Abstract:
In the present invention a semiconductor integrated circuit is described to perform signal detection in a data communication system. The circuit is configured such that the capacitors used in high pass filter and a low pass filter are CMOS capacitors. The capacitors are formed from transistors where the gate is one terminal of the capacitor and the source and drain connected together form the second terminal of the capacitor. The source and drain that are connected together are connected to a voltage bias in the circuit which prevents the capacitors from being in a “floating” circuit configuration. The signal detection is done in one stage where a high pass filter is in the source of the input transistors and a low pass filter is in the drain of the input transistors. A comparator connects to the drain circuitry of the input transistors which supplies and offset voltage to the comparator. The input signal must be of a specific frequency to be conducted through the filters and of specific amplitude to overcome the offset. The simplicity requires far few devices than previous signal detectors and facilitates the ability to handle high frequency signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to signal detection and in particular an integrated circuit signal detector for a data communication system. 
     2. Description of Related Art 
     The detection of signals at a specific frequency with a specific signal strength is often required in a data communication system. As an example, in an Ethernet system certain signals with an amplitude greater than 200 mv are required to be detected before a link between two stations can be established. It is desirable to have this detection capability implemented in an integrated circuit using typical digital circuit process steps. 
     In U.S. Pat. No. 5,940,400 (Eastmond et al.) a method and device is directed to provide collision presence detection in wireless intensity modulated binary coded transceivers. A measurement of the degree of correlation which exists between a transmitted signal and a received signal provides the basis for collision detection. In U.S. Pat. No. 5,717,720 (Jackson et al.) is directed to digital data receivers, methods and circuitry for differentiating between signals and data packets of varying protocols and frequencies transferred over a digital burst mode communications system. U.S. Pat. No. 5,199,049 (Wilson) is directed to a digital squelch circuit for detecting valid data signals in a burst mode communication system, e.g. a packet based LAN. A counter is started in a squelch circuit and input signals are detected at various interval of the counter. If there is an input signal transition a predetermined number of times as measured by the counter, the input signal is defined as valid. 
     A typical implementation of a signal detector is shown in FIG. 1. A differential input  10  is connected to a high pass filter  11 . The high pass filter  11  comprising circuit elements C 1 , C 2 , R 1  and R 2  is connected to a first operational amplifier  12  connected in differential mode. The output of the first operational amplifier  12  is connected to a low pass filter  13  comprising circuit elements C 3 , C 4 , R 3  and R 4 . The low pass filter  13  is further connected to a second operational amplifier  14  connected in differential mode. In the output circuitry of the second operational amplifier  14  is an offset circuit  15  comprising resistors R 5  and R 6  and current sources J 1  and J 2 . The offset is determined by the current from current source J 2  flowing through R 6 . A comparator  16  is connected to the offset circuitry  15  such that a signal from second operational amplifier  14  must be larger than the offset voltage to produce a signal at the output  17  of the comparator  16 . 
     If the cutoff frequency of the high pass filter  11  is lower than the cutoff frequency of the low pass filter  13 , then a signal at the differential input  10  with a frequency between the two cut off frequencies will produce an output from the second operational amplifier  14 . If the input signal has sufficient amplitude to overcome the offset voltage produced by the offset circuitry  15 , then the comparator will produce a pulse at the output  17 . 
     A problem with the circuitry of FIG. 1 is that it is difficult to integrate the circuitry into a chip containing digital circuitry. Capacitors C 1  and C 2  are connected in a “floating” configuration where they are not directly connected to ground or a circuit bias. Using a CMOS integrated circuit processes it is not easy to implement these capacitors. Either special silicon wafer steps are required that are not a part of typical CMOS digital circuit process steps, or a big area is required to facilitate a metal layer to metal layer capacitors. A second problem results from the need for multiple stages requiring relatively complicated circuitry in each stage which increases the cost of design and manufacture but also has a tendency to limit the circuit performance at high frequency signals. 
     OBJECTS OF THE INVENTION 
     It is an object of the present invention to produce a signal detector suitable for use in communication systems that can detect signals at a specific frequency and having a specific strength. 
