2. Here, a voltage-controlled oscillator (VCO)  27  feeds a frequency divider  29 , usually implemented as a digital counter. The output from the divider  29  provides one input to a phase detector  31 , the other input being taken from a reference frequency source  33 . The phase detector  31  outputs an error signal representing the phase difference between the two input signals. This error signal is fed to a low pass filter  35  and then to a summing circuit  37  that is fed back to a control input of the VCO  27 . A modulation signal is applied from a supply means  39  to a further input of the summing circuit  37 . The VCO  27  is directly modulated by the summation result, and the modulated output is taken as the output from the VCO. The loop bandwidth of the circuit of FIG. 2 is determined by a number of factors, and the modulation frequency or frequencies may be totally inside or outside of the bandwidth, or even partially inside and outside the bandwidth.  
           [0008]    Systems such as that shown in FIG. 2 have been in use for many years. They offer the advantage that complexity is minimized, and the component count is reduced so that power consumption can be kept at a minimum. The general layout of the circuit components of FIG. 2 will be recognized as being similar to a phase locked loop (PLL) and the bandwidth of the loop is chosen such that the modulation frequency lies outside the bandwidth. In many systems this is because the channel spacing requirements are such that in order to achieve adequate suppression of reference frequency components at the VCO, it is impractical if the attenuation curve of the low pass filter does not start at a low value. A general problem with such systems is that the frequency deviation needs to be set for each different modulation system on an individual basis because of the difficulties in producing a VCO in which the control voltage coefficient Kv (controlling the frequency change in Hertz per volt) is constant within a single production batch. Further problems appear since the value of Kv varies with supply voltage and/or temperature.  
         OBJECTS AND SUMMARY OF THE INVENTION  
         [0009]    The invention, in one sense, relates to a system for performing frequency modulation which includes a VCO and a phase detector configured to receive an output signal from the VCO, the phase detector being arranged to output an error signal representing the phase difference between the signal output from the VCO and a reference signal, the system also including control means arranged to monitor the error signal to derive an indication of the frequency deviation and, as a result of the derivation, to maintain the frequency deviation of the VCO substantially constant. In the context of the application, ‘VCO’ is intended to mean any circuit or system whereby the frequency of an output voltage from the circuit or system is dependent on a voltage (often referred to as a ‘control voltage’) inputted therein.  
           [0010]    According to a further aspect of the present invention, there is provided a frequency modulation system comprising: a VCO; means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO; a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a reference signal; and control means arranged (a) to receive the error signal and to derive an indication of the frequency deviation of the VCO; and (b) to vary the output from the summing means in accordance with the frequency deviation thereby to maintain the frequency deviation of the VCO substantially constant.  
           [0011]    The system may further include a controllable gain module arranged to output, to the summing means, an amplified version of the modulating input signal, the control means being arranged to vary the output from the summing means by controlling the amount of amplification applied to the modulating input signal.  
           [0012]    The control means may be arranged to generate, from the error signal, a signal representative of the frequency deviation of the VCO, and to generate a gain signal for controlling the gain applied by the controllable gain module.  
           [0013]    The control means may include means arranged to determine the amplitude of the error signal, and comparator means arranged to compare the determined amplitude with a predetermined reference voltage, the output from the comparator means comprising the gain signal for controlling the gain of the controllable gain module.  
           [0014]    The amplitude determining means comprises a rectifier. In digital modulation applications, the amplitude determining means may include a sample and hold circuit. The system may further comprise a frequency divider connected between the VCO and the phase detector.  
           [0015]    According to a further aspect of the invention, there is provided a frequency modulation system comprising: a VCO; means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO; a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a fixed frequency reference signal; and control means arranged (a) to receive the error signal and to derive a controlling signal that is proportional to the frequency deviation of the VCO, and (b) to vary the output from the summing means, using the controlling signal, thereby to maintain the frequency deviation substantially constant.  
