Abstract:
A temperature stabilized RF detector uses a series circuit including a detecting diode coupled in series with a similarly poled stabilizing diode so that a single current flows in a single direction through both diodes. The temperature related effects of the two diodes are mutually balancing so that temperature variations in the detecting diode do not adversely affect the detection of an RF signal.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an RF (radio frequency) detector using a semiconductor diode, and more particularly to a temperature stabilized RF detector. 
     In the RF detector, it is usual to employ a DC bias current flowing through a detecting diode in order to obtain linear detecting characteristics over the wide dynamic range of RF inputs. Since the forward voltage of the diode depends on its operating temperature, however, such an RF detector has a disadvantage in that the detected output voltage depends on the temperature. This effect is conspicuous specifically for small RF input 
     An RF detector having an improved temperature characteristics for small RF input is described, for example, in a paper entitled &#34;A TEMPERATURE STABILIZED RF DETECTOR WITH EXTENDED DYNAMIC RANGE&#34;, by R. J. Turner, 32nd IEEE Vehicular Technology Conference Record, May 23-26, 1982, pp. 231-242. The detail of this RF detector will be descrbed hereinafter. Basically, the Turner&#39;s detector cancels the change in detected output voltage due to the temperature variations by connecting a temperature stabilizing diode to the detecting diode so that their temperature environments are identical. As a result, the DC bias variations in the detecting diode due to temperature variations can be compensated. However, since the detecting current flows through only the detecting diode to which the RF signal is applied, the currents flowing through the detecting diode and the temperature stabilizing diode become different from each other. Therefore, the forward voltages across both diodes inevitably become different from each other. Thus, the complete temperature stabilization cannot be attained simply by resorting to the Turner&#39;s detector. 
     SUMMARY OF THE INVENTION 
     It it an object of the present invention to provide an RF detector circuit which is capable of precisely detecting an RF signal for a wide dynamic range of the RF inputs. 
     A temperature stabilized RF detector circuit according to the present invention comprises a detecting diode for detecting an RF signal, means for supplying the detecting diode with a DC bias, and a temperatue stabilizing diode. The detecting and temperature stabilizing diodes are arranged so that the same current flows through both diodes, in the same direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of an RF detector circuit according to the present invention; 
     FIG. 2 is a schematic diagram showing the specific circuit of FIG. 1; 
     FIG. 3 is a schematic diagram of a prior art RF detector circuit; 
     FIG. 4 is a diagram showing the idealized I-V characteristics of a didode at various operating temperatures; 
     FIG. 5 is a diagram showing the real I-V characteristics of a diode at various operating temperatures. 
     FIGS. 6A, 6B are the same as parts of FIGS. 1 and 2, respectively, with the polarity of the diodes reversed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     To facilitate an understanding of the present invention, the RF detector circuit described in the aforementioned aricle by Turner will first be explained referring to FIG. 3. 
     In FIG. 3, an RF voltage at an input terminal 3 is detected by a detecting diode 1, smoothed by a filter composed of capacitors 9 and 10 and a choke coil 11, and finally supplied as an output V out1  to a terminal 4. The bias voltage V B  is applied to the detecting diode 1 via a resistor 15 and an RF stopping circuit composed of a choke coil 7 and a capacitor 8. A voltage V out2  at a terminal 22 is the output of a subtracter 21, which voltage is the difference between the output voltage V out1  and the output voltage V REF  of a circuit composed of a diode 13, resistors 16 and 20, and capacitors 18 and 19, that is, 
     
         V.sub.out2 =V.sub.out1 -V.sub.REF                          (1) 
    
     Now, the resistance values of the resistors 15, 16, 17 and 20 will be denoted by R 15 , R 16 , R 17  and R 20 , respectively. The voltage across the resistor 17, that is, the voltage V out1  at the terminal 4, is the sum of a DC bias voltage divided by a voltage divider composed of the resistors 15 and 17, and the detected voltage V DET , 
     
         V.sub.out1 =(V.sub.B -V.sub.D1)(R.sub.17 /(R.sub.15 +R.sub.17))+V.sub.DET ( 2) 
    
     where V D1  denotes the forward voltage of the diode 1. 
     On the other hand, the voltage V REF  across the resistor 20 is given by, 
     
         V.sub.REF =(V.sub.B -V.sub.D13)(R.sub.20 /(R.sub.16 +R.sub.20)) (3) 
    
     where V D13  denotes the forward voltage of the diode 13. If R 15  =R 16  and R 17  =R 20 , the following expression is obtained from equations (1), (2) and (3), 
     
         V.sub.OUT2 =V.sub.OUT1 -V.sub.REF =(V.sub.D1 -V.sub.D13)(R.sub.17 /(R.sub.15 R.sub.17))+V.sub.DET                           (4) 
    
     If the diodes having the same temperature characteristics are used as diodes 1 and 13 in the identical temperature environment, the following relation holds, 
     
         V.sub.D1 =V.sub.D13                                        (5) 
    
     with the result that 
     
         V.sub.out2 =V.sub.DET                                      (6) 
    
