Low EMI shutdown circuit for modem applications

1. Field of the Invention

This invention relates generally to cable modem drivers, and more particularly to a low EMI shutdown driver circuit and method exhibiting very predictable and gentle cable modem start-up behavior.

2. Description of the Prior Art

Currently available technology relating to the present invention includes a Multimedia Cable Networking Standards (MCNS) compliant “external RF switch” architecture. By way of background, the Institute of Electronic and Electrical Engineering's (IEEE) 802.14 Cable TV Media Access Control and Physical Protocol Working Group was formed in 1994 to develop international standards for data communications over cable. The goal was to submit a cable modem Media Access Control and Physical Protocol standard to the IEEE in December 1995, but the delivery date slipped to late 1997.

Because of the delay in finalizing the IEEE 802.14 standard, certain cable operators, operating under a limited partnership, dubbed Multimedia Cable Network System Partners Ltd. (MCNS), proceeded to research and publish their own set of interface specifications for high-speed cable data services. MCNS released its Data Over Cable System Interface Specification (DOCSIS) for cable modem products to vendors in March 1997. Many vendors have announced plans to build products based on the MCNS DOCSIS standard.

There is a stringent requirement in the cable modem standard, DOCSIS, for cable modem drivers to transition between the power-up and power-down modes while keeping the disturbance on the line within a very tight limit. Existing solutions suppress the line glitch by using an external RF switch.

In view of the foregoing, a need exists for a bias current generator that exhibits very predictable and gentle start-up behavior and that mitigates the need for an external RF switch such as used with known solutions for suppressing line glitches.

To meet the above and other objectives, the present invention provides a bias current generator that exhibits very predictable and gentle start-up behavior. According to one embodiment, a TANH bias current generator has a ‘distributed’ output stage comprising a differential pair Q 1 /Q 2 in combination with a pair of resistors R 1 and R 2 that act as a potentiometer controlling the amount of voltage seen across the input of the differential pair. The distributed output stage has a ‘steering’ input that controls the amount of bias current flowing through each transistor (Q 1 and Q 2 ). For a given bandgap voltage, ramp voltage and slope, resistors R 1 and R 2 control the speed of transfer of bias current from transistor Q 1 to transistor Q 2 . For a linear voltage ramp, the voltage across base resistor R 1 will be approximately linear, and therefore the current transfer from Q 1 to Q 2 will take the form of a hyperbolic tangent that will allow a very gentle start-up.

According to another embodiment, a third transistor Q 1 ′ is added to provide a non-zero “power-down” current. This embodiment is used when the bias current is required to be moved between two non-zero amounts. Transistor Q 1 has an emitter area “A” and transistor Q 1 ′ has an emitter area “B”. The ratio of “power-up” to “power-down” current is set by emitter area and is equal to B/A.

In one aspect of the invention, a bias current generator is implemented that exhibits very predictable and gentle start-up behavior.

In another aspect of the invention, a bias current generator is implemented that accommodates moving a bias current between two non-zero amounts.

In yet another aspect of the invention, a bias current generator is implemented that has low EMI during enable and disable modes of operation.

In still another aspect of the invention, a bias current generator is implemented for use in cable modem drivers that are required to meet DOCSIS for cable modem products.

In still another aspect of the invention, a bias current generator is implemented for use with any circuits that are required to transition between low-power and high-power modes in a manner consistent with low EMI.

Other aspects and features of the present invention and many of the attendant advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description when considered in connection with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein:

FIG. 1 is a schematic diagram illustrating a standard bias current generator that is known in the art;

FIG. 2 is a schematic diagram illustrating a TANH bias current generator according to one embodiment of the present invention;

FIG. 3 is a diagram illustrating control, ramp and bias-out waveforms for the TANH bias current generator shown in FIG. 2;

FIG. 4 is a schematic diagram illustrating the TANH bias current generator shown in FIG. 2 modified to provide a non-zero “power-down” current; and

FIG. 5 is a schematic diagram illustrating a ramp generator suitable for use with the TANH bias current generators depicted in FIGS. 2 and 4 .

While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.

