FM DEMODULATOR

Abstract: The frequency modulation (FM) demodulator comprises a limiter (1) for converting an input signal into a rec­ tangular signal; first pulse generator (11) responsive to the transition points of said rectangular signal for generating a first pulse train having a first and a second states for the period of said rectangular signal, said first state having a predetermined period of time; second pulse generator (12) for generating a second pulse train having a third and fourth states for the period of said rectangular signal, the duration of said third state being reduced by a predetermined time length based on the duration of said second state of said first pulse train; and low-pass filter (13) for integrating said second pulse train to provide a demodu­ lated signal. This FM demodulator has a higher demodu­ lation sensitivity and is suitable for an FM signal having a narrow normalized bandwidth. (Fig. 4).

Description: BACKGROUND OF THE INVENTION The present invention relates to a frequency modulation (FM) demodulator and, more particularly, to a pulse count type FM demodulator. A pulse count type demodulator of the prior art, as will be described in more detail later, usually consists of a limiter circuit, a monostable multivibrator and a low-pass filter (LPF). The monostable multivibrator generates a pulse having a fixed time width in response to a transition point, for instance the leading edge point, of the limiter circuit's output. The LPF integrates the output of the monostable multivibrator to supply a demodulated output. Since the width from the trailing edge of the monostable multivibrator's output to its next leading edge is proportional to the frequency of the input signal, the output voltage of the LPF is proportional to the frequency of the input signal, so that FM demodulation is achieved. In the above described pulse count type demodulator of the prior art, the lower limit of the frequency band of demodulated signals is zero Hz (D.C.), and linearity is maintained over a wide band ranging from zero to the upper limit determined by the output pulse width of the monostable multivibrator. Though having such a wide frequency band, the prior art demodulator is poor in demodulation sensitivity. For this reason, where an FM signal whose maximum frequency deviation is extremely small relative to the center frequency, i.e., an FM signal whose normalized bandwidth is narrow, is to be demodulated, a demodulated signal is vulnerable to the adverse effect of external noise, such as source voltage fluctuation. The US-A-4,091,330 describes a demodulator with a multivibrator switchable from one state to another in response to transitions of an FM signal. Time delay means are provided for switching the multivibrator to a second state after a time delay determined by the time delay means. Therefore, an object of the present invention is to provide an FM demodulator having a higher demodulation sensitivity and being suitable for an FM signal having a narrow normalized bandwidth and for monolithic integration. This object is solved with the features of the claims. The above and other objects, features and advantages of the present invention will become more apparent from the detailed description hereunder taken in conjunction with the accompanying drawings, wherein: FIG. 1 is a block diagram illustrating a pulse count type FM demodulator of the prior art; FIGS. 2A to 2D are time charts for describing the operation of the demodulator of FIG. 1; FIG. 3 is a diagram showing the frequency vs. voltage (F/V) characteristic of the demodulator of FIG. 1; FIG. 4 is a schematic block diagram illustrating a pulse count type FM demodulator according to a preferred embodiment of the invention; FIGS. 5A to 5D are time charts for describing the operation of the demodulator of FIG. 4; FIG. 6 is a diagram showing the F/V characteristic of the demodulator of FIG. 4; FIG. 7 is a block diagram illustrating an FM demodulator according to another preferred embodiment of the invention; FIGS 8A to 8D are time charts for describing the operation of the demodulator of FIG. 7; FIG. 9 is a block diagram illustrating an FM demodulator according to still another preferred embodiment of the invention; FIG. 10 is a block diagram illustrating an FM demodulator according to yet another preferred embodiment of the invention; FIGS. 11A to 11D are time charts for describing the operation of the demodulator of FIG. 10; FIG. 12 is a schematic circuit diagram of the principal part of the second pulse generator circuit of the FM demodulator of FIG. 10; and FIG. 13 is a graph illustrating simulated F/V characteristics of the FM demodulator of FIG. 10, obtained by the use of the simulation program of SPICE-F. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS To facilitate understanding of the present invention, a pulse count type FM demodulator of the prior art will be described first with reference to FIGS. 1, 2A to 2D and 3. Referring to FIG. 1, the demodulator comprises a limiter 61, a monostable multivibrator 62 and a low-pass filter (LPF) 63. The limiter 61 amplitude-limits an input signal 2a to provide a rectangular signal 2b as shown in FIG. 2B. The monostable multivibrator 62 generates a pulse 2c (FIG. 2C) rising from a transition point of the rectangular signal 2b and having a fixed time duration (τ c ), and supplies it to the LPF 63, which provides a demodulated output 2d (FIG. 2D) by integrating the pulse 2c. Thus, the demodulated output of the demodulator of FIG. 1 is calculated by Equation (1) below: where Vo is the demodulated output; T, the period of the input signal (= 1/f in ); f in , the frequency of the input signal; Vp(t), the output pulse train signal = E, the amplitude of the output pulse. Integrating Equation (1) gives Equation (2) below: Vo = Eτ c f in (2) Equation (2) indicates that the demodulated output Vo is proportional to the input frequency f in , and its demodulation (F/V) characteristic is shown in FIG. 3. As may be apparent from the characteristic shown in FIG. 3, the lower limit of the demodulation band is zero Hz (D.C.), and linearity is maintained over a wide band ranging from zero to the upper limit determined by the pulse width τ c . This demodulation characteristic, however, has the disadvantages of low demodulation sensitivity and, where the normalized bandwidth of input FM signals is narrow, of vulnerability to external noise. FIG. 4 illustrates a pulse count type FM demodulator, which is a preferred embodiment of the present invention. In FIG. 4, an input terminal 10 is supplied with a rectangular signal 5a (FIG. 5A), similar to the rectangular output from the limiter 61 shown in FIG. 1. A first pulse generator circuit 11, like the monostable multivibrator 62 of FIG. 1, generates a pulse 5b (FIG. 5B) having a fixed pulse width commencing at the leading edge of the rectangular signal 5a. The pulse interval Δt of the pulse 5b of FIG. 5B is a function of the input frequency f in . Thus holds the relationship of Equation (3) below: Δt = T - τ c (3) A second pulse generator circuit 12, receiving the pulse 5b, narrows the pulse width τ c of the pulse 5b only for a period of time proportional to its interval Δt (the proportional constant being a , which is a positive real number), and thereby generates a pulse 5c having a pulse width of τ c - a Δt as shown in FIG. 5C. An LPF 13 integrates the pulse train 5c to provide a demodulated output, which is represented by Equation (4) below, derived from Equation (1) above: Vo = E {( a + 1) τ c f in - a } (4) The characteristic of Equation (4), as shown in FIG. 6, indicates a narrower demodulation band on the low frequency side and, correspondingly, an ( a + 1) times higher demodulation sensitivity than the F/V characteristic of the prior art illustrated in FIG. 3. Accordingly, even where the normalized bandwidth of an FM signal is narrow, the demodulator is hardly vulnerable to external noise. FIG. 7 illustrates a pulse count type FM demodulator, which is another preferred embodiment of the present invention. In FIG. 7, a monostable multivibrator 21, serving as a first pulse generator circuit, generates pulses 8b (FIG. 8B) having a pulse width τ c commencing at the leading and trailing edges of an input rectangular wave signal 8a to increase the demodulation sensitivity, and is so adjusted as to keep the Δt smaller than τ c . The output pulse 8b of the monostable multivibrator 21 is divided into two branches, of which one is directly fed to one of the inputs of an AND gate 28 and the other, to the other input of the AND gate 28 through a delay circuit 29. The delay circuit 29 is comprised of a first integration circuit comprising a resistor 22 and a capacitor 23, a first inverter 24 to receive the integrated output, a second integration circuit which comprises a resistor 25 and a capacitor 26 and receives the output of the first inverter 24, and a second inverter 27 to receive the output of this second integration circuit. The delay time τ₁ of the delay circuit 29 is set as represented by Inequality (5) below: Δt max < τ₁ < τ c (5) where Δt max is the pulse interval of the pulse 8b when the input frequency is at its minimum. The output 8d of the AND gate 28, as shown in FIG. 8D, has a pulse width smaller by Δt than the output pulse 8b of the monostable multivibrator 21. Integrating the pulse 8d with the LPF 13 gives the F/V characteristic of a = 1, in Fig. 6. FIG. 9 illustrates an FM demodulator, which is still another