           [0016]    According to further aspect of the invention, there is provided a method of performing frequency modulation in a system comprising a VCO, means for summing a modulating input signal and an error signal thereby to generate a signal for controlling the frequency of a signal outputted from the VCO, and a phase detector for generating the error signal based on the detected phase difference between the signal outputted from the VCO and a reference signal, wherein the method comprises: monitoring the error signal and deriving an estimate of the frequency deviation exhibited by the VCO; and controlling the output of the summing means in accordance with the estimated frequency deviation such that the frequency deviation of the VCO is maintained substantially constant.  
           [0017]    While the above-mentioned FM systems and methods can be used in radio system applications, the same also applies to optical system applications.  
           [0018]    Additional objects and advantages of the invention will be set forth in part in the description that follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.  
           [0019]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate at least one presently preferred embodiment of the invention as well as some alternative embodiments. These drawings, together with the description, serve to explain the principles of the invention but by no means are intended to be exhaustive of all of the possible manifestations of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a schematic circuit diagram of a known FM system that is useful for understanding the invention;  
         [0021]    [0021]FIG. 2 is a schematic circuit diagram of a further, known FM system, which is useful for understanding the invention;  
         [0022]    [0022]FIG. 3 is a schematic circuit diagram of a FM system in accordance with a first preferred embodiment of the invention;  
         [0023]    [0023]FIG. 4 is a circuit diagram showing part of the FM system shown in FIG. 3; and  
         [0024]    [0024]FIG. 5 is a schematic circuit diagram of a FM system in accordance with a second preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    Reference now will be made in detail to the presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, which is not restricted to the specifics of the examples. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention cover such modifications and variations as come within the scope of the appended claims and their equivalents. The same numerals are assigned to the same components throughout the drawings and description.  
         [0026]    As mentioned above, although the following description refers to a FM system, it should be remembered that such references are intended to cover PM systems also.  
         [0027]    Referring to FIG. 3, in a first preferred embodiment, a system for frequency modulating a signal comprises a VCO  43  that is connected to a frequency divider  45 , which can be implemented as a digital counter. The frequency divider  45  is itself connected to a phase detector  47 . The phase detector  47  also receives a reference frequency signal from a reference source  49  and generates an output signal in the form of a voltage level representing the instantaneous phase error between (a) the signal received from the VCO  43  via the divider  45 , and (b) the reference frequency. The error signal is passed to a low pass filter  53  and to a rectifier  51 , which rectifier forms part of a gain control circuit  62 , as will be explained below. The low pass filter  53  is connected to an adder  55 , the output of which is connected to the control input of the VCO  43 . As will be appreciated, the output frequency of the VCO  43  will be dependent on the control voltage. A modulating input source  63  provides the required modulating input signal. The modulating input signal is input to an input of the adder  55  by means of a gain controlled stage  61 . The FM output signal is taken from the output of the VCO  43 .  
         [0028]    As briefly mentioned above, a gain control circuit  62  is provided. The purpose of the gain control circuit  62  is to modify the gain controlled stage  61  so as to maintain the modulation index at a substantially constant Fig. Since the modulated VCO signal appears at the input to the phase detector  47 , it is noted that the outputted error signal from the phase detector contains a small a.c. component that is proportional to the frequency deviation of the VCO  43 . Since the modulation index is dependent on the frequency deviation, this error signal can be used to control subsequent stages of the system to maintain modulation index accuracy. It is comparatively easy to produce the phase detector  47  with a known transfer characteristic K□ (radians/volt). Accordingly, an accurate measure of the frequency deviation is given by the magnitude of the alternating voltage contained in the error signal from the phase detector  47 . Given the circuit arrangement already discussed, it will be clear that the amplitude of the modulating signal from the modulating input  63 , and thus the frequency deviation of the VCO  43 , is determined by the gain controlled stage  61 . Therefore, by controlling the gain-controlled stage  61  in accordance with the amplitude of the a.c. component in the error signal, the modulation index can be kept substantially constant.  
