AM transmitter with an offset voltage to the RF stage to compensate for switching time of the modulators

• US 4,122,415 A
• Filed: 12/09/1977
• Issued: 10/24/1978
• Est. Priority Date: 03/21/1977
• Status: Expired due to Term

• Current Assignee: RCA Corporation (General Electric Company)
• Original Assignee: RCA Corporation (General Electric Company)
• Inventors: Luther, Arch Clinton Jr., Sauer, David Arthur
• Primary Examiner(s): Grimm, Siegfried H.
• Application Number:
  US05/859,189
• Time in Patent Office: 319 Days
• Field of Search: 332/9 R, 332/9 T, 332/10, 332/31 R, 332/31 T, 332/37 R, 332/41, 325/139, 325/142, 325/144, 325/182
• US Class Current: 332/152
• CPC Class Codes: H03C 1/02 Details

Abstract

An AM transmitter of the type including apparatus for generating a subcarrier signal, pulse width modulating this subcarrier with audio, amplifying this resulting pulse width signal using active devices operating in the more efficient switching mode is disclosed. The amplified pulse width signal is passed through a low pass filter wherein the audio is recovered and used to amplitude modulate a radio frequency stage. The improvement herein includes a means for providing an offset voltage to the RF stage so that the system will develop zero carrier output when the pulse width of the modulating signal to the active devices has a relatively small finite value.

Claims

1. In an AM transmitter of the type including a pulse width modulator responsive to a local subcarrier and intelligence signals for producing pulse width modulated rectangular waves with the width of the waves dependent on the amplitude of the intelligence signals, amplifying means including active devices switched "on" and "off" in accordance with the width of said waves, an RF carrier stage, a source of biasing potential coupled to said RF carrier stage and said amplifying means, a low pass filter including series connected coils coupled between said amplifying means and said RF carrier stage for removing the subcarrier and applying high level DC varying at the audio rate to said carrier stage for modulating same, the improvement therewith for preventing distortion when the pulse width approaches zero width and the finite switch time of the amplifying means becomes significant comprising:

coupling means coupled between the amplifying means and said source of potential for coupling energy in the series connected coils of the filter when the amplifying means is in the "off" state, and

means coupled between said coupling means and said source of potential for providing a back biasing voltage to said RF carrier stage so that the carrier output goes to zero as the pulse width reaches a relatively small finite value.

2. In an AM transmitter of the type including a pulse width modulator responsive to a local subcarrier and intelligence signals for producing pulse width modulated rectangular waves with the width of the waves dependent on the amplitude of the intelligence signals, amplifying means including active devices switched "on" and "off" in accordance with the width of said waves, an RF carrier stage, a source of biasing potential coupled to said RF carrier stage and said amplifying means, a low pass filter including series connected coils coupled between said amplifying means and said RF carrier stage for removing the subcarrier and applying high level DC varying at the audio rate to said carrier stage for modulating same, the improvement therewith for preventing distortion when the pulse width approaches zero width and the finite switch time of the amplifying means becomes significant comprising:

a diode coupled between the amplifying means and said source of potential for coupling energy in the series connected coils of the low pass filter when the amplifying means in the "off" state, and

means coupled between said diode and said source of potential for providing a back biasing voltage to said diode so that a voltage more positive than the potential source is provided at the cathode of the diode so that the system will develop zero volts across the RF carrier stage and no carrier output as the pulse width of the signal to the amplifying means reaches a relatively small finite value.

3. The combination of claim 2 wherein said means for providing a back biasing voltage includes a zener diode.

4. The combination of claim 2 wherein said means for providing a back biasing voltage includes a resistor and a bypass capacitor.

5. The combination of claim 2 wherein said means for providing a back biasing voltage includes a power supply.

6. The combination of claim 5 wherein said power supply is a battery.

7. The combination of claim 5 wherein said back biasing voltage includes a resistor coupled across said power supply.