Abstract:
A current comparator is supplied with differential currents based upon an amplitude modulated radio frequency carrier. The comparator outputs are summed in a combining stage the output of which will contain a direct current proportional to the average carrier level and a current variation which represents the carrier modulation. The circuit provides very efficient detection of the modulated carrier.

Description:
BACKGROUND OF THE INVENTION 
     In my U.S. Pat. No. 4,069,460 which was issued Jan. 17, 1978, to the assignee of the present invention, a balanced current comparator is disclosed. The present invention makes use of that comparator to provide an amplitude modulated (AM) carrier wave detector. The detector has utility in AM radio receivers as a precision full wave detector. In particular, it has utility in AM stereo radio receivers where precision rectification is needed. Most present AM stereo radio receivers employ a conventional diode AM detector which acts as a peak rectifier. Such detectors must be operated at a high-signal level to avoid nonlinearities, and therefore, tend to produce carrier signal radiation. The peak rectification characteristic also makes them responsive to impulse noise. A suitable detector should be relatively immune to noise, produce as low radiation as possible, precisely rectify the AM carrier, and provide a carrier related direct-current (d-c) output. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a detector circuit which acts as a full wave rectifier and is amenable to integrated circuit (IC) fabrication. 
     It is a further object of the invention to provide an AM detector circuit having full wave output and highly precise rectification. 
     It is a still further object of the invention to provide an AM detector circuit based upon a current comparator which has its outputs summed in a precision rectifier configuration which provides outputs precisely related to the AM carrier and the carrier modulation. 
     These and other objects are achieved in a circuit configured as follows. An AM carrier wave signal is applied to a differential input stage which is coupled to a current comparator having a pair of input nodes. This comparator is configured to have a very small deadband range. The two nodes are coupled to a current summing stage, the output of which will contain current pulses that represent a full wave rectified version of the AM carrier. A low pass filter having an RF filter characteristic will yield the carrier modulation. A second low pass filter having a modulation filter characteristic will yield a dc voltage related to the carrier average value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of the circuit of the invention. 
     FIG. 2 is a series of graphs portraying the signals related to the operation of the circuit of FIG. 1. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the circuit of the invention in a form amenable to IC fabrication. A source of operating potential is connected between the +Vcc terminal 10 and ground terminal 11 as shown. Input terminals 12 and 13 are supplied with an AM carrier such as the one shown in FIG. 2. This signal would typically be obtained from the intermediate frequency stage of an AM radio receiver (not shown). Current source 14 provides current I 1  to differential pair transistors 15 and 16 such that nodes 17 and 18 are supplied with differentially related currents I 2  and I 3 . Thus I 2  =I 2  +I 3  and the FIG. 2 output showing represents |I 2  -I 3  |. Resistors 19 and 20 act to degenerate transistors 15 and 16 and can be employed to regulate the circuit temperature compensation. 
     Nodes 17 and 18 are coupled to symmetrical current mirror 21 which is made up of transistors 22 thru 25. Current mirror 21 operates as disclosed in my U.S. Pat. No. 4,069,460, referenced above, and acts to vary the voltages at nodes 17 and 18 in accordance with the input signal at terminals 12 and 13. Transistor 26 is shown as a dual emitter device with the emitters coupled to nodes 17 and 18. However transistor 26 could be two separate transistors with their collectors and bases commonly connected. 
     Diodes 27 thru 29 forward biased by current source 30, bias the base of transistor 26 at 3 V BE  above ground. While not shown, diodes 27 thru 29 could be realized using the emitter-base diodes of NPN transistors. Since current mirror 21 biases nodes 17 and 18 at 2 V BE  above ground, transistor 26 will be conductive and responsive to the input signal. FIG. 2 portrays the output current supplied by transistor 26 in load resistor 31. It can be seen that the circuit operates as a frequency doubling full wave rectifier. 
     Capacitor 32 is selected along with resistor 31 to provide a low pass filter that effectively removes the R-F carrier while leaving the modulation signal at output terminal 33. Resistor 34 and capacitor 35 form a second low pass filter that removes the modulation signal and provides a d-c output at average terminal 36. This d-c output provides an R-F carrier related signal for automatic gain control use in the related AM radio (not shown). 
     It can be seen that as the emitters of transistor 26 are driven negatively, conduction will increase, whereas, as they are driven positively the transistor action cuts off. This is the mechanism that produces signal rectification. However, the bias on transistor 26 can be likened to class AB amplifier transfer operation. This provides a characteristic that avoids a deadband in the signal transfer. This produces a linear transfer over a large dynamic range and avoids the distortion that would ordinarily accompany the modulation troughs, particularly for weak signals. Since nodes 17 and 18 always have associated shunt capacitance, such a deadband response can be aggravated particularly at the higher operating signal frequencies. 
     EXAMPLE 
     The circuit of FIG. 1 was constructed using conventional monolithic IC components. The NPN transistors were of standard vertical construction and the PNP transistors were of standard lateral construction. The following part values were employed: 
     
         ______________________________________Part             Value      Units______________________________________Source 14        1          milliamperesResistors 19 and 20            500        ohmsSource 30        5          microamperesResistor 31      2K         ohmsCapacitor 32     .002       microfaradsResistor 34      8K         ohmsCapacitor 35     10         microfarads______________________________________ 
    
     The circuit had less than 1% distortion in detecting a 455 kHz signal modulated at 90%. The output at terminal 36 was linearly related to the average signal value. 
     The circuit of the invention has been described, and an operating example given for the IC version. When a person skilled in the art reads the foregoing, alternatives and equivalents, within the spirit and intent of the invention, will occur to him. For example, all of the transistors and diodes of FIG. 1 could be complemented and the power supply polarity reversed. Accordingly it is intended that the scope of the invention be limited only by the following claims.