Abstract:
An apparatus is provided for improved temperature compensation of received RF signals. An RF receiver includes receiver circuitry which receives an RF signal and demodulates it to generate an output signal. Temperature compensation circuitry is coupled to the receiver circuitry and includes a diode having diode temperature characteristics. The temperature compensation circuitry adjusts the output signal of the receiver circuitry in response to the diode temperature characteristics to provide a tighter minimum trigger level.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to radio communication devices, and more particularly relates to an improved apparatus for voltage level temperature compensation in circuits for radio communication devices. 
   BACKGROUND OF THE INVENTION 
   The output of radio frequency (RF) receiver circuitry is temperature dependant. Conventionally, such RF receiver circuitry includes a thermistor coupled to the output signal line to compensate for temperature changes. A thermistor is a resistor which changes its value in response to changes in temperature. For land based radio equipment where the temperature changes are rarely dramatic, a thermistor-based temperature compensation circuit providing ±3 dB of compensation may be sufficient. However, avionic RF equipment, such as avionic Traffic Collision Avoidance System (TCAS) RF receivers and other avionic transponder RF receivers must meet governmental requirements, such as requirements by the United States&#39; Federal Aviation Administration (FAA) for minimum trigger levels (i.e., the variation in a received signal which would trigger a response by the avionic RF equipment). While conventional avionic transponders control the minimum trigger level by providing a thermistor to change the log-video amplifier gain over temperature, current FAA guidelines have set a tighter requirement for the temperature dependency of the minimum trigger level than the ±3 dB that thermistor circuitry can provide. 
   Thus, it is desirable to provide a temperature compensation circuit for RF receiver circuitry that provides a tighter output variation over temperature than ±3 dB. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention. 
   BRIEF SUMMARY OF THE INVENTION 
   A radio frequency (RF) receiver provides improved temperature compensation of received RF signals. The RF receiver includes receiver circuitry which receives an RF signal and demodulates it to generate an output signal. Temperature compensation circuitry is coupled to the receiver circuitry and includes a diode having diode temperature characteristics. The temperature compensation circuitry adjusts the output signal of the receiver circuitry in response to the diode temperature characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and 
       FIG. 1  is a block diagram of a radio frequency (RF) transponder in accordance with an embodiment of the present invention; and 
       FIG. 2  is a circuit and block diagram of the temperature compensation circuit of the RF transponder of  FIG. 1  in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. 
   Referring to  FIG. 1 , a radio frequency (RF) avionic transponder  100  in accordance with an embodiment of the present invention includes diversity antennas  102  and  104  coupled to a receive/transmit switch  106  operating under control of a signal from controller  108 . When the controller  108  signals the switch  106  to place it in the receive mode, the diversity antennas  102  and  104  are coupled to receiver circuitry  110  including a first receiver  112  and a second receiver  114 , the diversity antenna  102  being coupled to the first receiver  112  and the diversity antenna  104  being coupled to the second receiver  114 . 
   In accordance with typical transponder receiver circuitry  110  design, the first receiver  112  and the second receiver  114  generate respective log-video signals and limiting intermediate frequency (IF) signals. The limiting IF signals from the first receiver  112  and the second receiver  114  are provided to a digital phase shift keying (DPSK) analog-to-digital converter  116  in a conventional manner for generation of a digital DPSK signal for providing to the controller  108 . 
   The first and second log-video signals from the first and second receivers  112 ,  114  are provided to temperature compensation circuitry  122  including a first temperature compensation circuit  124  and a second temperature compensation circuit  126  coupled to receive the first and second log-video signals, respectively, and generate therefrom respective first and second temperature compensated log-video signals. From the temperature compensation circuitry  122 , the first and second temperature compensated log-video signals are provided to analog-to-digital converter  128  and analog-to-digital converter  130  for processing the signals before providing them to the controller  108  for processing by a video processor portion thereof. Analog-to-digital converters  128 ,  130  could alternatively be comparators to process the temperature compensated log-video signals, providing the resulting signals to the controller  108 . 
   As is well known to those skilled in the art, the controller  108  includes the video processor portion and interrogation reply logic. The interrogation reply logic of the controller  108  determines if the processed temperature compensated log-video signals have a variation greater than a minimum trigger level (where the minimum trigger level is the amount of variation permitted due to changes in temperature) and generates a trigger signal in response thereto. The trigger signal is provided to transmitter circuitry  132  for conversion and modulation thereof and a transmission enable signal is provided to the switch  106  for transmission of an RF signal from one of the antennas  102 ,  104 . 
   Referring to  FIG. 2 , in accordance with the embodiment of the present invention, the temperature compensation circuitry  122  is shown in greater detail. Since the structure of both temperature compensation circuits  124 ,  126  is the same, only one temperature compensation circuit  200  is shown. The temperature compensation circuit includes diode circuitry  201  and a current source  211  coupled to the log-video signal output from the receiver circuits  112 ,  114 . In the diversity receiver construction described hereinabove, the diode circuitry includes a first diode coupled to the first receiver  112  and a second diode coupled to the second receiver  114 . 
   The diode circuitry  201  includes a diode  202 , a first resistor  204  and a second resistor  206  all connected to a common node  208 . The diode  202  has diode temperature characteristics. The log-video signal has temperature dependant DC offset voltage characteristics, and the first resistor  204  is connected between the log-video signal outputted from the receiver circuitry  110  and the common node  208 . The second resistor  206  is connected between the common node  208  and a constant voltage source  210 . The first resistor  204  has a first resistive value and the second resistor  206  has a second resistive value. While the current source  211  can be any current source, and may be located elsewhere in the circuitry or even in software, in accordance with the embodiment of the present invention, the current source  211  includes a variable resistor  212  and a third resistor  214  connected in series between the log-video signal and the constant voltage source  210 . The tap for the variable resistor  212  is connected to a node between the variable resistor  212  and the third resistor  214 . In accordance with the embodiment of the present invention, the current source  211  makes adjustment of the minimum trigger level possible by enabling DC offset voltage adjustment of the log-video signal by adjusting the variable resistor  212 . 
   A summing buffer  215  has a first input of an operational amplifier  216  thereof receiving the log-video signal and a second input of the operational amplifier  216  coupled to ground voltage. The summing buffer  215  generates a temperature compensated log-video signal at the output of the operational amplifier  216 , thereby providing the RF avionic transponder  100  with a tighter minimum trigger level. In accordance with the present invention, the temperature compensated log-video signal is generated by the temperature compensation circuit  200  adjusting the log-video signal in response to the diode temperature characteristics of the diode  202 , the temperature dependant DC offset voltage characteristics, the constant voltage  210 , and the first and second resistive values of the first and second resistors  204 ,  206 . In addition, in accordance with the embodiment of the present invention, the current source  211  also functions to center the output of the operational amplifier  216  in the optimal range of the analog-to-digital converters  128 ,  130 . 
   Accordingly, the present invention provides an improved temperature compensation circuit  200  which provides an output variation of less than ±1 dB. In addition, the temperature compensation circuit  200  provides a linear variation over temperature as opposed to the exponential variation of conventional thermistor-based temperature compensation circuits, in that, in accordance with the present invention, the temperature compensation circuit  200  provides a bias to the log-video signal that is a constant millivolt per degree Centigrade. In addition, the amount of temperature compensation can advantageously be modified by changing the resistive values of one or both of the first resistor  204  and the second resistor  206  and the minimum trigger level can be adjusted by adjusting the variable resistor  212 . 
   While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. For example, while the embodiment of the present invention described hereinabove is an RF avionic transponder  100 , the present invention may be implemented in any RF receiver, providing temperature compensation therefore. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.