RF-AM transmitter with pulse width modulator

• US 4,776,036 A
• Filed: 11/13/1987
• Issued: 10/04/1988
• Est. Priority Date: 02/25/1986
• Status: Expired due to Fees

• Current Assignee: Continental Electronics Corp.
• Original Assignee: Varian Associates, Inc. (Siemens AG)
• Inventors: Weaver, Bryan A., Hulsey, Gilmer L.
• Primary Examiner(s): Masinick, Michael A.
• Application Number:
  US07/119,868
• Time in Patent Office: 326 Days
• Field of Search: 455/108, 455/109, 455/116, 375/22, 332/31 R, 332/37 R, 332/9 R, 330/10
• US Class Current: 455/108
• CPC Class Codes: H03C 1/16 by means of discharge device having at least three electrodes H03C1/28 - H03C1/34, H03C1/50, H03C1/52, H03C1/62 take precedence

Abstract

An RF-AM transmitter includes a pulse amplifier responsive to a pulse width modulated (PWM) signal derived in response to an information signal. A low pass filter converts pulses derived by the power amplifier into a variable amplitude signal that is DC coupled to a power supply for an RF amplifier. An amplifier system including the pulse and RF amplifiers has stray capacitance coupled to a modulator which derives the PWM signal; the stray capacitance lengthens narrow modulator output pulses so they are excessively long. The amplitude of only the narrow pulses is reduced by a resistor-capacitor timing circuit in series with a switch to substantially preserve the narrow pulse area. The switch provides a low impedance path from the timing circuit to a reference potential terminal when current initially flows in the circuit during pulse transitions and throughout the length of each of the short duration pulses. Thereby, the power amplifier is forward biased but not driven into saturation during the short duration pulses. The switch provides a higher impedance path from the timing circuit to the reference potential terminal after the transition has elapsed, enabling the power amplifier to be saturated during pulses longer than the short duration pulses. The RF power amplifier tube has a tendency to load the power amplifier tube out of saturation during wide pulses. The ampitude of the wide pulses is maintained substantially constant by a resistor shunted by an inductor between the modulator and the power amplifier.

Claims

1. A radio transmitter for transmitting an RF carrier amplitude modulated by an information signal comprising a pulse width modulator responsive to the information signal for deriving a pulse width modulated signal having pulses with widths representing the amplitude of the information signal, an amplifying system including a power amplifier including a first tube having a grid responsive to the pulse width modulated signal and an anode-cathode path DC coupled to an anode-cathode circuit of a second tube of an RF power amplifier, the RF power amplifier being responsive to the carrier, said amplifying system including low pass filter means for converting pulses derived by the first tube into a variable amplitude signal DC coupled to the anode-cathode circuit, means for biasing the first tube so it is driven between saturation and cutoff in response to transitions in the pulses derived by the pulse width modulator so the first tube is driven toward saturation while pulses are derived from the modulator and the first tube is at cutoff while no pulses are being derived from the modulator, the amplifier system having stray capacitance including shunt capacity of the first tube, the stray capacitance causing transitions of the pulses coupled to the grid to be changed in shape so that narrow ones of said pulses having a length less than a predetermined duration are excessively long for an accurate representation of the amplitude of the information signal and tend to distort the carrier to have excessively high instantaneous amplitude, means coupled to the amplifying system for reducing the amplitude of only said narrow pulses so that the tendency thereof to cause the carrier to have excessively high amplitude is substantially obviated, said RF power amplifier tube having a tendency to load said first tube out of saturation during portions of pulses wider than the narrow pulses subsequent to the narrow pulse lengths so that the wider pulses have a tendency to have an inadequate amplitude and cause the carrier to have lower amplitude than it should have, and means coupled to the amplifying system for maintaining the amplitude of said wider pulses at a level to maintain the first tube in saturation throughout the duration thereof so that the tendency of the carrier to have a lower amplitude than it should have is substantially overcome.