As stated herein before, there is a stringent requirement in the cable modem standard, DOCSIS, for cable modem drivers to transition between the power-up and power-down modes while keeping the line disturbance within a very tight limit. Existing solutions suppress the line glitch by using an external RF switch. Looking now at FIG. 1, a schematic diagram illustrates a standard bias current generator 10 that is known in the art. Amplifier Op 1 ( 12 ) and transistor Q 1 ( 14 ) operate to buffer bandgap voltage Vbg ( 16 ). The bandgap voltage 16 can therefore be made to appear across Rbg ( 18 ). This causes a current Vbg/Rbg ( 20 ) to flow into the emitter of Q 1 ( 14 ) and out through its collector where it appears as the output bias current.

FIG. 2 is a schematic diagram illustrating a TANH bias current generator 100 according to one embodiment of the present invention. The TANH bias current generator 100 modifies the standard bias current generator 10 by using a ‘distributed’ output stage 102 comprising transistor Q 1 , transistor Q 2 , resistor R 1 and resistor R 2 . The distributed output stage 102 has a ‘steering’ input 104 that controls the amount of the bias current (I-Bias=Vbg/Rbg) 106 flowing through each of Q 1 and Q 2 . Resistors R 1 and R 2 act as a potentiometer controlling the amount of voltage seen across the input of the differential pair Q 1 /Q 2 . For a given bandgap voltage 108 , ramp voltage 110 and slope 112 therefore, R 1 /R 2 will control the speed of transfer of bias current 106 from Q 1 to Q 2 . For a linear voltage ramp (slope) 112 , the voltage across R 1 will be approximately linear; and thus the current transfer from Q 1 to Q 2 (Bias-Out 120 ) will take the form of a hyperbolic tangent such as seen in FIG. 3 . FIG. 3 illustrates control 116 , ramp 118 and bias-out 120 waveforms for the TANH bias current generator 100 shown in FIG. 2 .

FIG. 4 is a schematic diagram illustrating another TANH bias current generator 200 modified to provide a non-zero “power-down” current. This bias current generator 200 can be used when the bias current is required to be moved between two non-zero amounts. It is used in the same manner as the TANH bias current generator 100 depicted in FIG. 2, but Q 1 ′ ( 202 ) is included to provide a non-zero “power-down” current. The ratio of “power-up” to “power-down” current is set by emitter area and is equal to B/A for the bias current generator 200 illustrated in FIG. 4 . The Bias-Out current 204 is therefore B/A*I-Bias or simply I-Bias (ignoring h fe effects).

FIG. 5 is a schematic diagram illustrating a ramp generator 300 suitable for use with the TANH bias current generators 100 , 200 depicted in FIGS. 2 and 4 respectively. The “power-down” signal 302 can be seen to be a logic signal that switches between Vcc and ground ( 0 ) voltage levels. Capacitor 304 is therefore charged by a current (Vcc−V beQ1 )/R 1 and discharged by a current (Vcc−V beQ3 /R 2 . Simplifying, V beQ1 =V beQ3 =V be , and R 1 =R 2 =R; then

dv/dt=(Vcc−V be )/RC.  (1)

The present inventors found that when this ramp generator 300 is used as the ramp generator for the bias current generators 100 , 200 illustrated in FIGS. 2 and 4 respectively, the impedance of capacitor 304 at the switching frequency must be kept considerably lower than R 2 for the voltage across R 1 to be maintained as a linear ramp.

The bias current generators 100 , 200 were found to exhibit low EMI during the enable and disable modes of operation. This feature is necessary to implement cable modem drivers or any other circuits that are required to generate low EMI when transitioning between low-power and high-power modes, such as cable modem drivers that are required to meet DOCSIS for cable modem products.

In view of the above, it can be seen the present invention presents a significant advancement in the art of cable modem driver technology. Further, this invention has been described in considerable detail in order to provide those skilled in the data communication art with the information needed to apply the novel principles and to construct and use such specialized components as are required. In view of the foregoing descriptions, it should further be apparent that the present invention represents a significant departure from the prior art in construction and operation. However, while particular embodiments of the present invention have been described herein in detail, it is to be understood that various alterations, modifications and substitutions can be made therein without departing in any way from the spirit and scope of the present invention, as defined in the claims which follow. For example, although various embodiments have been presented herein with reference to particular functional architectures and characteristics, the present inventive structures and characteristics are not necessarily limited to particular circuit architectures or sets of characteristics as used herein. It shall be understood the embodiments described herein above can easily be implemented using many diverse signal processing elements so long as the combinations achieve a bias current generator according to the inventive principles set forth herein above.