GB2226200A - FM discriminator - Google Patents

Abstract

In an FM discriminator, a switching circuit 6-24 is responsive to the leading or trailing edge of an amplitude limited FM signal to produce a rectangular pulse waveform V2 and a voltage V1 developed by a capacitance 26 charging from a current source 16 and a differential amplifier 32 changes state at instants determined by the relative polarity changes between the rectangular waveform and the capacitance voltage to produce pulses of fixed width. These pulses have variable spacing and are integrated at 34 to recover the original modulation. Capacitance 26 is alternately charged by source 16 via transistor 18 and discharged via transistor 24 to form voltage V1 and the pulse waveform V2 is produced by the current flowing into resister 28 from source 16 or 30. A large part of the discriminator can be formed as an integrated circuit and the current source may be arranged to compensate for temperature induced changes in the capacitance value. The discriminator may be part of an FM receiver incorporated in a wrist watch. <IMAGE>

Description

PULSE COUNTING DETECTOR FM DISCRIMINATOR The present invention relates to an FM discriminator which uses a pulse count detecting technique and is suitable for high-level integration.

It is proposed to incorporate FM receivers for receiving FM audio broadcasts in devices as small as wrist-watches. For example, our pending application < > (F20607) discloses an architecture for an FM receiver suitable for high level integration for incorporating in a wrist-watch. One of the requirements for high level integration is that of simplicity of construction to enable a low component count.

It is an object of the present invention to produce an FM discriminator suitable for high level integration.

The present invention operates on the known principle of generating fixed width pulses from the amplitude limited version of the FM signal and then integrating the pulses in order to derive an envelope which represents the FM modulated signal. Such a discriminator is described for example on pp 155, 156 of "Principles of Communications Systems", Taub & Schilling, McGraw-Hill, 1971.

The present invention provides an FM discriminator comprising input switch means for receiving an amplitude limited version of an FM modulated signal, current source means coupled to the switch means for providing electrical current to a capacitance, means coupled to the input switch means for providing a rectangular pulse waveform, and a differential amplifier having its respective inputs coupled to respond to the rectangular pulse waveform and a voltage developed by the capacitance, the arrangement being such that in operation in response to leading or trailing edges of the FM signal, the input switch means is switched to initiate generation of said rectangular pulse waveform and current flow in said capacitance, whereby the voltage level of the rectangular pulse waveform changes in polarity relatively to the said voltage of the capacitance to cause a change of state of the differential amplifier, and at a subsequent time instant determined by the time constant of the capacitance, said voltage of the capacitance changes in polarity relative to the voltage of the rectangular pulse waveform to restore the differential amplifier to its initial state, so that the output of the differential amplifier provides a pulse of a fixed width.

Thus in accordance with the invention, the differential amplifier changes its state for a predetermined time period in response to the leading or trailing edges of the input amplitude limited FM modulated signal, thereby to provide a pulse of a fixed width within each cycle of the FM signal, the width being determined by the time constant of the capacitance. The advantage of such a circuit is simplicity and suitability for high level integration. Further the circuit is accurate in operation in that it responds directly to an edge transition of an input signal, and changes back to its original condition in response to the next edge transition of the input signal.

This is an inherently more accurate mode of operation than for example employing a monostable multivibrator to generate a fixed width pulse, since this would require an AC coupling to the trigger input and would be wholly reliant for its operation on detecting a single edge transition in each cycle of the input FM signal.

Although it is normally difficult to determine the accuracy of an on-chip capacitance to better than i 15%, this is not important since the absolute value of the capacitance and thus the fixed pulse width is unimportant. It is however possible to construct the current source means with a temperature coefficient which compensates for temperature induced changes in capacitance value, thereby ensuring the pulse width remains constant in operation. The discriminator may thus track other circuits employing capacitances incorporated on the same chip, for example gyrator based circuits in oscillators or filters.