2. The transmitter of claim 1 wherein the amplitude reducing means includes a resistor-capacitor timing circuit and a switch controlled by said timing circuit in shunt with the grid, and the means for maintaining the amplitude of the wider pulses includes a resistor shunted by an inductor between the pulse width modulator and the grid.

3. The transmitter of claim 1 wherein the amplitude reducing means includes a resistor-capacitor timing circuit shunting the grid, switch means responsive to current flowing through said timing circuit for providing (a) a low impedance path from said timing circuit to a reference potential terminal when current initially flows in said timing circuit during a transition in each of said pulses and throughout the length of each of said short duration pulses so that the power amplifier tube is forward biased but not driven into saturation during said short duration pulses and (b) a higher impedance path from said timing circuit to the reference potential terminal after the transition so that the first tube is driven into saturation during pulses longer than said short duration pulses.

4. The transmitter of claim 2 wherein the switch means includes a path in series with said timing circuit and a control terminal for controlling opening and closing of said path, means for coupling current from said timing circuit to said control terminal so that said path of the switch means is closed in response to initial relatively high current flowing in said timing circuit during said transitions and is open in response to the relatively low current flowing in the circuit after the transition and the duration of the short pulse have occurred.

5. The transmitter of claim 1 wherein the amplitude reducing means includes a resistor-capacitor timing circuit responsive to transitions and pulses derived from said modulator, switch means responsive to current flowing through said timing circuit for providing (a) a low impedance path from said grid to a reference potential terminal when current initially flows in said timing circuit during a transition in each of said pulses and throughout the length of each of said short duration pulses so that the power amplifier tube is forward biased but not driven into saturation during said short duration pulses and (b) a higher impedance path from said grid to the reference potential terminal after the transition so that the first tube is driven into saturation during pulses longer than said short duration pulses.

6. A radio transmitter for transmitting an RF carrier amplitude modulated by an information signal comprising a pulse width modulator responsive to the information signal for deriving a pulse width modulated signal having pulses with widths representing the amplitude of the information signal, an amplifying system including a power amplifier including a first tube having a grid responsive to the pulse width modulated signal and an anode-cathode path DC coupled to an anode-cathode circuit of a second tube of an RF power amplifier, the RF power amplifier being responsive to the carrier, said amplifying system including low pass filter means for converting pulses derived by the first tube into a variable amplitude signal DC coupled to the anode-cathode circuit, means for biasing the first tube so it is driven between saturation and cutoff in response to transitions in the pulses derived by the pulse width modulator so the first tube is driven toward saturation while pulses are derived from the modulator and the first tube is at cutoff while no pulses are being derived from the modulator, the amplifier system having stray capacitance including shunt capacity of the first tube, the stray capacitance causing transitions of the pulses coupled to the grid to be changed in shape so that narrow ones of said pulses having a length less than a predetermined duration are excessively long for an accurate representation of the amplitude of the information signal and tend to distort the carrier to have excessively high instantaneous amplitude, and means coupled to the amplifying system for reducing the amplitude of only said narrow pulses so that the tendency thereof to cause the carrier to have excessively high amplitude is substantially obviated.

7. The transmitter of claim 6 wherein the amplitude reducing means includes a resistor-capacitor timing circuit connected to respond to the transitions and pulses derived by the modulator, switch means responsive to current flowing through said timing circuit for providing (a) a low impedance path from said grid to a reference potential terminal when current initially flows in said timing circuit during a transition in each of said pulses and throughout the length of each of said short duration pulses so that the power amplifier tube is forward biased but not driven into saturation during said short duration pulses and (b) a higher impedance path from said grid to the reference potential terminal after the transition so that the first tube is driven into saturation during pulses longer than said short duration pulses.

