MXPA01000788A - Network amplifier monitoring circuit - Google Patents

Abstract

A monitoring circuit is provided. The monitoring circuit can be used to monitor signals in a cable network. The monitoring circuit includes first and second stages. The first stage has an input and an output. The input is coupled to an external circuit. The first stage scales a voltage received at its input. The second stage is coupled to the output of the first stage. The second stage has a high input impedance and a low output impedance. The second stage buffers a signal at the output of the first stage to an output of the second stage.

Description

NETWORK AMPLIFIER MONITORING CIRCUIT TECHNICAL FIELD OF THE INVENTION The present invention relates generally to the field of communications and, in particular, to circuits and methods for a monitoring circuit in a network amplifier.

BACKGROUND Coaxial cable networks are a common means used to distribute higher quality audio and video programming to consumers, which is typically achieved by using conventional antennas connected to a television in each user's direction. A cable network typically includes a head end that receives inputs or programming from many content providers, for example, ABC, NBC, CBS, Fox, CNN, ESPN, etc. The head end is typically connected to a distribution network that distributes head end programming to, for example, television sets of many end users. The distribution network may include coaxial cable alone, or in combination with optical fiber, or other distribution medium. The radio frequency signals that are transmitted on a portion of the coaxial cable of the distribution network tend to be attenuated as a function of the distance from the head end. This means that the radio frequency signals decrease in quality while they are more distant from the head end. Cable networks typically include network amplifiers that are selectively distributed across the network to compensate for that attenuation. These amplifiers receive and transmit the radiofrequency signals at selected points in the network so that the signals provided to each end user provide an acceptable level of quality. Network amplifiers typically include one or more "monitoring circuits" to allow the network operator to monitor the radio frequency signals that are transmitted over the network and within the amplifier. Conventionally, a monitoring circuit uses a directional coupler that is placed in the path of the radio frequency signal of the amplifier. The directional coupler includes a transformer, as is a balanced double-opening device. The directional coupler couples the radio frequency signals monitored from the radiofrequency signal path of the amplifier to, for example, a test point connector, or an input or other circuit. The monitored radiofrequency signals are scaled down by a factor selected at the test point connector, for example, 20 decibels (dB). Network amplifiers conventionally include a monitoring circuit that is associated with the input gate and a monitoring circuit that is associated with the output gate of the amplifier. In addition, the amplifier may also include other monitoring circuits that are used to provide internal feedback signals to the circuits of the network amplifier such as an automatic gain control circuit (AGC). The AGC circuit adjusts the gain level of the network amplifier so that the output is within an acceptable range. A drawback with conventional monitoring circuits is an insertion loss of maximum 1 dB in the path of the amplifier signal through the monitoring circuit. This means that the monitoring circuit reduces or attenuates the output of the radio frequency signal through the amplifier. This loss in signal strength reduces the effectiveness of the amplifier. This insertion loss is additive and applies to each monitoring circuit. Thus, a network amplifier with input, output and AGC circuits produces a 3 dB reduction in signal strength. Additionally, the monitoring circuits increase the reduction of the radiofrequency signal more. Conventional attempts to overcome the reduction of the radio frequency signal can add or complicate other problems with the amplifier. For example, Simply increasing the gain of the amplifier can be used to compensate for the 1 dB reduction caused by a monitoring circuit. However, the intermodulation distortion of the amplifier increases 2 or 3 dB per 1 dB in increasing the output level of the network amplifier. The intermodulation distortion is important for the perceived video fidelity of the composite signal in the cable network. The use of the conventional monitoring circuit at the input of a network amplifier can add up to 1 dB of noise figure to the noise figure of the preamplifier for the network amplifier. The figure of noise in operation is an important factor in determining where to place the amplifiers in a given network. An additional problem with conventional monitoring circuits is that the directional couplers used have a limited bandwidth. That is, the directional coupler can only handle signals over a limited frequency range; typically, a plain of 5 to 860 MHZ with ± 0.25 dB. Currently, cable networks plan to expand the frequency range of the channels offered to go as high as 1000 MHZ. This will make the directional coupler a limiting factor in the monitoring circuits. For the reasons stated above, and for Other reasons set forth below that will be apparent to those skilled in the art by reading and understanding the present specification, there is a need in the art for a circuit for monitoring radio frequency signals in a network amplifier with reduced insertion loss and high capacity. broadband SUMMARY OF THE INVENTION The aforementioned problems with network amplifiers and other problems are addressed by the present invention and will be understood by reading and studying the following specifications. A monitoring circuit is described that uses an active coupler to monitor signals on a network amplifier. In particular, an illustrative embodiment of the present invention includes a monitoring circuit. The monitoring circuit can be used to monitor signals in a cable network. The monitoring circuit includes a first and second stage. The first stage has an entrance and an exit. The input is coupled to an external circuit. The first stage scales a voltage that is received at its input. The second stage is coupled to the output of the first stage. The second stage has a high input impedance and a low output impedance. The second stage dampens a signal at the output of the first stage at an output of the second stage. In another modality, a monitoring circuit is provided. The monitor circuit includes a voltage divider. The voltage divider has an input to be coupled to a node of an external circuit and has an output. The voltage divider also includes a damper circuit. The damper circuit is coupled to the output of the voltage divider. The damper circuit includes a field effect transistor with a gate coupled to the output of the voltage divider. The transistor is configured as a common purge amplifier so that the source of the transmitter provides an output of the monitoring circuit. In another embodiment, a network amplifier is provided. The network amplifier includes an input gate, an output gate and an amplifier circuit. The amplifier circuit is coupled between the input gate and the output gate along a signal path of the network amplifier. At least one monitoring circuit is coupled to the signal path. The monitoring circuit includes a first stage and a second stage. The first stage has an entrance and an exit. The input of the first stage is coupled to the path of the signal. The first stage scales a voltage that is received at its input. The second stage is coupled to the output of the first stage. The second stage has a high input impedance and a low output impedance to damp a signal at the output of the first stage to an output of the second stage. In another embodiment, a method for monitoring a signal in a signal path of a cable network is provided. The method includes deriving the signal from the signal path of the cable network. The level of the derived signal is reduced and damped to provide a circuit output.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram of one embodiment of a network amplifier in accordance with the teachings of the present invention. Figure 2 is a schematic diagram of an active coupler mode for monitoring signals in a network amplifier in accordance with the teachings of the present invention.

