JPH0340528B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to pulse width modulation (PWM) systems, and more particularly to systems designed to supply a current of varying amplitude to a radio frequency stage of a wireless transmitter.
Such transmitters are sometimes required to operate at very high power levels, and this requirement can create problems.

One such problem is the so-called "carrier compression" problem, in which changes in the demand for current from the power supply cause variations in the output voltage of the power supply, thereby reducing the amplitude of the carrier wave. Changes in current demand result from changes in the amplitude of the modulating signal.

Another problem arises from the risk of electrical damage to the switching tube and/or audio frequency transformer or other components required to handle the total current delivered to the high frequency stage. Such destruction not only damages related components, but can also overload and seriously damage the power supply. Therefore, the power supply must be equipped with protective devices (e.g., circuit breakers) that can handle large currents and yet respond quickly when the current exceeds a maximum safe value. However, providing such a protection device itself poses a problem. This is because the associated current value is actually very large.

The present invention is a circuit for supplying a current with a varying amplitude to a high frequency stage of a wireless transmitter, the circuit comprising: a first power source arranged to supply current to the high frequency stage; a pulse width encoder for converting a signal into a pulse width modulated form; an amplifying means for amplifying the pulse width modulated signal; and an amplifying means for amplifying the pulse width modulated signal; a circuit comprising means for storing energy for converting it back into a form in which the signal is amplified; and means for applying an amplified signal of varying amplitude to said high frequency stage so as to modulate the current supplied thereto; In this, the amplification means is connected to receive power for amplification from a second power source different from the first power source, and transmits a signal whose amplified amplitude changes to a high frequency stage. Said means in addition to provide a circuit characterized in that it comprises a DC blocking capacitor.

For purposes of explanation, the two separate power supplies are defined as power supplies such that a change in the current drawn from one of them does not substantially affect the other.
Both power supplies are ultimately powered by a common main power supply, for example, a mains power supply. Both said power supplies use individual devices to convert the mains voltage into the required rectified voltage, so that there is no significant coupling between said power supplies, i.e. there is no change in the current drawn from one of them. The two power supplies are separate in the specific sense that, even if one occurs, it does not significantly affect the other. If both supplies include rectifiers, there is no power feedback to either supply, and no current circulates between the supplies either under normal conditions or (especially) under fault conditions.
In particular, no sustained fault current flows from a higher power supply to a lower power supply.

The first power supply that supplies the current forming the carrier wave modulated by the signal from the amplifier is independent of the power supply for the amplifier, so that variations in the amplifier's current demand affect the carrier wave power. will not be affected. In this way, carrier compression problems can be reduced or eliminated.

Since two power sources are used in the present invention, two respective protection devices are required. The first protection device (which protects the first power supply and/or its load circuit) may be a relatively simple device, as it typically does not need to respond very quickly. Therefore, it does not pose any serious cost or reliability problems. The second protection device (which protects the second power supply and the parts of the circuit directly connected thereto) needs to respond quickly. However, according to the invention, this protection device handles only a portion (sideband) of the power applied to the load and therefore only one power supply (consisting of two components connected in series). can be used to provide higher reliability than existing interlocking protection devices included in systems that provide full power to the load.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

Referring now to FIG. 1, one output line of a first power supply 1 is connected to a grounded conductor 2, and the other output line 3 has a rectified voltage V applied thereto. The current flowing in this line 3 is monitored by a current sensing device 4, which
gives a control signal when the current exceeds a safe value.
Additional general sensing devices may be installed in other parts of the circuit to generate similar control signals when other types of defects occur. The control signal acts to open a contact of circuit breaker 5, and in the particular system shown here, opens a further contact 5A.
It also works to close the . These ingredients 4, 5 and 5
A is collectively referred to as the "protection device". This device does not need to operate particularly quickly. This is because the audio frequency blocking inductor 6 prevents the current from rising too quickly. The power supply 1 serves to generate a steady current that flows through an audio frequency blocking inductor 6 and a load 7 to provide power at a high frequency carrier frequency.