8. The transmitter of claim 7 wherein the timing circuit and the switch means both are in shunt with the grid.

9. The transmitter of claim 8 wherein the switch means includes a path in series with said timing circuit and a control terminal for controlling opening and closing of said path, means for coupling current from said timing circuit to said control terminal so that said path of the switch means is closed in response to initial relatively high current flowing in said timing circuit during said transitions and said short duration pulses and is open in response to the relatively low current flowing in the circuit after the transition and short duration pulse interval have been completed.

10. The transmitter of claim 8 wherein the switch means includes a field effect transistor having a gate connected to respond to current flowing in the resistor-capacitor circuit and a source drain path for providing the high and low impedance paths, diode means connected between the circuit and the gate for coupling positive going ones of said transistions to the gate and for preventing coupling of negative going ones of said transitions to the gate, another capacitor shunted by another resistor connected between said diode means and the gate, a resistive voltage divider having a tap connected to the gate and connected to be responsive to current flowing through said another capacitor, and a zener diode connected between one electrode of said source drain path and the gate for fixing the voltage between them.

11. A radio transmitter for transmitting an RF carrier amplitude modulated by an information signal comprising a pulse width modulator responsive to the information signal for deriving a pulse width modulated signal having pulses with widths representing the amplitude of the information signals, an amplifying system including a pulse power amplifier responsive to the pulse width modulated signal and a variable impedance path for DC biasing an RF amplifier, the RF amplifier being responsive to the carrier, said amplifying system including low pass filter means for converting pulses derived by the power amplifier into a variable amplitude DC bias for the RF amplifier, the amplifier system having stray capacity causing transitions of the pulses coupled to a control electrode of the pulse power amplifier to be changed in shape so that narrow ones of said pulses having a length less than a predetermined duration are excessively long for an accurate representation of the amplitude of the information signal and tend to distort the carrier to have excessively high instantaneous amplitude, and means coupled to the amplifying system for reducing the amplitude of only said narrow pulses so that the tendency thereof to cause the carrier to have excessively high amplitude is substantially obviated.

12. The transmitter of claim 11 wherein the amplitude reducing means includes a resistor-capacitor timing circuit and a switch controlled by said circuit, said switch being in shunt with the electrode of said pulse power amplifier for reducing the amplitude of current supplied by the power amplifier to the low pass filter during said short duration pulses relative to the amplitude of constant current supplied by the power pulse amplifier to the filter during pulses longer than the short duration pulses.

13. The transmitter of claim 11 wherein the amplitude reducing means includes a resistor-capacitor timing circuit responsive to transitions and the short duration pulses derived from the modulator, switch means responsive to current flowing through said timing circuit for providing (a) a low impedance path from the electrode of said pulse power amplifier to a reference potential terminal when current initially flows in said timing circuit during a transition in each of said pulses and throughout the length of each of said short duration pulses so that the pulse power amplifier is forward biased but not driven into saturation during said short duration pulses and (b) a higher impedance path from said control electrode to the reference potential terminal after the transition so that the pulse power amplifier is driven into saturation during pulses longer than said short duration pulses.

14. The transmitter of claim 13 wherein the timing circuit is in shunt with said electrode and the switch means includes a path in series with said circuit and a control terminal for controlling opening and closing of said path, means for coupling current from said timing circuit to said control terminal so that said path is closed in response to initial relatively high current flowing in said timing circuit during said transitions and is open in response to the relatively low current flowing in the timing circuit after the transition and the duration of the short pulse have occurred.

15. The transmitter of claim 13 wherein the timing circuit is in shunt with said electrode and the switch means includes a field effect transistor having a gate connected to respond to current flowing in the timing circuit and a source drain path for providing the high and low impedance paths, diode means connected between the timing circuit and the gate for coupling positive going ones of said transitions to the gate and for preventing coupling of negative going ones of said transistions to the gate, another capacitor shunted by another resistor connected between said diode means and the gate, a resistive voltage divider having a tap connected to the gate and connected to be responsive to current flowing through said another capacitor, and a zener diode connected between one electrode of said source drain path and the gate for fixing the voltage between them.