     It is another an object of the present invention to provide a signal detector that can be integrated into a CMOS digital integrated circuit using typical CMOS process steps and requiring a small area for implementation. 
     It is further an object of the present invention to provide a detection circuit with relatively few components and having minimal effect on the performance of processing high frequency signals. 
     SUMMARY OF THE INVENTION 
     In the present invention a single stage circuit is used to filter out signals of all frequencies except for an input signal with a specific frequency. The single stage circuit also produces an offset voltage to be used by a subsequent comparator circuit to determine the strength of the signal with the specific frequency. The single stage circuit comprises two transistors operating in parallel and receiving a differential signal. Each transistor has a high pass filter in the source circuitry and a low pass filter in the drain circuitry. The two filters are designed such that the cut off frequency of the high pass filter is below the cut off frequency of the low pass filter which allows a specific frequency from the input to be amplified through to the output. The output is derived in the drain circuitry of the two transistors in such a way that an offset voltage is presented to a comparator connected to the output. The offset voltage allows the comparator to detect a signal of a specific frequency and at a specific strength at the output of the two transistors which is higher than the offset voltage 
     The low pass and high pass filters are formed by using semiconductor capacitors. The capacitors are formed from CMOS transistors where the source and drain are connected together and the capacitance is formed from the gate to source. The gate of the CMOS capacitors is connected to signal nodes and the drain-source connection of the capacitors is connected to the source bias of the single stage circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram of a conventional signal detection circuit, 
     FIG. 2 is a circuit diagram of the signal detector circuit of the present invention, 
     FIG. 3 shows a circuit simulation of the voltage gain versus frequency for the circuit of the present invention, 
     FIGS. 4 a-c  show the input and output results from circuit simulation of the circuit of the present invention with inputs being sinusoidal waveforms of different frequencies and having the same amplitude, and 
     FIG. 5 shows the input and output results from circuit simulation of the circuit of the present invention with low amplitude inputs at a frequency within the bandpass of both the low pass and high pass filters. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 2 is shown a circuit diagram of the circuit of the present invention. The invention provides a signal detector that can detect signals at a specific frequency and having a specific strength. In FIG. 2, the output  37  produces a pulse when an input signal at VIP  30  and VIN  31  are at a specific frequency and above a specific differential amplitude with respect to each other. The specific frequency is a frequency that falls within the bandpass of both a high pass filter and a low pass filter in the circuit of the invention. 
     As shown in FIG. 2, a differential input signal comprising two signals, VIP  30  and VIN  31 , is connected to a pair of transistors, M 1  and M 2 , operating in parallel as a single circuit stage. The source of transistor M 1  is connected to transistor M 3  which is further connected to transistor M 5 . Transistors M 3  and M 5  operate as constant current sources controlled by their input voltages VB 1   32  and VB 2   33 . The source of transistor M 5  is connected to a circuit bias V SS . Also connected to the source of transistor M 1  is the gate of a transistor C 1  that forms a capacitance between its gate and source. The source and drain of transistor C 1  are connected together and further connected to circuit bias V SS . The gate to source capacitance of transistor C 1  forms a high pass filter in the source circuitry of transistor M 1 . The source of transistor M 2  is connected to transistor M 4  which is further connected to transistor M 6 . Transistors M 4  and M 6  operate as constant current sources controlled by their input voltages VB 1   32  and VB 2   33 . The source of transistor M 6  is connected to a circuit bias V SS . Also connected to the source of transistor M 2  is the gate of a transistor C 2  that forms a capacitance between its gate and source. The source and drain of transistor C 2  are connected together and further connected to circuit bias V SS . The gate to source capacitance of transistor C 2  forms a high pass filter in the source circuitry of transistor M 2 . 