Load 7 is the radio transmitter's high frequency vacuum tube, the anode of which needs to be maintained at a steady voltage of about V volts to operate properly under unmodulated carrier conditions, and this need 1
satisfied by.

The transmitter is amplitude modulated by applying an audio voltage to the anode of the high frequency vacuum tube, ie point F in FIG. The largest audio signal to be processed by the transmitter is at the anode of the high frequency vacuum tube.
When it has a peak-to-peak amplitude of 2V, that is, point F
The maximum efficiency of modulation is obtained when the potential of is varied between zero volts (earth potential) and a peak value of 2V based on the audio signal. This is 100%
This is called amplitude modulation. This is usually important in broadcast transmitters to avoid or minimize audio distortion.

The anode voltage is modulated by an audio signal introduced at point A shown in FIG. 1, which signal is shown schematically in FIG. 2A. This signal A
is amplified by a pulse-width modulation amplifier 8, which is powered by a second power supply 9, which is notable for supplying power to the second power supply by means of a DC discharge path. 1 power supply. For the circuit shown here, one output line of the second power supply 9 is connected to the grounded conductor 2, and the other output line 10 carries the rectified voltage 2V +
δV is maintained. Line 10 has current sensor 1
1 is installed, and this sensor receives a current that can be safely applied by the power source 9 or a switching device 1 described later.
3 and 14, a control signal is provided which activates fast-acting circuit breaker 12 and faster-acting switch 12A. Similar control signals may be generated by one or more further general fault sensing devices located elsewhere in the circuit. The combination of components 11, 12 and 12A constitutes the second protection device. However, the protection device 4, which allows the inductor 6 to respond relatively slowly to excessive current in order to limit the rate of current rise;
5 and 5A, the protection devices 11, 12 and 12A must be able to respond very quickly. However, it should be noted that this second protection device does not handle all the power applied to the load. This is because most of this power is supplied from the power supply 1. Devices 11, 12 and 1
2A can be designed to operate reliably because it does not handle all the power, and the difficulty involved is considerably greater than if it had to handle all the power consumed by the load. few.

The pulse width modulation amplifier 8 will be described below.
The input signal A is shown in FIG. 2 and consists of a portion where the voltage is above zero and a portion where the voltage is less than zero. This waveform A is converted into a series of pulses B (FIGS. 1 and 2) by a pulse width encoder PWE having a structure of known type,
The width of each pulse B is based on the instantaneous voltage level of waveform A. Therefore, the width of pulse B is at its minimum at the time when it coincides with the trough of waveform A, and at its maximum at the time when waveform A is at its positive peak. The width of pulse B when waveform A is zero is approximately half the interval between pulses. For this particular pulse width encoder, the maximum pulse width is made slightly shorter than the pulse repetition period to eliminate distortion that may occur if the pulses are not perfectly shaped. Similarly, the minimum pulse width is made greater than zero, also to eliminate distortion.

In the system shown here, the pulse width encoder PWE outputs an output C having a waveform that is an inversion of the waveform of the output B, that is, a waveform in which the peak portions of the waveform of the output B are respectively zero portions, and the zero portions are each replaced by peak portions. also occurs. These signals B and C are used to switch tubes 13 and 14; when waveform B is high, tube 13 is turned on and when waveform C is high, tube 14 is turned on. When the vacuum tube 13 is turned on, its resistance becomes very small, so the voltage at point D is 2V + δV supplied by the power supply 9.
Slightly lower than the bolt. At other times, when tube 14 is turned on, the voltage at point D will be slightly higher than zero volts. This is because this point is actually switched to the grounded line 2. Therefore, the voltage at point D varies between slightly less than 2V+δV volts and slightly more than zero volts, as shown in FIG. 2D. This follows the waveform of signal B. Therefore, the average voltage at point D follows waveform A, or is considerably amplified.