         [0029]    The gain control circuit  62  comprises the rectifier  51 , which is connected to a voltage comparator  57 . The voltage comparator  57  also receives a reference voltage  59 . The output of the voltage comparator  57  is fed to the gain controlled stage  61 . The a.c. component of the error signal from the phase detector  47  is rectified by the rectifier  51 , and applied to the voltage comparator  57  wherein the voltage is compared with the reference voltage  59 . The output from the voltage comparator  57  is fed to the gain controlled stage  61  so as to maintain the frequency deviation caused by modulation of the VCO  43  constant. Note that, in practice, integrating capacitors (not shown) would be associated with the rectifier and a low pass filter (not shown) would be provided between the comparator  57  and the gain-controlled stage  61 .  
         [0030]    Reference is made to FIG. 4, which shows a circuit-level implementation of the gain control circuit  62  and the gain controlled stage  61 . An operational amplifier  70  forms the basis of the voltage comparator  57 . The output from the rectifier  51  (which may include integrating capacitors, if necessary) is connected to the non-inverting input of the operational amplifier  70  by means of a first resistor  67 . The inverting input of the operational amplifier  70  is connected, by means of a second resistor  68 , to the reference voltage source  59 . A feedback resistor  69  is connected between the inverting input and the output of the operational amplifier  70 , and as will be understood, the ratio of the feedback resistor and the second resistor  68  set the gain of the operational amplifier. The output of the operational amplifier  70  is connected to a low-pass filter  71 , the output of which is connected to one input of a differential amplifier stage. The differential amplifier stage forms part of the gain controlled stage  61 . The differential amplifier stage comprises first and second transistors  72 ,  73  connected in a long-tailed pair arrangement. The output of the low-pass filter  71  is connected to the first transistor  72 . The base terminal of the second transistor  73  of the long-tailed pair arrangement is connected to the constant reference voltage source  59 . A tail transistor  75  receives, at its base terminal, the modulating input from the modulating input source  63 . A third resistor  76 , which can be provided to allow for level shifting, is connected between the emitter terminal of the tail transistor  75  and a negative supply line, although as an alternative, a grounded line could be used.  
         [0031]    By varying the voltage at the base terminal of the first transistor  72  (whilst the voltage at the base terminal of the second transistor  73  is held constant) the collector current of the tail transistor  75  is diverted either through a load resistor  74  (connected between a positive supply line and the collector terminal of the second transistor  73 ) or through the collector terminal of the first transistor  72 , which is connected to the positive supply line. Thus, a variable gain circuit is achieved. The output from the collector terminal of the second transistor  73  provides the output signal to the adder  55 . In practice, means would be provided to prevent a d.c. level shift occurring when a change in gain occurs.  
         [0032]    Other circuit arrangements for varying the amplitude of the modulating input signal will be known to those skilled in the art.  
         [0033]    It will be clear to those skilled in the art that variations in the value of N (the amount by which the frequency is divided in the frequency divider  45 ), required when the frequency of the VCO  43  is changed for purposes such as the selection of another operating frequency, will cause changes in the frequency deviation. This may be accommodated either by providing an allowable tolerance on the deviation for those cases where the change in N is small, or by alteration of the reference voltage, from the reference voltage source  59 , in sympathy with the changing value of N. Alternatively, this can be achieved by controlling the gain of the gain-controlled stage  61  in sympathy with the value of N programmed in the frequency divider  45 .  
         [0034]    Although the embodiment shown in FIG. 3 comprises a simple frequency divider  45 , the same function may be achieved in a number of alternative ways. For example, a two modulus divider could be used, as could part of a fractional N divider. Both types of divider are well known in the art.  
         [0035]    Referring now to FIG. 5, a second preferred frequency modulating system is shown. The system is the same as that shown in FIG. 3, except that a different gain control circuit  66  is shown. The gain control circuit  66  comprises a sample and hold circuit  64  which is connected to a voltage comparator  67 . The reference voltage source  59  supplies the reference voltage to an adder/subtractor  65 . The adder/subtractor  65  is also connected to the output of the low pass filter  53  and to the modulating input  65 . The output from the adder/subtractor  65  is passed to a further input of the voltage comparator  67 . The output from the voltage comparator  67  is connected to the gain controlled stage  61 .  
         [0036]    The second preferred form of system shown in FIG. 5 is advantageous where the modulation is of digital form, such as in two-level FSK modulation (2-FSK). However, the system is not limited to such digital modulation schemes and can be used with analogue modulation schemes.  