     Continuing to refer to FIG. 2, the drain of transistor M 1  is connected to a resistor R 1  which is further connected to a resistor R 3  and the gate of a transistor C 3 . The juncture between R 1 , R 3  and C 3  forms a circuit output from the first half of the signal detection circuit and produces a detection circuit output signal, Vo 1 . Transistor C 3  is used to produce a semiconductor capacitor between its gate and source. The source and drain are connected together and further connected to circuit bias Vss. The gate to source capacitance of transistor C 3  in conjunction with R 3  forms a low pass filter in the drain circuitry of transistor M 1 . Resistor R 3  is further connected to a bias transistor M 7  which is used to provide power from V DD  to the signal detection circuit under the control of the voltage VPD  34 . The voltage VPD  34  is used to power off the signal detection circuit by disconnecting the voltage V DD  from resistors R 3  and R 4 . The drain of transistor M 2  is connected to a resistor R 2  which is further connected to a resistor R 4 . Resistor R 4  is further connected to transistor M 7 . The gate of a transistor C 4  used to produce a semiconductor capacitor is connected to the juncture between resistors R 2  and the drain of transistor M 2 . The source and drain of the transistor capacitor C 4  is connected to circuit bias Vss. The gate to source capacitance of transistor C 4  in conjunction with R 2  and R 4  forms a low pass filter in the drain circuitry of transistor M 2 . The connection between R 2 , C 4  and the drain of M 2  forms a circuit output from the second half of the signal detection circuit and produces a signal detection circuit output signal, Vo 2 . 
     Continuing to refer to FIG. 2, the “+” signal input of a comparator  35  is connected the gate of C 4  and the juncture between resistor R 2  and the drain of transistor M 2 , and the “−” signal input to comparator  35  is connected to the gate of transistor C 3  and the juncture of resistors R 1  and R 3 . A voltage, VPD  34 , is connected to the comparator  35  to allow the comparator to be turned off when not in use. The comparator is connected to voltage bias V DD  and voltage bias VB 3   36 . The output  37  of the comparator  35  produces a pulse when the input signal at VIP  30  and VIN  31  are at a specific frequency and above a specific differential amplitude with respect to each other. The specific frequency is a frequency that falls within the bandpass of both the high pass filter in the source circuitry of transistors M 1  and M 2  and the low pass filter in the drain circuitry of transistors M 1  and M 2 . 
     Continuing to refer to FIG. 2, a Laplace transform analysis shows that the present invention produces the same results as that of the prior art. The current flowing in one of the two transistors, M 1 , of the single stage circuitry is Igm1=Vgs1×Gm1, where Vgs1 is the gate to source voltage and Gm1 is the transconductance of the transistor M 1 . The gate to source voltage in terms of an input voltage, VIP, connected to the gate of M 1  is Vgs1=VIP−Igm1×(S×C 1 ), where C 1  is a semiconductor capacitor in the source of the transistor M 1 . The output voltage, Vo 1 , of the transistor M 1  in terms of the transistor current is Vo 1 =−Igm1/(S×C 3 +1/R 3 ), where R 3  is a resistor in the drain circuitry of M 1 , and C 3  is a semiconductor capacitor in the drain circuitry of M 1 . Substituting for Igm1 yields Vo 1 /VIP=K1×S/((S+1/(R 3 ×C 3 ))×(S+Gm1/C 1 )), where K1=−Gm1/C 3  The transfer function Vo 1 /VIN provides the same results as that obtained for the circuit of prior art shown in FIG.  1 . 
     Continuing to refer to FIG. 2, a similar Laplace transform analysis result can be obtained for the transfer function for transistor M 2 . The current flowing in transistor, M 2 , is Igm2=Vgs2×Gm2, where Vgs2 is the gate to source voltage and Gm2 is the transconductance of the transistor M 2 . The gate to source voltage in terms of an input voltage, VIN, connected to the gate of M 2  is Vgs2=VIN−Igm2×(S×C 2 ), where C 2  is a semiconductor capacitor in the source of transistor M 2 . The output voltage, Vo 2 , of transistor M 2  in terms of the transistor current is Vo 2 =−Igm2/(S×C 4 +1/R), where R=R 2 +R 4  is the resistance of the resistors in the drain circuitry of transistor M 2 , and C 4  is the semiconductor capacitance in the drain circuitry of M 2 . Substituting for Igm2 yields Vo 2 /VIN=K2×S/((S+1/(R×C 4 ))×(S+Gm2/C 2 )), where K2=−Gm2/C 4  The transfer function Vo 2 /VIN provides the same results as that obtained for transistor M 1  and the circuit of prior art shown in FIG.  1 . The time constant R×C 4  for the drain circuitry for transistor M 2  is designed to be the same as the time constant R 3 ×C 3  for the drain circuitry for transistor M 1 . The amplitude of “+” and “−” inputs of the comparator  35  can be made to be the same by adjusting the drain circuitry of M 1  and M 2 , but since the comparator inputs are differential, it is not necessary that the amplitude of the “+” and “−” input signals to the comparator have an identical amplitude. 