In an alternative embodiment (not shown), only signal C is generated by a pulse width encoder and applied to vacuum tube 14 as in FIG.
The grid control circuit for vacuum tube 13 consists of two vacuum tubes 1
3 and 14 are connected to the anode of the vacuum tube 14 so as to conduct in opposite phases. There are many known ways to do this.

An energy storage device in the form of an inductor 15 is used to provide a voltage E whose value represents the short-term average voltage at point D. Further energy storage components of the general type may be added to improve the degree of filtering of the pulses. When there is no audio signal at point A, the DC voltage at point E is approximately δV/
2 is greater than the voltage V, but the inductor 15
No direct current flows through capacitor 1, and this current flows through capacitor 1.
6. If an audio signal is applied at point A where the value of amplitude modulation is the maximum value allowed for pulse width modulation amplifier 8, then the voltage at point E will respond to changes in the average voltage at point D. Approximately δV/
It varies between 2 and 2V+δV/2. Therefore, even if 100% pulse width modulation (with distortion) is not used, the change in voltage E can be equal to 2V. This 2V voltage change at point E is applied to load 7 via capacitor 16, thus modulating the voltage at the load between 0 and 2V as shown in FIG. 2F.

In the circuit shown in FIG. 1, the first power supply 1 is required to supply a steady current to the load 7. However, this is not important, and the invention can also be applied in cases where the voltage level of the first power supply 1 for supplying the carrier current to the load 8 is controlled.

In another variant, the amplifier 8 is not connected to the grounded line 2 and the part of the line 2 shown in dotted lines is removed. Instead, line 1
0 may be grounded, or a ground connection may be made to the center tap of the power supply 9 as shown by the dotted line. The advantage of such a center tap is that the maximum voltage between each output of the power supply and ground is slightly greater than V volts, rather than more than 2 V volts.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a circuit constructed according to the present invention, and FIG. 2 is a diagram showing various waveforms appearing at various locations in the circuit shown in FIG. 1... First power supply, 2... Grounded conductor,
4...Current sensing device, 5...Circuit breaker, 5A
...Contact, 6...Inductor, 7...Load, 8...
...Pulse width modulation amplifier, 9...Second power supply, 11
...Current sensor, 12...Circuit breaker, 13,
14... Switching device (vacuum tube), 15...
inductor.

Claims (1)

[Scope of Claims] 1. A circuit for supplying a current of varying amplitude to a high frequency stage of a wireless transmitter, the circuit comprising: a first power supply arranged to supply current to the high frequency stage;
a pulse width encoder for converting a signal of varying amplitude into a pulse width modulated signal; an amplification means for amplifying the pulse width modulated signal; and amplification means for amplifying the pulse width modulated signal. means for storing energy for converting it back into a changed amplitude form; and means for applying said amplified amplitude changing signal to said radio frequency stage to modulate the current supplied thereto; In the circuit comprising: the width increasing means is connected to receive power for increasing the width from a second power source 9 different from the first power source 1, and A circuit characterized in that said means for applying a varying signal E to the high frequency stage comprises a DC blocking capacitor 16. 2. A first protection device 4, 5, 5A for protecting the first power supply 1 against overload and a second protection device for protecting the second power supply 9 against overload.
The circuit according to claim 1, wherein the circuit is provided with protection devices 11, 12, 12A. 3. The second protection device 11, 12, 12A is
The circuit according to claim 2, which operates faster than said first protection device 4, 5, 5A. 4 between the high frequency stage and the first power source 1,
The first power source is connected to the DC blocking capacitor 16.
4. A circuit according to claim 1, wherein an inductor (6) is connected for isolating the amplified amplitude varying signal E applied to the high frequency stage through the inductor (6). 5 The width increasing means includes two switching devices 13 and 14 connected in series to the second power supply 9 terminal.
, the switching device is switched by a pulse of the pulse width modulated signal,
When one switching device is open,
5. The device according to claim 1, wherein the other switching device is closed, and the energy storage means are connected to a point between the switching devices 13, 14. or the circuit described in item 1. 6. The circuit according to any one of claims 1 to 5, wherein the energy storage means includes an inductor 15.