RF amplifier protector and method

An amplifier protector and method for preventing power overload of a power amplifier and destruction of transmission circuitry and printed wiring board. The protector continually monitors the peak forward and reflected voltages of the transmitter output which are proportional to the forward and reflected power. An associated processor determines whether the forward voltage is below a threshold voltage which indicates a circuit malfunction. If the circuit malfunction is detected, the processor via an automatic level control circuit turns off the power amplifier in a timely fashion to prevent damage to the transmission circuitry.

BACKGROUND OF THE INVENTION 
The present invention pertains to RF (radio frequency) circuitry and more 
particularly detection of circuit failures and protection of RF amplifiers 
and the RF path due to overloading caused by circuit failure. 
When circuitry associated with an RF power amplifier fails, such as PIN 
diode switches, harmonic filters or power sensors, the automatic level 
control (ALC) loop that controls the power amplifier output level becomes 
an open loop. The result of this type of component failure is that the 
power amplifier attempts to deliver maximum power output into the failed 
circuitry. One result of this failure may be that various components are 
damaged by the power overload. More extensive damage may result from 
damage to the printed circuit board on which the components are mounted. 
Printed circuit board damages are very expensive to repair or in some 
cases non-repairable. 
In the open loop situation (when a malfunction occurs), the ALC demands 
maximum power. As a result, the components become thermally damaged. 
It is highly desirable therefore to provide a RF power amplifier overload 
protector and method for monitoring the RF power output of a power 
amplifier of a transmitter and detecting a malfunction of this type and 
turning off the power amplifier. 
SUMMARY OF THE INVENTION 
In accordance with the present invention a novel RF amplifier protector and 
method is shown. An amplifier protection arrangement for coupling 
modulated signals to an antenna for transmission of these signals includes 
an amplifier for receiving the modulated signals and producing amplified 
signals. A power sensor determines the power of the amplified signals and 
provides first and second voltage indicators proportional to the power of 
the amplified signal. The power sensor is coupled to the amplifier and to 
the antenna. A controller adjusts the power of the amplified signals. The 
controller is coupled to the amplifier and to the power sensor. An 
amplifier protector turns off the amplifier via the controller, if the 
first and second voltage indicators show the power of the amplified 
signals being substantially below a threshold voltage. The amplifier 
protector is coupled to the power sensor and to the controller.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a block diagram of a radio circuit 10 including a RF 
amplifier protector arrangement is shown. Frequency synthesizer 12 
provides the output carrier waveform at the appropriate frequency. 
Frequency synthesizer 12 is connected to voltage controlled amplifier 14. 
The gain of voltage controlled amplifier 14 is adjusted by a DC voltage on 
the control lead 49 from the automatic level control (ALC) 48. Voltage 
controlled amplifier 14 is further connected to power amplifier 16. Power 
amplifier 16 boosts the output signal to a high level suitable for voice 
or data transmission. Power amplifier 16 is connected to PIN switches 21 
and 22. These PIN switches control the selection of band pass filters 24, 
25, 26 to attenuate harmonics before transmission of the output signal. 
Although three filters are shown more or fewer filters may be included in 
the radio. The proper filter is selected by means of PIN diode switches 21 
and 22. The output of PIN switch 22 is connected to power sensor 30. Power 
sensor 30 includes a directional coupler and two diode detectors. The 
output of power sensor 30 is coupled to transmission antenna 32. Power 
sensor 30 also provides two outputs VF and VR which provide outputs 
relative to the forward and reflected power levels respectively of sensor 
30. 
Power sensor 30 is connected to automatic level control 48 via the VF and 
VR leads. Power sensor 30 is connected to peak detector 34 via the VF lead 
and to peak detector 36 via the VR lead. Peak detector 34 is connected to 
low pass filter 40. Peak detector 36 is connected to low pass filter 42. 
Low pass filters 40 and 42 are coupled through analog-to-digital converter 
44 to processor 46. Processor 46 may be implemented using a microprocessor 
or various other types of processors. Processor 46 generates the necessary 
timing and read/write function required for proper operation of the 
amplifier protector arrangement. 
