Abstract:
The present invention provides means for deriving a single frequency signal from logic signals A(t) and B(t) by first applying the logic signals to a combining arrangement, which includes logic circuits and can be built using only standard integrated circuit components, e.g., flip-flops, gates, and amplifiers. The output of the combining means is the sum and difference frequencies, in digital form, of the logic signals A(t) and B(t). The single frequency is selected from the frequencies present at the output of the combining means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to techniques used for generating or deriving single frequency signals as may be required in communication systems, as for example for pilot carrier frequencies, signaling tones, and the like; and more particularly to means and methods by which such signals may be derived from available logic signals of different frequencies. 
     2. Description of Prior Art 
     Heretofore, single frequency signals of the character described have been obtained by subdividing the frequency of a clock source, as by means of a counter, or by modulation wherein a beat signal is obtained having a frequency equal to the sum or difference of the frequencies of the two more basic signals used in the modulation process. The frequency division technique may be used only where the frequency division requirement corresponds with an available counter division ratio. Thus, given say a 720 kHz clock source, an 8 kHz signal may be obtained by using a divide-by-90 counter. On the other hand, if there was available in the system say a 160 kHz signal and a 72 kHz signal was required, derivation of the latter by a division technique is not feasible. However, if in the same system an 8 kHz signal is available, the 160 kHz signal may be reduced to an 80 kHz signal by a divide-by-2 counter and then modulated with the 8 kHz signal to produce the required 72 kHz signal. 
     The usual method for obtaining the product P(t) = sin 2π(f m  ± f n )t is accomplished by the well-known process of modulation wherein the frequency of a single frequency wave (normally called the carrier wave) is varied in step with the instantaneous value of a second signal, called the modulating wave. The signals to be modulated A(t) = sin 2πf m  t and B(t) = sinπ2 f n  t may be either sine waves as indicated or square waves. These are fed into a modulator usually made up of diodes and/or transistors and quite often transformers; and a filter is placed at the output of the modulator to select the desired output signal sin 2π(f m  + f n ) or sin 2π(f m  - f n )t. In equipment which uses mainly integrated digital logic circuits it is advantageous to use standard logic circuits whenever possible and to minimize the use of the discrete, analog components such as required by the usual modulator. 
     SUMMARY OF INVENTION 
     The present invention provides means for deriving a single frequency signal from logic signals A(t) and B(t) by first applying the logic signals to a combining arrangement, which includes logic circuits and can be built using only standard integrated circuit components, e.g., flip-flops, gates, and amplifiers. The output of the combining means is the sum and difference frequencies, in digital form, of the logic signals A(t) and B(t). The single frequency is selected from the frequencies present at the output of the combining means. Briefly then, the present invention may be used to generate a signal having a frequency f c  which is the sum or difference of the frequencies f a  and f b  of signals available in logic level form. In this sense, the present invention differs from modulators in general in that the means and method of the present invention is used to provide a signal of single frequency. 
     The invention possesses other objects and features of advantage, some of which of the foregoing will be set forth in the following description of the preferred form of the invention which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by the said drawings and description may be adopted within the scope of the invention as set forth in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to said drawings: 
     FIG. 1 is a graph of ideal waveforms in a modulation process using a unipolar carrier signal. 
     FIG. 2 is a graph of ideal waveforms in a modulation process using a bipolar carrier signal. 
     FIG. 3 is a schematic diagram of a signal derivation means constructed in accordance with the present invention. 
     FIG. 4 is a schematic diagram of a modified form of the invention. 
     FIG. 5 is a graph showing signal waveforms involved in the structure of FIG. 3. 
     FIG. 6 is a graph showing signal waveforms involved in the structure of FIG. 4. 
     DETAILED DESCRIPTION OF INVENTION 
     Typical modulation waveforms using a unipolar carrier signal B(t) are illustrated in FIG. 1 wherein it may be seen that the modulated product M(t) is of the commonly understood form which may be expressed: M(t) = A(t) . B(t). 
     In a similar manner, ideal waveforms involved in a modulation process of signals A(t) and B(t), wherein B(t) is a bipolar signal, are shown in FIG. 2. As will be observed, the modulation signal M(t) = A(t) . B(t). 
     The object of the present invention is to provide the aforementioned modulation signals M(t) not by the usual modulator but by the use of integrated circuit components as illustrated in FIGS. 3 and 4. The method of the present invention for obtaining a signal M(t) having a frequency equal to the sum or difference of a pair of logic signals A(t) and B(t) here consists briefly in the combining of logic signals A(t) and B(t) in a plurality of coincidence circuits wherein each coincidence circuit produces one specified output condition only when the input logic signals are of one like state and produces the other output condition for all other combinations of the logic signals. The appearance of logic signals A(t) and B(t) must naturally occur during an assigned time interval which interval is established by a conventional clock signal, not shown. 