DETAILED DESCRIPTION The following detailed description refers to the accompanying drawings that form a part of the specification. The drawings show, and the description describes, by way of illustration, the specific illustrative embodiments in which the invention can be practiced. These modalities are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be used and logical, mechanical and electrical changes may be made without departing from the scope of the present invention. The following description is not, therefore, to be taken in a limiting sense. Figure 1 is a block diagram of a mode of a network amplifier, generally indicated at 100, constructed in accordance with the teachings of the present invention. The network amplifier 100 is used to amplify signals in a network such as a cable television network. The network amplifier 100 includes many monitoring circuits 102a, 102b and 102c that monitor the signals in the amplifier 100. For example, the monitor circuit 102a monitors the signals in the main path 104 in the input gate 106 of the amplifier 100. Similarly , the monitor circuit 102c monitors signals in the main path 104 in the output gate 108. The monitoring circuit 102b monitors signals in the main path 104 and provides the signals to the automatic gain control circuit 110. The AGC circuit 110 controls the gain of the network amplifier 100 in response to the signals monitored in the path 104. Each monitoring circuit 102a, 102b and 102c uses a field effect transistor to monitor the signals in the path 104 in order to avoid problems with the conventional monitoring circuits described in detail above. The network amplifier 100 includes, in one embodiment, a fixed equalizer 107, a first amplifier 105, a variable equalizer 109 and a second amplifier 111 that are coupled in series along the path 104. It is understood that these elements in the path 104 are provided by means of illustration and not by means of limitation. Other arrays and other elements in this path can be used to appropriately amplify the signal in path 104. In addition, the number of monitoring circuits can be varied without departing from the scope of the present invention. Figure 2 is a schematic diagram of a monitoring circuit, generally indicated at 200, and constructed in accordance with the teachings of the present invention. Conveniently, the monitoring circuit 200 provides a non-invasive broadband technique for monitoring radio frequency signals in an amplifier of a cable network with substantially no insertion loss. The monitoring circuit 200 includes two main portions; namely, the voltage divider circuit 202 and the damper circuit 204. The monitoring circuit 200 is For example, in a printed circuit board with the input node (IN) coupled to the main path of the signal 206 of, for example, an amplifier circuit such as the amplifier 100 of FIG. input IN comprises an electrical connection between the path of the signal 206 and the voltage divider circuit 202. In this embodiment, the voltage divider 202 comprises a high impedance signal shunt for the monitoring circuit 200. The voltage divider circuit 202 is used to reduce the signal level at the IN input node by lowering it to an appropriate level for the equipment of monitoring, for example, -20 dB. The voltage divider circuit 202 includes a first and a second rheostat R1 and R2 which are connected in series between the IN and the ground so that the rheostats Rx and R2 form a voltage divider with an output at the node 208. The values of R1 and R2 determine the voltage division factor, K, for the voltage divider circuit 202 in accordance with the following equation: R. K i and R.