Automatic level control 48 is coupled to voltage controlled amplifier 14 to 
form a feedback loop for adjusting the power output by the radio circuit 
10. ALC 48 also has an input from the voice and data circuitry (not shown) 
for the AM modulation input. Automatic level control (ALC) 48 may include 
a loop filter, a voltage reference, and current drivers. ALC 48 controls 
the comparison and adjustment of the desired output versus feedback 
levels. The output power is controlled by a digital input signal from 
processor 46 on lead 47. The selected input level transmitted from 
processor 46 to ALC 48 is stored in the ALC for subsequent use. Processor 
46 is also connected to frequency synthesizer 12. 
Processor 46 is connected to PIN switches 21 and 22 to control the 
selection of the appropriate filter 24-26. Processor 46 is connected to 
display 50. Display provides for indication to an operator that a 
malfunction has occurred and that power amplifier 16 has been turned off. 
Processor 46 is connected to memory 52. Memory 52 includes prestored and 
calibration data associated with the various power levels for different 
modulation types for the power amplifier 16. Processor 46 has a 
PUSH-TO-TALK input which is generated when the operation keys the radio 
for transmission by external circuitry (not shown). 
During normal operation, processor 46 selects the appropriate frequency and 
modulation type for frequency synthesizer 12; the band pass filter 
settings of PIN switches 21 and 22 for selecting filters 24-26 and the 
power output setting for ALC 48. Processor 46 sets the appropriate 
frequency and transmits this to frequency synthesizer 12. The band pass 
filter settings are transmitted to the PIN switches 21 and 22. The power 
output setting is transmitted from processor 46 to ALC 48 via lead 47. The 
closed loop operation of the ALC 48 compares the measured value of 
transmitted forward voltage VF of power sensor 30 to the power level 
transmitted to ALC 48 via processor 46. Based upon the result of this 
comparison, the ALC 48 adjusts the control signal on lead 49 to control 
the voltage controlled amplifier 14 to set the appropriate output level. 
As a result, power amplifier 16 is driven at a level which corresponds to 
the desired level indicated by the processor 46. Processor 46 continually 
samples, at a rate of 10 times per second (every 100 ms.), the forward 
(VFWD) and reflected (VRFL) power levels measured by peak detectors 34 and 
36, respectively. The processor must sample at a fast enough rate in order 
to turn off the power amplifier rapidly enough to protect the circuit 
components and printed wiring board from thermal damage, if a circuit 
malfunction occurs. Processor 46 then determines from the previously 
mentioned settings a particular threshold level. Processor 46 then 
compares the threshold with the measured (VFWD) value. For a failure of 
one of the components in the power amplifier path, from power amplifier 16 
to antenna 32, the value of VFWD will measure a lower value than 
acceptable. Processor 46 detects this as a fault condition and via ALC 48 
and voltage controlled amplifier 14 turns off power amplifier 16, thereby 
protecting the circuitry and printed wiring board from excessive power and 
heat. 
The transmitted output may be either an AM or FM signal. As a result, the 
output of power sensor 30 must be held at peak for the AM case. The peak 
detectors 34 and 36 are transparent in the FM mode. The waveforms for the 
AM case are depicted in FIG. 2. The threshold value for the output signal 
is shown as waveform 100. The power sensor voltage is the sinusoidal 
voltage displayed as waveform 102. The peak detected and filtered voltage 
is shown as waveform 104. Charge and discharge times of the peak detectors 
34 and 36 and low pass filters accommodate modulation rates for the AM 
case from approximately 30 Hz to 8 KHz. This discharge time for the peak 
detectors 34 and 36 and low pass filters 40 and 42 must allow the 
circuitry to hold the low frequency signal above threshold between cycles 
of the modulation rate. The charge time must be fast enough to track the 
peak values of the highest modulation rate. The threshold voltage is set 
according to measurements made at the time the radio is initially tuned 
and aligned. These measurements are stored in memory 52. 
FIG. 3 is a block diagram of the processor method for RF amplifier 
protection. When processor 46 is initiated it enters block 120 and 
constantly performs the following method. First, the processor 46 
determines whether the transmitter function of the radio has been keyed. 
This is indicated to processor 46 by the PUSH-TO-TALK signal being sent. 
