GB1566545A - Modulators - Google Patents

Description

(54) MODULATORS (71) We, COMMUNICATIONS PATENTS LIMITED, a British Company, of Carlton House, Lower Regent Street, London, SWlY 4LS do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment:--- The present invention relates to modulators and in particular to modulators for use in amplitude modulated transmitters.

In amplitude modulated transmitters long established practice for generating the modulated wave has been to impress the audio waveform on the HT supply to the final, P.A. output stage. In the case of transistor transmitters this practice has not proved satisfactory because even with the collector-emitter voltage at zero considerable "leak through" of the applied carrier wave occurs so that the troughs of the modulated output are distorted relative to the peaks.

It has become common practice in amplitude modulated transistor transmitters to modulate both the P.A. and P.A. driver stages and in some cases one or more of the pre-driver stages as well in order to obtain satisfactory linearity of the modulated output. The P.A. stage has usually had full modulation applied to it while the driver stage has had only a portion applied to it, most commonly through networks of resistors and diodes or sometimes from a tap on the modulation transformer. These fixed components have precluded any adjustment of the modulation linearity in individual transmitters and in consequence the quality of transmitters has varied from one to another in dependence upon the spread of transistor characteristics to the extent that matched sets of transisors for the various stages have even been used in some designs.

It is an object of the present invention to obviate or mitigate the above problems.

According to the present invention, there is provided an AM transmitter comprising an output stage including at least one pair of semi-conductor switches connected in series across a D.C. source, a driver stage in respect of each switch in the said pair, a main modulator for modulating the D.C. source, and an auxiliary modulator for modulating power supplied to the driver stages, the auxiliary modulator comprising a modulating transistor a control terminal of which receives signals from the main modulator, said modulating transistor being supplied from two independently controllable sources of potential which may be controlled to determine the peak and trough auxiliary modulation points.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic block diagram of an AM transmitter incorporating the present invention; Figs. 2 and 3 illustrate waveforms appearing in the transmitter according to claim 1; Fig. 4 is a schematic block diagram of a modulator according to the present in invention; and Figs. 5 and 6 illustrate details of the modulator of Fig. 4.

Fig. 1 is a schematic diagram of an RF transmitter comprising an audio modulator 1 providing a pulse width modulated audio input 2 to a main HT modulator 3. An auxiliary modulator 4 is also provided. The main modulator 3 provides an HT output modulated by the pulse width modulated input 2 to transistor switches 5, 6 via a current detecting circuit 7 the purpose of which is described below. The switch 5 is connected in series with a further switch 8, and the switch 6 is connected in series with a further switch 9. The switches 5, 6, 8 and 9 thus form a power amplifier of bridge form.

If switches 5 and 9 are turned on and switches 6 and 8 are turned off, current is drawn from the output of modulator 3 to earth via switch 5, a load 10, a further current detecting circuit 11 and switch 9.

If switches 6 and 8 are turned on and switches 5 and 9 are turned off, current is drawn from the output of modulator 3 to earth via switch 6, circuit 11, load 10 and switch 8, ie. the direction of current flow is reversed. As the output of modulator 3 operates essentially as a AM voltage source the amplitude of which is rep resentative of the audio input, the output load (ie. an aerial) transmits an AM modulated signal at a frequency controlled by the switching frequency of the switches 5, 6, 8 and 9.

The switches 5, 6, 8 and 9 are operated at an r.f. frequency determined by a switching signal generator which provides an r.f. square wave output A as shown in Fig. 2A. The output A is doubled in frequency by unit 13 to provide output B (Fig. 2B) which is applied to a pulse generator in the form of a monostable multivibrator 14. The multivibrator 14 provides an output C (Fig. 2C) to a logic circuit 15 which also receives input A (Fig. 2A) from source 12, the mark space ratio of the pulses of output C being dependent upon input 16 to the multivibrator 14 as will be described hereinafter. The signal A (Fig. 2A) applied to the logic circuit 15 acts as a synchronising signal to ensure that, in a transmitter which includes many modules of the type illustrated operating in parallel, all of the modules operate in phase.

The logic circuit 15 provides outputs D (Fig. 2D) and E (Fig. 2E) to respective driver stages 17 and 18 for the switches 5, 6, 8 and 9, each of the output D and E comprising portions at logic "0" which correspond to periods during which it is desired to turn on the switches controlled by the respective drivers 17 and 18. The drivers 17, 18 receive an auxiliary modulation input from modulator 4. The drivers 17, 18 amplify the logic level pulses at the output of logic circuit 15 to a level sufficient to drive the switches 5, 6, 8 and 9.