preferred embodiment of the present invention. In FIG. 9 the output of a monostable multivibrator 21 is the same as that of the corresponding one in the embodiment of FIG. 7. Delay circuits 31 to 34 are connected in tandem, and the respective outputs of these delay circuits and of the monostable multivibrator 21 are applied to an AND gate 35. For the optimal design, it is recommended to set the values of the delay time τ₁ of each of the delay circuit 31 to 34 and the number n of the delay circuits as represented by the following Equation-Inequality pair (6): The demodulation sensitivity of the demodulator of FIG. 9, set as described above, is ( n + 1) times that of the conventional demodulator of FIG. 1. FIG. 10 illustrates an FM demodulator, which is yet another preferred embodiment of the present invention. In FIG. 10, the output of a monostable multivibrator 21 is the same as that of the corresponding one in the embodiment of FIG. 7. A switching circuit 41 is intended to turn on and off constant current sources 42 and 43. When an input signal 11b is at its "high" level, the switching circuit 41 keeps the constant current source 42 on while holding the constant current source 43 off and open, and vice versa when the input signal 11b is at its "low" level. Therefore, when the input 11b to the switching circuit 41 is "high", the constant current source 42 charges a capacitor 44 by a high-potential power source 48. Conversely, when the input 11b to the switching circuit 41 is "low", the constant current source 43 discharges the capacitor 44 to a low-potential power source 49 (a ground potential in this particular instance). The higher potential between the two electrodes of capacitor 44 is clamped at a fixed potential by a clamp circuit 45. The relationship of Equation (7) below is to be maintained between the output current I₁ of the constant current source 42 and the output current I₂ of the constant current source 43. I₂ = n I₁ (7) Since the capacitor 44 is charged and discharged by the constant currents I₁ and I₂, respectively, the inclinations of its charge and discharge waveforms, shown in FIG. 11C, are constant as represented by Equation (8) below: where c is the capacity of the capacitor 44. If the upper limit of these charge and discharge waveforms is clamped at the clamp potential Vc of the clamp circuit 45, the potential variation δV (FIG. 11C) during the period of discharge by the current I₂ will be represented by Equation (9) below: where, as is evident from FIG. 11C, δt₂ equals Δt. Therefore, δV is given by Equation (10) below: Then, the time δt₁ required for returning the potential variation δV to V c during the period of charge by the current I₁ is calculated as follows: From Equation (11), According to Equation (10), Equation (12) can be developed as follows: According to Equation (7), Equation (13) can be converted into Equation (14) below: δt₁ = n Δt (14) Therefore, by waveform-shaping the charge and discharge waveforms shown in FIG. 11C with a reference voltage obtained from a voltage source 46, which is slightly lower than the clamp voltage V c , there is provided a pulse (FIG. 11D) having a width of τ c - n Δt. Thus, by varying the ratio n between the constant currents I₁ and I₂, the demodulation sensitivity can be changed. Integrating the pulse shown in FIG. 11D with the LPF 13 will provide the desired demodulation output. FIG. 12 is a more specific circuit diagram of an integration circuit 100 of Fig. 10 which comprises the switching circuit 41, the constant current sources 42 and 43, and the clamp circuit 45. An input signal 11b is supplied to the base of a first transistor 51 whose emitter is connected to the emitter of a second transistor 52 and to a constant current circuit 53. The collector of the first transistor 51 is connected to the cathode of a first diode 54. The base of the second transistor 52 is grounded through a bias constant voltage source 56, and the collector of same is connected to the cathode of a second diode 55. The anodes of both the first and second diodes 54 and 55 are connected to a power source. Further, the collectors of the first and second transistors 51 and 52 are connected to the bases of third and fourth transistors 57 and 58, respectively, whose emitters are both connected to the power source and collectors are connected to those of fifth and sixth transistors 59 and 510, respectively. The firth transistor 59, with its collector and base being short-circuited, is used as a diode, and to their connecting point is further connected the base of the sixth transistor 510, the emitters of both the fifth and sixth transistors being grounded. Being so connected, the fifth and sixth transistors 59 and 510 serve as a current mirror