If the transmitter function has not been keyed, block 22 iterates the 
transmitter key test via the N path. If the transmitter has been keyed, 
block 122 transfers control to block 124 via the Y path. Block 124 waits a 
time of 100 milliseconds. This 100 millisecond wait allows sufficient time 
for malfunction detection without thermal damage to the components or 
printed wiring board. Next, processor 46 reads the values of the forward 
voltage (VFWD) from detector 34 and the reflective voltage (VRFL) from 
peak detector 36. Next, processor 46 determines the threshold voltage for 
the power output selected, block 128. The calculation of the threshold 
voltage is made as follows: 
EQU VTH=0.4.times.VPOT-K.times.VRFL 
VPOT is an empirically determined voltage for each operating power output 
level for each radio. It is a function of the tolerances and the 
individual components selected. A set of VPOT values for each output power 
level selected are stored in memory 52 and read from memory 52 depending 
on whether low, medium or high power modes of transmission are selected in 
the AM or FM mode. Voltage values associated with the above mentioned 
power outputs are stored in memory 52 for both AM and FM cases. See Table 
1 below which shows an example of the power output for each case. The 
actual voltage values stored may vary from radio to radio. 
TABLE 1 
______________________________________ 
POWER 
OUTPUT MODULATION TYPE 
MODE AM FM 
______________________________________ 
HIGH 10 WATTS 10 WATTS 
MEDIUM -- 5 WATTS 
LOW 5 WATTS .1 WATTS 
______________________________________ 
In the preferred embodiment, 40% of the selected VPOT is determined by 
processor 46 to be VTH (the threshold voltage). Depending upon the 
accuracy required for the particular application of the protector circuit, 
other percentages of VPOT may be taken. 
The reflected voltage VRFL read from peak detector 36 is multiplied by a 
constant K which is empirically determined. The constant K is also stored 
in memory 52 for each radio. Values of the constant K are typically equal 
approximately equal to one-fourth. If VRFL is less than or equal to a 
predetermined amount (0.5.times.VTH) block 130, then the ALC reflected 
power circuit included in ALC 48 has not reduced the forward power output 
and control is transferred from block 130 to block 134. If VRFL is greater 
than (0.5.times.VTH), then block 130 transfers control to block 132 which 
subtracts the constant (K.times.VRFL) from the threshold voltage (VTH). 
This adjusts the threshold for lower output power due to ALC reflected 
power circuit operation. Next, block 134 determines whether VFWD, the 
forward voltage, is greater than the threshold voltage VTH. If it is, the 
radio is properly operating and block 134 transfers control to block 122 
via the Y path. Block 122 tests for the next transmission keying. If the 
forward voltage measured is less than or equal to the threshold voltage, a 
malfunction has been detected in the power amplifier path (from power 
amplifier 16 through power sensor 30). In this case, control is 
transferred from block 134 to block 136 via the N path. Processor 46 then 
through a command to ALC 48 causes voltage controlled amplifier 14 to turn 
off the signal to power amplifier 16, block 136. The processor 46 also 
issues a command to turn off the power amplifier 16. Processor 46 then 
displays an error indication upon display unit 50, block 138. This error 
indication may be, for example, a typed message on a CRT or teleprinter or 
a lighted lamp on a display panel, etc. Next block 140 determines whether 
the transmitter is unkeyed. That is a determination is made whether the 
PUSH-TO-TALK signal has been reset. If the transmitter is unkeyed, another 
transmission may be attempted and block 140 transfers control via the Y 
path to block 122. If the transmitter remains keyed, block 140 waits until 
the transmitter has been unkeyed. 
As can be seen from the above explanation, a protector arrangement and 
method for preventing circuit overload and excessive power and heat from 
being applied to the power amplifier path and printed wiring board of a 
radio has been shown. The power amplifier protector constantly monitors 
the power output and rapidly turns off the power amplifier to prevent 
damage, if a circuit malfunction is detected. 
Although the preferred embodiment of the invention has been illustrated, 
and that form described in detail, it will be readily apparent to those 
skilled in the art that various modifications may be made therein without 
departing from the spirit of the invention or from the scope of the 
appended claims.