In one embodiment, the input switch means comprises a first longtailed pair controlling charging of said capacitance and a second longtailed pair controlling current flow through a resistance for generation of the rectangular pulse waveform. In response to the leading edge (positive-going) of an input signal, the capacitance is changed to a value somewhat less than the voltage on the resistance.

In response to the trailing edge of the input signal, the voltage across the resistance immediately drops to a value less than that on the capacitance to cause a change in state of the differential amplifier.

The capacitance voltage subsequently drops to a value less than that across the resistance thereby to restore the state of the differential amplifier and generate the fixed width pulse.

In another embodiment, the input switch means comprises a transistor having its emitter coupled to further first and second switch transistors, the first transistor controlling current flow in a capacitance from a first current source, and the second transistor controlling the generation of a rectangular pulse waveform, the waveforms on the collectors of the first and second transistors being applied to the inputs of a differential amplifier.

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a circuit diagram of a first embodiment of the invention; Figures 2 and 3 are waveform diagrams for the circuit of figure 1; Figure 4 is a circuit diagram of a second embodiment of the invention; and Figure 5 is a waveform diagram for the circuit of figure 4.

Referring to figure 1 of the drawings, the first embodiment comprises differential input terminals 2, 4 for receiving from a limiter amplifier the amplitude limited version of an FM signal.

Input terminal 2 is coupled to transistors 6, 8 of first and second long-tailed pairs 10, 12 which include current sources 14, 16 in the respective tails of the pairs. Input terminal 4 is coupled to transistors 18, 20 of the long-tailed pairs. The collectors of transistors 6, 8 are connected in common to a diode 22 and the base of a transistor 24. The collector of transistor 24 is connected in common with the collector of transistor 18 to a charging capacitor 26 The collector of transistor 20 is connected to a charging resistor 28.

A current source 30 is connected to resistor 28 and the inverting input of a differential amplifier 32 is connected to resistor 28. The non-inverting input of amplifier 32 is coupled to capacitor 26. The output of the differential amplifier 32 is coupled to a low pass filter 34.

Figures 2 and 3 indicate the manner of operation of figure 1.

The input applied to terminal 4 is indicated in figure 2(a) as a rectangular pulse waveform. On a positive going edge of the waveform, transistors 18, 20 are switched on and transistors 6, 8 are switched off. Switching on of transistor 18 causes charging of capacitor 26 by source 16 and also establishes a voltage across resistor 28 by virtue of current flow through transistor 20 from current source 14. As shown in Figure 2(b), capacitor 26 charges to a value less than the voltage established across resistor 28. The circuit remains in this condition until the negative going edge of the input waveform causes transistors 18, 20 to be switched off and transistors 6, 8 to be switched on. The switching off of transistor 20 stops current flow from source 16 through resistor 28, but causes current flow through resistor 28 from current source 30. The resultant waveform is shown in figure 2(b) with the voltage across resistor 28 immediately changing to a less positive value.

Upon the switching off of transistor 18, capacitor 26 discharges through transistor 24 until it reaches a value which is more negative than the voltage now established on resistor 28. This is shown in figure 2(b) and it may be seen the voltage on capacitor 26 "crosses over" the voltage on resistor 28 after time interval t from the reception of the negative-going edge of the input signal. The effect of this on differential amplifier 32 is indicated in figure 2(c) where in time interval t in which a positive going edge is received by the circuit, the voltage on resistor 28 is greater than that across capacitor 26, and differential amplifier 32 remains switched off.At time instant q, the voltage on resistor 28 drops below that on capacitor 28 and hence the non-inverting input of amplifier 32 carries a greater voltage than that on the inverting input; hence amplifier 32 is switched on for a period t until the voltage on capacitor 28 again becomes less than that on resistor 28 at time instant r whereupon the differential amplifier 32 is again switched off. The effect of this is thus to generate a series of pulses of width t which are applied to a low pass filter 34.

Hence as shown in figure 3 if the IF output has a frequency of f dF as in conventional FM, then the fixed width pulses are generated as shown in figure 3(b) having a variable spacing. The integrated or filtered value is shown in figure 3(c) which represents the modulating signal.

