Method and apparatus for providing transmitter protection

An RF transmitter protection circuit for providing protection to a power amplifier includes circuitry for sensing the presence of an input excitation signal applied to the input of the power amplifier. Circuitry is provided for monitoring an output signal generated by the power amplifier at the output of the power amplifier and for generating a signal proportional to the output signal. Circuitry is further provided for applying the signal generated by the monitoring circuit to the input of the power amplifier in response to a loss of input signal detected by the sensor.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to radio frequency transmitters, and more 
particularly to a method and apparatus for providing protection to power 
amplifiers utilized with such transmitters. 
BACKGROUND OF THE INVENTION 
A high-power transmitter with a high-Q antenna such as, for example, solid 
state fixed position VLF station transmitters, are susceptible to severe 
damage if the radio frequency (RF) input excitation to the transmitter is 
suddenly removed or drastically changed in frequency. As a result, the 
high energy stored in the high-Q antenna and the antenna impedance 
matching circuit is subsequently dissipated in the power amplifier output 
devices at a level well above the maximum rating of such power amplifiers, 
thereby causing massive power output device failures. This failure results 
from the energy stored in high-Q antennas. 
A need has thus arisen for a circuit protection device for protecting power 
amplifiers utilized with RF transmitters in the event of a loss in RF 
input excitation to the transmitter. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, an RF transmitter protection 
circuit for providing protection to a power amplifier is provided. The 
circuit includes circuitry for sensing the presence of an input excitation 
signal applied to the input of the power amplifier. Circuitry is provided 
for monitoring an output signal generated by the power amplifier at the 
output of the power amplifier and for generating a signal proportional to 
the output signal. Circuitry is further provided for applying the signal 
generated by the monitoring circuit to the input of the power amplifier in 
response to a loss of input signal detected by the sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the Figure, a diagram of a transmitter 10 utilizing the 
present invention is illustrated. Transmitter 10 receives excitation from 
an RF input 12 which is applied to a power amplifier 14. The output of 
power amplifier 14 is applied to an antenna matching network 16 whose 
output is applied to an antenna 18. RF input 12 represents a generator 
which supplies RF signals in each of the frequency bands in which the 
transmitter 10 is to be operated. These RF signals are amplified by power 
amplifier 14. 
In the event of loss of RF input 12 to power amplifier 14, or if RF input 
12 falls below a minimum level, operation of amplifier 14 will terminate 
and present a low impedance state at its output. As a result, a large 
amount of RF power will be reflected back from antenna 18 through antenna 
matching network 16 to power amplifier 14. This high reflective power can 
damage power amplifier 14, and therefore the present invention provides 
for a method and apparatus for protecting power amplifier 14 in the event 
of loss or otherwise corruption of the RF excitation input 12 to 
transmitter 10. 
Power amplifier 14 may comprise, for example, a solid state high-efficiency 
class D high-power amplifier. A sine wave input signal is converted to a 
square wave signal which is power amplified by amplifier 14. The output 
signal is a square wave voltage source that has a sine wave current due to 
the series resonant type load presented by antenna matching network 16. 
Antenna matching network 16 is a 90.degree. phase shift type of network 
implementing a standard technique used to broadband the power applied to 
high-Q antennas. 
In accordance with the present invention, an input signal sensor 20 is 
utilized for monitoring the presence or absence, or below a minimum level, 
of the RF input signal 12. Input signal sensor 20 may comprise, for 
example, a phase locked loop that is locked to the input signal 12. When 
the input signal 12 is removed, the loop error signal would indicate an 
out of lock condition. Input signal sensor 20 may also comprise, for 
example, a full wave rectifier. Further, sensor 20 may operate to sample 
the input signal 12 at multiple points of the cycle and verify that the 
sample point of the signal is of correct magnitude and polarity. Input 
signal sensor 20 responds within a fraction of a cycle to the absence of 
input signal 12, and provides an actuation signal via line 22 to a control 
switch 24. Switch 24 may comprise, for example, an analog type high speed 
integrated circuit switch such as, for example, a model DG301A 
manufactured and sold by Siliconix, Inc. of Santa Clara, Calif. 
The output of power amplifier 14 is monitored by a current monitor 26 which 
monitors the RF current at the output of power amplifier 14. Current 
monitor 26 may comprise, for example, a current monitor Model No. 310 
manufactured and sold by Pearson Electronics, Inc. of Palo Alto, Calif. 
Current monitor 26 generates a voltage output that is in phase with the 
current being monitored from power amplifier 14. Current monitor 26 may 
also comprise a current transformer in which the burden resistor is 
sufficiently low enough in resistance to prevent phase shift between the 
monitored current and the voltage output. Current monitor 26 may also 
comprise a Hall-effect sensor. 
The voltage sample, derived from the output current of power amplifier 14, 
and generated by current monitor 26 is applied via signal line 28 to 
control switch 24. When actuated, control switch 24 provides the voltage 
sample derived from the output current of power amplifier 14 to the input 
of power amplifier 14 via signal line 30. This voltage acts as a 
substitute RF excitation for the RF input signal 12 in order to 
continually provide input to power amplifier 14 thereby preventing damage 
to power amplifier 14 in the event of removal of RF input signal 12. In 
the event of loss of RF input signal 12, control switch 24 is actuated to 
provide an input to power amplifier 14 which represents a voltage sample 
derived from power amplifier 14 output current to be used as a substitute 
RF input 12 for transmitter 10. After this substitution has been made, 
transmitter 10 may be reduced in power at a controlled rate by other 
transmitter 10 systems. 
The present protection apparatus must operate to sense a loss of RF input 
signal 12 in a short period of time; for example, less than one-fourth of 
an RF cycle to limit the transient to a reasonable value in order to 
prevent the large current build-up at the output of power amplifier 14. 
The time allowable for sensor 20 to sense a lack of RF input 12 is 
dependent on the amount of energy stored in antenna 18 and how fast the 
current rises when RF drive power to antenna 18 is removed. Voltage is 
reapplied via signal line 30 to power amplifier 14 of continuous phase as 
that of the removed RF input signal 12 and should have a center frequency 
compared to the upper and lower modulation frequency even though the 
interruption to RF input signal 12 may have occurred at the upper or lower 
modulation frequency. 
It therefore can be seen that the present invention provides for a method 
and apparatus for protecting power amplifiers of transmitters in the event 
of loss of excitation RF signal. 
Whereas the present invention has been described with respect to specific 
embodiments thereof, it will be understood that various changes and 
modifications will be suggested to one skilled in the art and it is 
intended to encompass such changes and modifications as fall within the 
scope of the appended claims.