The values of the rheostats Rx and R2 are chosen so that R1 is much larger than R2. In addition, the value of coupling for voltage divider circuit 202 is calculated according to the following equation: X = 20 log (K) In order not to lower the radiofrequency signal in the input node IN, the series combinations of the resistors Rx and R2 must be at least 10 times the characteristic impedance of the radiofrequency input (path 206), for example 10. times 75 ohms. The voltage divider circuit 202 also provides transient overvoltage protection for the damper circuit 204 by reducing the current with Rx and the voltage with the K factor. Optionally, the capacitor Cx is coupled in parallel with the rheostat Rx. The circumstances in which capacitor C is used are described in detail below. The damper circuit 204 is included to reduce the charging effects of the monitoring equipment at the output of the voltage divider 202. For this purpose, the damper circuit 204 includes a high-input impedance field effect rheostat Q1. In one embodiment, the rheostat Q is a metal semiconductor field effect (MESFET) rheostat of gallium arsenide (GaAs) with a built-in static protection diode. You can use others appropriate rheostats with high input impedance instead of the gallium arsenide MESFET. To provide a high input impedance and a low output impedance, the MESFET rheostat is configured as a common purge amplifier. A gate of the rheostat Q1 is coupled to the node 208. A source of the rheostat Q is connected to the ground through the rheostat R3. The source of the rheostat Q1 is coupled to an output node (OUT) through a blocking capacitor C4. The signal at the output node (OUT) can be passed to, for example, a 75 ohm test probe or other appropriate load. The capacitors C2 and C3 are coupled in parallel between the drain of the rheostat Q1 and the ground to provide a broadband radio frequency deviation for a damper circuit 204. The inductor L ?; is a shutter to prevent the radio frequency signals of the monitoring circuit 200 from affecting the power supply, Vcc. Due to the combined parasitic capacitance of the gate (Cgs) and the static protection diode in the MESFET mode, the output of the monitoring circuit 200 at the output node may not be uniform throughout the frequency spectrum of the processed signals by the associated amplifier. The capacitor C of the voltage divider circuit 202 can be adjusted to improve the uniformity of the output of the monitor circuit 200.

In operation, the monitoring circuit 200 passes a signal from the path 206 at the input node (IN) to the output node (OUT) with a specified attenuation to monitor signals in the path of the signal 206. In one embodiment, the Attenuation is 20 dB to allow the conventional monitoring / test equipment to be coupled to an output node (OUT). The voltage divider circuit 208 divides the signal from input node IN down to an attenuated voltage at node 208. This signal is passed to output node OUT by the rheostat Qx where it is provided to a test probe or other circuit appropriate.

CONCLUSION Although the specific modalities have been described and illustrated here, those of ordinary skill in the art will notice that any arrangement programmed to achieve the same purpose could be substituted by the specific modality shown. It is intended that this application cover any adaptation or variation of the present invention. For example, the monitoring circuit can be used in many applications. For example, the monitoring circuit can be used to monitor an output signal to determine the automatic gain / decay control settings for a distribution amplifier. In addition, the monitoring circuit can be used as a monitoring point in either the input or the output of an amplifier. The monitor circuit can also be used as the point of elimination of monitoring the state of the front path to receive control signals from the head end. The monitoring circuit can be used in other appropriate applications. The blocking capacitor (C4) can be removed. In addition, the adjustable capacitor (C) of Figure 2 can be removed when the parasitic capacitance at node 208 is considered small enough.

Claims (30)

1. A monitor circuit comprising: a voltage divider having an input for coupling to a node of an external circuit and having an output, a snubber circuit, coupled to the output of the voltage divider, and wherein the snubber circuit includes a Field effect rheostat with a gate coupled to the output of the voltage divider, the rheostat configured as a common drain amplifier so that the rheostat source provides an output of the monitoring circuit.

2. The monitoring circuit of claim 1, wherein the field effect transmitter comprises a metal semiconductor field effect rheostat of gallium arsenide.

3. The monitoring circuit of claim 2, wherein the rheostat includes a static protection diode between the source and the gate of the transistor.

The monitoring circuit of claim 1, wherein the voltage divider includes an adjustable capacitor to compensate for the parasitic capacitance of the transistor.