It will be seen that there is a delay period of length tm between the turning off of one set of switches and the turning on of the other set. This is to compensate for the combined effects of the delay time between an "on" command to a switch and the initiation of main current through the transistor and the storage time between an "off" command and the interruption of the main current.

The length of the delay period tm is controlled,.by the input 16 to the multivibrator 14. The input 16 is in turn controlled by a simultaneous conduction detection circuit 19 which receives inputs from detection circuits 7 and 11.

The detection circuit 7 provides an input waveform 20 to circuit 19 which is representative of the combined collector currents of the switches 5, 6, 8 and 9.

Ideally this is a cusp waveform (Fig. 3A), but in the event of simultaneous conduction of switches 5 and 8 or 6 and 9 the cusp waveform is modified by the addition of narrow spikes at the switching transitions (Fig. 3B).

The detection circuit 11 provides an input waveform 21 to the circuit 19 which is representative of the load current. The waveform 21 is full wave rectified to provide a true cusp waveform which is not influenced by simultaneous conduction and does not contain any spikes as these do not appear in the load current. The spikes cannot appear in the load current in any event as the load is resonant.

The circuit 19 subtracts its rectified input 21 from its input 20. The result of this subtraction is zero if there is no simultaneous conduction in the switches and in such circumstances the input 16 to the multivibrator 14 is held at a reference level determined by a voltage source 22. If there is simultaneous conduction however the result of the subtraction is not zero and the input 16 is modified accordingly to increase the width of the pulses (Fig. 2C) provided by the multivibrator. Thus the delay tm between the application of "on" command pulses to the switches is increased to automatically adjust the condition of the circuitry to stop the simultaneous conduction.

The voltage source 22 also limits the maximum pulse width of the multivibrator output to prevent the pulse width exceeding one half cycle of the carrier frequency determined by signal A (Fig. 2A).

The modulation of the output stages of the transmitter will now be described with reference to Figs. 4, 5 and 6. Fig. 4 is a simpilfied diagram of circuits shown in detail in Figs. 5 and 6.

Referring to Fig. 4, the main modulator corresponding to modulator 3 of Fig. 1 comprises a transistor 23 the base of which is driven by a pulse width modulated audio signal via terminal 24. A flywheel diode 25, inductor 26 and capacitor 27 are connected in known manner between the modulation transistor 23 and the positive supply rail 28. Output terminals 29 and 30 are connected to terminals of the output power amplifier (Fig. 1).

Square wave pulses appearing at the collector of transistor 23 are applied via a base drive circuit 31 to an auxiliary modulation transistor 32. A flywheel diode 33, inductor 34 and capacitor 35 are again provided but the auxiliary modulation transistor is supplied via terminals 36, 37 with two d.c. reference potentials which can be adjusted when the transmitter is set up. The auxiliary modulator output is passed via adjustable inductor 38 to output terminals 39, 40 which are connected to the supply and earth rails respectively of the PA drivers 17, 18 (Fig. 1).

The potential applied to terminal 37 determines the minimum excursion of the auxiliary modulator output, that is the trough modulation level. The potential applied to terminal 36 determines the peak modulation level. Thus by adjustment of these two potentials the modulation linearity of the transmitter can be adjusted.

The inductor 38 of Fig. 4 forms an element of a low pass filter including capacitive elements (not shown) incorporated in the drivers 17, 18 (Fig. 1).

Variations in the characteristics of the output switches 5, 6, 8, 9 (Fig. 1) causes some variation in the d.c. input impedance of the driver supply rail which is supplied by the auxiliary modulator. These impedance variations are however made insignificant by a shunt load resistor 41 (Fig. 4) which maintains the termination impedance for the low pass filter.

Referring now to Fig. 5, details of the main modulator shown in the lower half of Fig. 4 are shown. The input to terminal 24 is provided via an integrated circuit 42 from a balanced pulse width modulated audio input line 43. The remaining circuitry of Fig. 5 operates to amplify the pulse width modulated signal to a level such that the output of transistor 23 is appropriate to high level modulation. A signal is passed to the auxiliary modulator via terminal 44.

Referring to Fig. 6, details of the auxiliary modulator are shown.