         [0037]    In the second embodiment shown in FIG. 5, the output from the sample and hold circuit  64  will be greater or less than the average voltage at the output of the low pass filter  53 , the difference being dependent upon whether the modulating signal is a ‘1’ or a ‘0’ and the polarity of the signals transmitted, i.e. if the transmission of a ‘1’ is determined by a frequency that is higher or lower than the center frequency. The magnitude of the difference will be dependent on the amount of phase deviation at the input of the phase detector  47 . The signal that is fed to the voltage comparator  67 , which may be similar to that described for the previous embodiment, is derived by summing the average value of the control voltage of the VCO  43  with a signal to either increase or decrease the result of the summation in sympathy with the modulating signal. The magnitude of such increase or decrease is determined by the value of the reference voltage from the reference voltage source  59 . The output from the adder/subtractor  65  thus varies in sympathy with the modulating input signal, with the variation in level being fixed by the value of the reference voltage and the absolute value being fixed by the average value of the control voltage applied to the VCO  43 . The absolute value of the signal at the output of the sample and hold circuit  64  will be dependent both upon the modulation value (i.e. a ‘1’ or a ‘0’) and the control voltage applied to the VCO  43 . Thus, both signals applied to the voltage comparator  67  will vary together, and, provided delays in the VCO  43 , the divider  45 , and the phase detector  47 , the output of the voltage comparator  67  will be such as to enable the gain controlled stage  61  to vary the level of modulating signal such that the deviation of the VCO with modulation is automatically controlled to a given level, preset by the value of the reference voltage from the reference voltage source  59 .  
         [0038]    It will be noted that the rectifier  51  used in the circuit of FIG. 3 is replaced by the sample and hold circuit  65 . By sampling the amplitude of the error signal, outputted from the phase detector  47 , at a suitable symbol rate that is synchronous with the digital modulation signal, the instantaneous voltage present at the output of the phase detector represents the instantaneous frequency of the VCO  43  (albeit delayed by the propagation delay of the frequency divider  45 , a delay which can be readily compensated for by well known techniques). The absolute value of this sampled voltage depends on the operating frequency, and depending on whether a “1” or “0” is to be transmitted, will be above or below the average value of the control voltage. The adder/subtractor  65  therefore adds the transmitted “1” or “0” or vice versa. The addition and subtraction function is controlled by the transmitted data and the output of the adder/subtractor  65  then accurately represents the required voltage at the VCO  43 . The output of the phase detector  47  is sampled at the modulation bit-rate by the sample and hold circuit  65 , and the voltage compared with that at the output of the adder/subtractor  65  in the voltage comparator  67  so as to vary the gain of the gain controlled stage  61  to maintain the frequency deviation at the required level.  
         [0039]    Where the modulation is of multiple levels, such as in M-level FSK, the reference voltage may be varied in sympathy with the modulation to produce the necessary number of levels and so a required, substantially constant, modulation index.  
         [0040]    In the systems shown in FIGS. 3 and 5, the gain controlled stage  61  may be implemented as a variable gain amplifier, the gain being adjusted in accordance with the gain control circuit  62 ,  66  of the respective systems.  
         [0041]    As will be appreciated, the reference voltage of the gain control circuit  62 ,  66  can be adjusted in order to control the modulation index. Indeed, either the gain of the gain controlled stage  61 , or the reference voltage from the reference voltage source  59 , can be controlled in sympathy with the programmed frequency to maintain a substantially constant modulation index. In this context, “programmed frequency” takes its usual meaning in relation to frequency synthesizers, i.e. it is that VCO frequency produced by the programming of any particular number N in the divider, i.e. the divider  45 , in this case.  
         [0042]    It will be appreciated that the general configuration of the modulation systems shown in FIGS. 1 and 2 is similar to that of a PLL.  
         [0043]    The above two systems enable accurate control of the frequency deviation exhibited by the VCO and so allow the modulation index to be maintained substantially constant. This is performed in such a way that the modulation index is independent of variations in the value of Kv, i.e. the control voltage coefficient of the VCO  43 .  
         [0044]    While at least one presently preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.