It will be understood that in certain applications, low pass filter 34 may have an effect on the filtering of the IF signal introduced to terminals 2, 4 in that it rejects unwanted signals resulting from image frequencies or adjacent channels having frequencies above that of the filter cut off.

Referring to the second embodiment shown in figure 4, in the simplified version as shown in figure 5 an FM input signal from an amplitude limiter 48 is applied to the base of an input switching transistor 50. This transistor is coupled at its emitter to a resistor 52 and a current source 54. The resistor 52 is coupled to the base of a first switching transistor 56 and, via a resistor 58 and current source 60 to the base of a second switching transistor 62. The emitter of transistor 62 is coupled to a current source 64 and the emitter of transistor 56 is coupled to a current source 66 and a capacitor 68.

Output signals are provided on lines 70, 72 from the emitters of switching transistors 56, 62 to a differential amplifier (not shown).

In operation, an input rectangular waveform is applied to the base of transistor 50 which causes switching of transistors 56, 62 in synchronisation with the input waveform. The switching of transistor 62 generates a rectangular waveform on line 70 as indicated at B in figure 5. The switching of transistor 56 generates a waveform on line 72 as indicated at A in figure 5, with a sloping leading edge caused by the charging of capacitor 68. The voltage values of these two waveforms A, B are selected such that in response to the positive going edge of the input signal at transistor, the waveform on line 70 likewise provides a positive going edge and the capacitor 68 begins to charge to develop a more positive value.

Thus as shown in figure 5 the voltage on line 70 immediately changes in value to a value more positive than that on line 72.

Subsequently as capacitor 68 charges, the voltage at line 72 again becomes more positive than that on line 70 after a time interval t at a cross-over point x. During this time interval t, a differential amplifier coupled to lines 70, 72 provides a pulse of fixed width.

The arrangement is shown in more detail in figure 4, wherein like parts are indicated by the same reference numerals. It may be seen the signals on lines 70, 72 are applied to the inverting and noninverting inputs of an amplifier 74.

Claims (6)

1. An FM discriminator comprising input switch means for receiving an amplitude limited version of an FM modulated signal, current source means coupled to the switch means for providing electrical current to a capacitance, means coupled to the input switch means for providing a rectangular pulse waveform, and a differential amplifier having its respective inputs coupled to respond to the rectangular pulse waveform and a voltage developed by the capacitance, the arrangement being such that in operation in response to the leading or trailing edge of the FM signal, the input switch means is switched to initiate generation of said rectangular pulse waveform and current flow in said capacitance, whereby the voltage level of the rectangular pulse waveform changes in polarity relatively to the said voltage of the capacitance to cause a change of state of the differential amplifier, and at a subsequent time instant determined by the time constant of the capacitance, said voltage of the capacitance changes in polarity relative to the voltage of the rectangular pulse waveform to restore the differential amplifier to its initial state, so that the output of the differential amplifier provides a pulse of a fixed width.

2. An FM discriminator according to claim 1 wherein the input switch means comprises a differential amplifier means comprising first and second longtailed pairs, with said capacitance coupled to a collector circuit of the first longtailed pair and the rectangular pulse wavegorm means coupled to a collector circuit of the second longtailed pair.

3. An FM discriminator according to claim 2 wherein the rectangular pulse waveform means comprises a resistance coupled to first and second current sources which are switched by the second differential pair to establish different voltage levels across the resistance.

4. An FM discriminator according to claim 1 wherein the input switch means comprises a first transistor having its output circuit controlling second and third transistors, the output circuit of the second transistor controlling the changing of said capacitance, and the output circuit of the third transistor controlling a current source to establish a voltage level on an output line to provide said rectangular pulse waveform.

5. An FM discriminator as claimed in claim 1 wherein the differential amplifier is coupled to filter means to integrate said pulses of fixed width.

6. FM discriminators substantially as described with reference to the accompanying drawings.