The circuit is basically similar to that of the main modulator of Fig. 9 except that a positive earthed supply system is employed instead of a negative earthed system. This arrangement is more convenient with respect to the coupling of drive signals. In addition the modulator transistor employed is a PNP type to match the supply polarity. It should also be noted that the input signal applied at terminal 102 is AC coupled and then DC restored by means of the rectifier connected between the amplifier side of a capacitor connected to terminal 102 and the supply terminal 93. This form of connection prevents the occurrence of damage to the RF driver stage and auxiliary modulator as a result of failures occurring in the main modulator such, for example, as a collector to emitter short in the transistor 80.

WHAT WE CLAIM IS: 1. An amplitude modulated transmitter comprising an output stage including at least one pair of semi-conductor switches connected in series across an D.C. source, a driver stage in respect of each switch in the said pair, a main modulator for modulating the D.C. source, and an auxiliary modulator for modulating power supplied to the driver stages, the auxiliary modulator comprising a modulating transistor a control terminal of which receives signals from the main modulator, said modulating transistor being supplied from two independently controllable sources of potential which may be controlled to determine the peak and trough auxiliary modulation points.

2. An amplitude modulated transmitter according to claim 1, wherein the modulating transistor is conneceted in series with a flywheel diode between the two independently controllable sources of potential.

3. An amplitude modulated transmitter comprising main and auxiliary modulators substantially as hereinbefore described with reference to the accompanyina drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   30 are connected to terminals of the output power amplifier (Fig. 1).

   Square wave pulses appearing at the collector of transistor 23 are applied via a base drive circuit 31 to an auxiliary modulation transistor 32. A flywheel diode 33, inductor 34 and capacitor 35 are again provided but the auxiliary modulation transistor is supplied via terminals 36, 37 with two d.c. reference potentials which can be adjusted when the transmitter is set up. The auxiliary modulator output is passed via adjustable inductor 38 to output terminals 39, 40 which are connected to the supply and earth rails respectively of the PA drivers 17, 18 (Fig. 1).

   The potential applied to terminal 37 determines the minimum excursion of the auxiliary modulator output, that is the trough modulation level. The potential applied to terminal 36 determines the peak modulation level. Thus by adjustment of these two potentials the modulation linearity of the transmitter can be adjusted.

   The inductor 38 of Fig. 4 forms an element of a low pass filter including capacitive elements (not shown) incorporated in the drivers 17, 18 (Fig. 1).

   Variations in the characteristics of the output switches 5, 6, 8, 9 (Fig. 1) causes some variation in the d.c. input impedance of the driver supply rail which is supplied by the auxiliary modulator. These impedance variations are however made insignificant by a shunt load resistor 41 (Fig. 4) which maintains the termination impedance for the low pass filter.

   Referring now to Fig. 5, details of the main modulator shown in the lower half of Fig. 4 are shown. The input to terminal 24 is provided via an integrated circuit 42 from a balanced pulse width modulated audio input line 43. The remaining circuitry of Fig. 5 operates to amplify the pulse width modulated signal to a level such that the output of transistor 23 is appropriate to high level modulation. A signal is passed to the auxiliary modulator via terminal 44.

   Referring to Fig. 6, details of the auxiliary modulator are shown.

   The circuit is basically similar to that of the main modulator of Fig. 9 except that a positive earthed supply system is employed instead of a negative earthed system. This arrangement is more convenient with respect to the coupling of drive signals. In addition the modulator transistor employed is a PNP type to match the supply polarity. It should also be noted that the input signal applied at terminal 102 is AC coupled and then DC restored by means of the rectifier connected between the amplifier side of a capacitor connected to terminal 102 and the supply terminal 93. This form of connection prevents the occurrence of damage to the RF driver stage and auxiliary modulator as a result of failures occurring in the main modulator such, for example, as a collector to emitter short in the transistor 80.

   WHAT WE CLAIM IS: 1. An amplitude modulated transmitter comprising an output stage including at least one pair of semi-conductor switches connected in series across an D.C. source, a driver stage in respect of each switch in the said pair, a main modulator for modulating the D.C. source, and an auxiliary modulator for modulating power supplied to the driver stages, the auxiliary modulator comprising a modulating transistor a control terminal of which receives signals from the main modulator, said modulating transistor being supplied from two independently controllable sources of potential which may be controlled to determine the peak and trough auxiliary modulation points.
2. 2. An amplitude modulated transmitter according to claim 1, wherein the modulating transistor is conneceted in series with a flywheel diode between the two independently controllable sources of potential.
3. 3. An amplitude modulated transmitter comprising main and auxiliary modulators substantially as hereinbefore described with reference to the accompanyina drawings.