circuit. Further, the emitter area of the sixth transistor 510 is made n times as great as that of the fifth transistor 59. The collector of the sixth transistor 510 serving as the output point is connected to a first terminal of the capacitor 44 whose second terminal is grounded. In parallel to the capacitor 44 is connected a series circuit of a third diode 511 and a reference voltage source 512. The above described structure enables the capacitor 44 to be charged and discharged by switching the first and second transistors 51 and 52 with the input signal 11b and the ratio between the charging and discharging currents to be determined by that between the emitter areas of the fifth and sixth transistors 59 and 510. The third diode 511 and the reference voltage source 512 constitute the clamp circuit 45, whose clamp value is represented by V ref + V r , where V r represents the on-voltage of the diode 511 and V ref , the voltage of the reference voltage source 512. FIG. 13 shows the F/V characteristics, simulated by the simulation program of SPICE-F, of an FM demodulator circuit composed of the bipolar transistor circuit illustrated in FIGS. 10 and 12. The abscissa represents the frequency, and the ordinate, the D.C. component of the demodulated output. The characteristics were simulated with an intermediate frequency of 455 kHz, with temperature variations from -20° to +70° taken into consideration. As is evident from these simulated characteristics, a demodulation performance sufficiently close to linearity can be achieved in the temperature range of -20° to +70° and in the frequency (f) range of 390 kHz to 490 kHz. Incidentally, the ratio between the constant currents I₁ and I₂ is set at 1.75. As hitherto described, an FM demodulator according to the present invention makes it possible to increase the demodulation sensitivity. This is achieved by adding a circuit which subjects the pulse with τ c of a monostable multivibrator output to pulse width modulation by the pulse interval Δt to alter the pulse width to τ c - a Δt. An FM demodulator according to the invention is especially suitable for monolithic integration of a demodulator circuit for use with input signals whose normalized bandwidth is comparatively narrow.

Claims: A frequency modulation (FM) demodulator comprising: a) limiter means (61) for converting an input signal into a rectangular signal; b) first pulse generator means (11) responsive to the transition points of said rectangular signal for generating a first pulse train having a first and a second states for the period of said rectangular signal, said first state commencing at the leading edge of the rectangular signal and having a predetermined period of time; and c) low-pass filter means (13) for integrating a second pulse train to provide a demodulated signal, characterized by, d) second pulse generator means (12) for generating the second pulse train having a third and fourth states for the period of said rectangular signal, the duration of said third state corresponding to the duration of the first state being reduced by a time length based on the duration of said second state. An FM demodulator as claimed in Claim 1, wherein said first pulse generator means comprises a monostable multivibrator. An FM demodulator as claimed in Claim 1 or 2, wherein said second pulse generator means comprises a delay circuit for giving a prescribed delay to said first pulse train; and an AND gate for ANDing said first pulse train and the output of said delay circuit to provide an output as the output of said second pulse generator means. An FM demodulator as claimed in Claim 3, wherein said delay circuit comprises a pair of series circuits of an integration circuit and an inverter, each of said integration circuits comprising a resistor and a capacitor, and each of said inverters inverting the polarity of the each output of said integration circuits. An FM demodulator as claimed in Claim 1 or 2, wherein said second pulse generator means comprises a plurality of delay circuits for successively giving prescribed delays to said first pulse train and an AND gate for ANDing said first pulse train and the respective outputs of said plurality of delay circuits to provide an output as the output of said second pulse generator means. An FM demodulator as claimed in Claim 5, wherein the delays by said plurality of delay circuits are equal to each other. An FM demodulator as claimed in any of Claims 1 to 6, wherein said second pulse generator means comprises an integration circuit for receiving said first pulse train and integrating it by different time constants at the leading and trailing edges of said first pulse train, respectively; and a comparator for converting the output of said integration circuit