     One circuit arrangement for practising the general method is shown in FIG. 3 which may be used with a unipolar, i.e., binary, logic (carrier) signal B(t). A pair of logic signals, modulation signal A(t) and carrier signal B(t), are applied directly to the inputs of the first of a pair of coincidence circuits. The first coincidence circuit has a logic 1 output except when the true form of logic signals A(t) and B(t) are both logic 1 during any particular assigned time interval. The second coincidence circuit has inputs of the true form of the carrier signal B(t) and the complement of the modulating signal A(t), i.e., A(t). As shown in FIG. 3, the second coincidence circuit has a 1 output except when both inputs, i.e., A(t) and B(t), are both logic 1 during any assigned time interval. The outputs of the two coincidence circuits are then summed. The summing operation is here shown accomplished in FIG. 3 by the use of a summing amplifier 11 having plus and minus inputs connected to outputs C and D of the respective first and second coincidence circuits, as shown. The amplifier 11 output at E is equal to the input differences, i.e., D input minus C input. As will be demonstrated, output E contains the desired modulation signal M(t) = A(t) . B(t). A narrow-band filter 16 selects the sum or difference frequency from the signals present at E so that the desired single frequency signal is available at F. 
     The aforementioned coincidence circuits here comprise simple NAND gates 12 and 13 which provide selective outputs C and D in accordance with the following truth table: 
     
         A(t)       B(t)          C______________________________________0          0             10          1             11          0             11          1              0.______________________________________ 
    
     Gate 13 functions in an identical manner to selectively provide output D for various inputs of A(t) and B(t) as shown in the truth table. In FIG. 3 the signal A(t) and its complement A(t) are shown originating from a flip-flop 14. This is a convenient way of obtaining the true and complementary waveforms if a source voltage F(t) is available having a frequency of two times the frequency of signal A(t). However, if signal A(t) is directly available, A(t) can be obtained by running signal A(t) through an inverter. The essential thing insofar as the present invention is concerned is to provide the true and complement forms of one of the pair of signals to be modulated. The flip-flop 14 does this but also acts as a frequency divider. As will be seen from FIG. 3, signals A(t) and A(t) are applied to the inputs of NAND gates 12 and 13 as is also the second (carrier) signal B(t); and the outputs C and D of the NAND gates 12 and 13 are connected to the minus and plus inputs of summing amplifier 11. A filter 16 is connected to the output of amplifier 11 to pick off the desired modulation product. 
     The operation of the apparatus of FIG. 3 may be better understood by reference to the waveforms illustrated in FIG. 5. The top three waveforms show the basic wave trains B(t), A(t), and A(t). The wave train depicted at C corresponds with the output C of NAND gate 12 and results from the NANDING of signals A(t) and B(t). The wave train depicted at D corresponds with the output of NAND gate 13 and results from the NANDING of signals A(t) and B(t). The wave train depicted at E corresponds with the output of amplifier 11 and is the result of summing the waveforms shown at C and D. Most importantly, it will be noted that the wave train shown at E corresponds with the wave train M(t) of FIG. 1 and hence the desired modulation has been accomplished using integrated circuit components rather than the usual modulator. 
     A modification of the circuitry is illustrated in FIG. 4 for obtaining modulation of signal A(t) by bipolar signal B(t) as shown in FIG. 2. In this case the true and complementary forms of both signals are used. As shown in FIG. 4, the true signals A(t) and B(t) are connected to a first coincidence circuit 18 providing an output C having a logical 1 in all cases except when both of the signals applied thereto are logical 1. Similarly, the complementary signals A(t) and B(t) are connected to a second coincidence circuit 19 providing an output D having a logical 1 in all cases except when both of the input signals are a logical 1. Outputs C and D are then connected to a third coincidence circuit 20 having an output E again providing an output having a logical 1 in all cases except when outputs C and D are a logical 1. Coincidence circuits 18, 19, and 20 may comprise simple NAND gates providing a selective output in accordance with the truth table hereinabove set forth. The true and complementary forms of signals A(t) and B(t) may be obtained as here shown from a pair of flip-flops 22 and 23 as in the first described embodiment where signals F(t) and S(t) are available having frequencies of two times the frequencies of A(t) and B(t), respectively. A filter 24 is connected to output E of NAND gate 20 for picking off the desired modulation product. 
     The operation of the circuit of FIG. 4 may be better understood with reference to the waveforms shown in FIG. 6. The true and complementary forms of signals A(t) and B(t) are the waveforms shown on the first four lines of the FIGURE. The wave train depicted at C corresponds with the output C of NAND gate 18 and results from the NANDING of signals A(t) and B(t). The wave train as depicted at D corresponds with the output D of NAND gate 19 and results from the NANDING of signals A(t) and B(t). The wave train depicted at E corresponds with the output E of NAND gate 20 and results from the NANDING of outputs C and D. Most importantly, it will be noted that the waveform shown at E is the same as that shown at M(t) of FIG. 2 thus demonstrating the successful modulation of a bipolar carrier signal by use of integrated circuit components rather than the conventional modulator. There is a DC offset which is immaterial to the circuit application and has no consequence after the signal passes through filter 24.