     Referring to FIG. 3, a gain versus frequency plot is shown obtained from circuit simulation for the signal detection circuit of the present invention. The vertical axis is gain and the horizontal axis is a logarithmic scale of frequency where e4=10 4 , e6=10 6 , e8=10 8  and e10=10 10 . For this particular plot the curve peaks at approximately 31 MHz. Output signals resulting from input signals above or below the center frequency are substantially attenuated as compared to a center frequency of approximately 31 MHz. Other center frequencies can be obtained by changing the cutoff frequency of the high pass filters in the source circuitry of transistors M 1  and M 2  and the cutoff frequency of the low pass filters in the drain circuitry of transistors M 1  and M 2 . 
     In FIG. 4 a  is shown results of an output voltage  40  at the circuit output  37  with input signals  41  and  42  at circuit inputs  30  and  31 . An approximate +1.8V offset voltage is applied to the inputs VIP and VIN along with the sinusoidal signals  41  and  42 . The input signals  41  and  42  are of a specific frequency to pass through the high pass filters in the source circuitry of transistors M 1  and M 2  and to pass through the low pass filters in the drain circuitry of transistors M 1  and M 2 . The input signals  41  and  42  have an amplitude sufficient to create a differential signal at the input to the comparator  35  sufficient to cause the comparator  35  to switch from a low state to a high state as shown with the output waveform  40 . The output waveform switches from a low voltage state to a high voltage state when the input sinusoidal signal  41  is high and input sinusoidal signal  42  is low. The width of the pulses in the output signal is determined by the amount of time that the differential voltage at the “+” and “−” inputs to the comparator  35 , caused by input signals  41  and  42 , is greater than the minimum differential voltage needed to cause the output of the comparator to switch from a low voltage state to a high voltage state. 
     In FIG. 4 b  the amplitude of the two input signals  43  and  44 , applied to the inputs VIP and VIN, are the same as signals  41  and  42 , but the frequency of the two input signals  43  and  44  is lower than the frequency of the two input signals  41  and  42  of FIG. 4 a . The output voltage  45  of the comparator  35  remains in a low voltage state because the lower frequency input signals  43  and  44  cannot get through the high pass filter with sufficient amplitude to produce a differential voltage at the input of the comparator  35  that is sufficient to switch the output of the comparator  35  from a low voltage state to a high voltage state. In FIG. 4 c  amplitude of the two input signals  46  and  47 , applied to the inputs VIP and VIN, are the same as signals  41  and  42 , but the frequency of the two input signals  46  and  47  is higher than the frequency of the two input signals  41  and  42  of FIG. 4 a . The output voltage  48  of the comparator  35  remains in a low voltage state because the higher frequency input signals  46  and  47  cannot get through the low pass filter with sufficient amplitude to produce a differential voltage at the input of the comparator  35  that is sufficient to switch the output of the comparator  35  from a low voltage state to a high voltage state. 
     In FIG. 5 is shown input signals  61  and  62  which are connected to VIP and VIN of the signal detection circuit shown in FIG.  2 . The frequency of the two input signals  61  and  62  are the same frequency as signals  41  and  42  shown in FIG. 4 a , but the differential amplitude of signals  61  and  62  smaller than the differential amplitude of signals  41  and  42 . This is emphasized by the signals appearing to be the same amplitude as signals  41  and  42 , but on a magnified vertical scale in FIG.  5 . The two input signals  61  and  62 , although at a proper frequency to pass through the low pass filters in the drain circuitry of transistors M 1  and M 2  and the high pass filters in the source circuitry of transistors M 1  and M 2 , do not have a sufficient differential voltage at the input to the comparator  35  to cause the output signal  63  of the comparator  35  to switch from a low voltage state to a high voltage state. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.