into a rectangular signal. An FM demodulator as claimed in Claim 7, wherein said integration circuit comprises first and second constant current sources for supplying first and second currents, respectively; a switching circuit responsive to the rise of said first pulse train for turning said first constant current source on and said second constant current source off, and responsive to the fall of said first pulse train for turning said first constant current source off and said second constant current source on; a capacitor coupled to said first and second constant current sources, charged by said first current from said first constant current source, and discharged by said second current from said second constant current source; and a clamp circuit for clamping a voltage between the terminals of said capacitor at a prescribed level. An FM demodulator as claimed in Claim 8, wherein a ratio between said first and second currents in n , n being a positive real number. An FM demodulator as claimed in Claim 9, wherein said switching circuit comprises a differential amplifier whose input terminals receive said first pulse train and which is comprised of first and second transistors, and wherein said first and second constant current sources comprises third and fourth transistors whose bases are connected to the respective collectors of said first and second transistors; and a current mirror including fifth and sixth transistors which are respectively coupled to said third and fourth transistors and whose emitter areas are in the ratio of n, the collector of said sixth transistor being coupled to said capacitor. An FM demodulator as claimed in Claim 8 or 9, wherein said clamp circuit comprises a diode. A method of demodulating an FM signal comprising the steps of: a) responsive to the transition points of said FM signal, generating a first pulse train having first and second states for one period of said FM signal, said first state commencing at the leading edge of the FM signal and having a predetermined period of time; b) converting a second pulse train into a D.C. voltage, characterized by c) comprising the step of generating the second pulse train having third and fourth states, the duration of said third state corresponding to the duration of the first state reduced by a time length based on the duration of said second state. A method as claimed in Claim 12, further comprising the step of amplitude limiting said FM signal. A method as claimed in Claim 13, wherein said step of generating said first pulse train comprises the step of generating, as said first pulse train, a pulse that rises at the leading and trailing edges of the amplitude-limited FM signal, and wherein said step of generating said second pulse train comprises the steps of; delaying said first pulse train by a predetermined time length; and generating said second pulse train by taking the logical product of said first pulse train and the delayed first pulse train, and wherein said step of converting into said D.C. voltage comprises the step of low-pass filtering said second pulse train. A method as claimed in Claim 13, wherein said step of generating said first pulse train comprises the step of generating, as said first pulse train, a pulse that rises at the leading and trailing edges of the amplitude-limited FM signal, and wherein said step of generating said second pulse train comprises the step of; successively delaying said first pulse train plural times; and generating said second pulse train by taking the logical products of said first pulse train and the successively delayed plurality of pulse trains, and wherein said step of converting into said D.C. voltage comprises the step of low-pass filtering said second pulse train. A method as claimed in Claim 13, wherein said step of generating said first pulse train comprises the step of generating, as said first pulse train, a pulse that rises at the leading and trailing edges of the amplitude-limited FM signal, and wherein said step of generating said second pulse train comprises the steps of: supplying a capacitor with first and second constant currents in response to the rise and fall, respectively, of said first pulse train; clamping the upper limit of the voltage charged at said capacitor at a prescribed level; and comparing a voltage between the two terminals of said capacity and a reference voltage, which is slightly lower than said prescribed level, to generate said second pulse train, and wherein said step of converting into said D.C. voltage comprises the step of low-pass filtering said second pulse train.

Inventors: ISHII, EIICHI, YOSHIZAWA, SHIGEO
Applicants: NEC CORPORATION
Application year: 1987
Main classification: H03D3
IPC codes: H03K17, H03K9, H03D3