5. The monitoring circuit of claim 1, wherein the damper circuit further includes a sealing inductor coupled to the purge of the transistor.

6. The monitoring circuit of claim 1, wherein the damper circuit includes a blocking capacitor at its output.

The monitoring circuit of claim 1, wherein the voltage divider comprises a first and a second rheostat coupled in series between an input to the voltage divider and the ground, wherein the first rheostat has a greater resistance than the second The rheostat and the sum of the first and second rheostat are at least 10 times the characteristic impedance of the external circuit.

8. A monitoring circuit, comprising: a first stage having an input and an output, the input coupled to an external circuit. the first stage scales the voltage that is received at its input, and a second stage, coupled to the output of the first stage, the second stage having a high input impedance and a low output impedance to damp a signal at the output from the first stage to an exit from the second stage.

9. The monitoring circuit of the Claim 8, wherein the first stage includes a high impedance signal shunt.

The monitoring circuit of claim 8, wherein the first stage comprises a voltage divider.

The monitoring circuit of claim 10, wherein the voltage divider comprises a first and a second rheostat coupled in series between an input to the voltage divider and the ground, wherein the first rheostat has a greater resistance than the second The rheostat and the sum of the first and second rheostat are at least 10 times the characteristic impedance of the external circuit.

The monitoring circuit of claim 8, wherein the second stage comprises a field effect rheostat with a gate coupled to the output of the first stage, the rheostat being configured as a common purge amplifier so that the source of the rheostat provides an output of the monitoring circuit.

The monitoring circuit of claim 12, wherein the field effect rheostat comprises a metal semiconductor field effect rheostat of gallium arsenide.

The monitoring circuit of claim 13, wherein the rheostat includes a static protection diode between the source and the gate of the rheostat .

The monitoring circuit of claim 14, wherein the first stage includes an adjustable capacitor to compensate for the parasitic capacitances of the field effect rheostat.

16. The monitoring circuit of claim 12, wherein the second stage further includes a seal inductor coupled to the purge of the rheostat.

The monitoring circuit of claim 8, wherein the second stage includes a blocking capacitor at its output.

18. A network amplifier comprising: an input gate. an exit gate. an amplifier circuit coupled between the input gate and the output gate along a signal path of the network amplifier, and at least one monitoring circuit coupled to the signal path, including the monitoring circuit: a first stage having an input and an output, coupled the output to the signal path. the first stage scales a voltage that is received at its input, and a second stage, coupled to the output of the first stage, the second stage that has an impedance of high input and a low output impedance to damp a signal at the output of the first stage to an output of the second stage.

The network amplifier of claim 18, wherein at least one monitoring circuit includes a monitoring circuit that is coupled to the signal path in the input gate of the network amplifier.

The network amplifier of claim 18, wherein at least one monitoring circuit includes a monitoring circuit with an input coupled to the signal path and an output coupled to an input of an automatic gain control circuit, coupled the automatic control circuit of gain to a control input of the amplifier circuit.

21. The network amplifier of the claim 18, wherein the first stage includes a high impedance signal bypass.

22. The network amplifier of claim 18, wherein the first stage comprises a voltage divider.

The network amplifier of claim 22, wherein the voltage divider comprises a first and a second resistor coupled in series between an input to the voltage divider and the ground, wherein the first rheostat has a greater resistance than the second rheostat and the sum of the first and second rheostat is at least 10 times the characteristic impedance of the external circuit.

The network amplifier of claim 18, wherein the second stage comprises a field effect rheostat with a gate coupled to the output of the first gate, the rheostat configured as a common purge amplifier so that the source of the Rheostat provides an output of the monitoring circuit.

25. The network amplifier of claim 24, wherein the field effect rheostat comprises a metal semiconductor field effect rheostat of gallium arsenide.

26. The network amplifier of the claim 25, wherein the rheostat includes a static protection diode between the source and the rheostat gate.

27. The network amplifier of the claim 26, wherein the first stage includes an adjustable capacitor to compensate for the parasitic capacitance of the rheostat.

28. A method for monitoring a signal in a signal path of a cable network, the method comprising: deriving the signal from the signal path of the cable network, reducing the level of the signal derived from the signal path , Y dampen the signal to provide it to another circuit.

The method of claim 28, wherein reducing the level of the signal derived from the signal path comprises dividing the voltage of the signal with a voltage divider circuit.

30. The method of claim 28, wherein dampening the signal comprises buffering the signal with a field effect rheostat in a common purge configuration.