     Therefore, the use of the diode 13 can eliminate the change in the DC bias due to temperature variations and thus suppress the corresponding change in the final detected voltage. 
     In the aforementioned detector circuit (FIG. 3) by Turner, it is assumed that the forward voltage of a diode depends only on its operating temperature and not on the current flowing therethrough, as is shown in FIG. 4. Under this assumption, the temperature stabilization can be completely realized by means of the circuit in FIG. 3. However, the real diode does not have the I-V characteristics shown in FIG. 4, but has the characteristics shown in FIG. 5. In other words, the forward voltage V d  of a diode is given by 
     
         V.sub.d =V.sub.d (i,T), 
    
     where T denotes the operating temperature of the diode. 
     Assuming that the operating temperature T is constant, a constant current i D13  flows through the diode 13 (FIG. 3), which current is equal to the current i D1  through the diode 1 if the RF input signal is absent. Otherwise, the detected current flows through the diode 1 in addition to the current i D13 . That is, 
     
         i.sub.D1 =i.sub.D13 +i.sub.DET                             (7) 
    
     where i DET  denotes the DC component of the ripple current produced by the detection. As is obvious from equation (7), i D1  ≠i D13 , so that 
     
         V.sub.d (i.sub.D1, T)≠V.sub.d (i.sub.D13, T) 
    
     Thus, the assumption of equation (5) does not hold, which means that the temperature stabilization is incomplete for the circuit shown in FIG. 3. 
     If the values of the resistors 15, 16, 17 and 20 are properly selected, the relation i D1  =i D13  holds only for a certain RF input level. However, it does not hold for other RF input levels, again causing an incomplete temperature stabilization. 
     Now, the present invention will be explained with reference to FIG. 1. In FIG. 1, an RF detector circuit comprises a detecting diode 1 and a filter 26 composed of capacitors 9 and 10 and a choke coil 11, by which an RF signal applied at a terminal 3 is detected and smoothed to be supplied to a terminal 4. An RF stopping circuit 23 is composed of a choke coil 7 and a capacitor 8. The series connection of a resistor 5 and a compensating diode 2 is connected between the terminal 4 and ground. A bias voltage V B1  is produced by summing a voltage at the connection point of the resistor 5 and the diode 2, and a bias voltage V B  applied at a terminal 12. This summing is accomplished by means of an adder 6. This bias voltage V B1  is applied to the detecting diode 1 through RF stopping circuit 23. 
     The detecting diode 1 and the compensating diode 2 have the same polarity with respect to the current loop of the bias voltage V B1  applied to the diode 1. Moreover, the same current is made to flow through both diodes. In other words, the current i D1  through the diode 1 is always equal to the current i D2  through the diode 2. Therefore, if the diodes 1 and 2 are thermally coupled so as to be subjected to the same temperature effect, the forward voltages of the diodes 1 and 2 become 
     
         V.sub.D1 =V.sub.d (i.sub.D1, T)=V.sub.D2 =V.sub.d (i.sub.D2, T) 
    
     Thus, an output voltage V out  at the terminal 4 is given by, 
     
         V.sub.out =V.sub.B1 -V.sub.D1 +V.sub.DET 
    
     where V DET  denotes the detected voltage. Since 
     
         V.sub.B1 =V.sub.B +V.sub.D2 =V.sub.B +V.sub.D1 
    
     the output voltage becomes 
     
         V.sub.out =V.sub.B +V.sub.D1 -V.sub.D1 +V.sub.DET =V.sub.B +V.sub.DET (8) 
    
     Consequently, the detected output voltage V out  is not affected by the variation in the forward voltage of the diode for any RF input signal level and any operating temperature. The voltage V B1  and the value of the resistor 5 are determined so that the linear operation of the diode is assumed and that the dynamic range of the output voltage V out  lies in the desired range. 
     In the circuit shown in FIG. 2, the adder 6 in the circuit of FIG. 1 is realized by a circuit using the operational amplifiers. The operational amplifier 24 is a high input-impedance inverting adder, while operational amplifier 25 is a low output-impedance inverter acting as a low-impedance voltage source. If all the values of the resistors determining the gain of the amplifiers 24 and 25 are chosen to be identically R, it is obvious that the result of equation (8) can be obtained. It is preferable that an external circuit connected to the terminal 4 is a voltage-driven circuit having substantially infinite input-impedance. In practice, to the terminal 4 may be connected an external circuit whose input impedance is large enough so that substantially no current flows from the RF detector to the external circuit. 
     In the above explanation, the RF stopping circuit 23 and the filter 26 for the detected voltage in FIG. 1 are not limited to those circuits which are actually shown. Any circuit can be used if the current flowing through the diodes 1 and 2 are mutually equal with respect to the DC loop of the bias voltage V B1 . The bias voltage V B1  in FIGS. 1 and 2 is assumed to be positive. The polarities of the diodes 1 and 2 should be reversed as shown in FIGS. 6A, 6B, a negative bias voltage is used. 
     As is explained above, the detector circuit according to the present invention comprises the serial connection of the compensating diode and a resistor connected between the detecting diode and ground so that the same DC current flows through both the detecting diode and the compensating diode. The detector circuit supplies the detecting diode through an RF stopping circuit with a sum of the voltage at the connection point of the serial connection and the predetermined DC voltage. As a result, the final detected voltage is not affected by the diode forward voltage. Accordingly, the effect due to the operating temperature variations is excluded. An extremely precise RF detection can be attained for any RF input signal level.