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Title:
METHOD AND APPARATUS FOR HIGH-EFFICIENCY CURRENT SOURCE AND AMPLIFIER
Document Type and Number:
WIPO Patent Application WO/2016/163976
Kind Code:
A1
Abstract:
Systems and methods for providing efficient power transfer between a source (610, 604) and a load (612). An impedance imbalance is used to reduce the power dissipated in the source (610, 604) with respect to the power dissipated in the load (612). An impedance transformer (622) is used to mitigate the effects of the impedance imbalance. The impedance transformer has one port matched to the impedance of the load (612) and the other port has an impedance that is between the impedance of the load (612) and the impedance of the source (610, 604). The electrical length of the connection from the source (612) to the impedance transformer (622) is short to reduce distortion.

Inventors:
PETROVIC BRANISLAV (US)
YONGHUANG ZENG (US)
LINGAM SASIDHAR (US)
DUNCAN RALPH (US)
Application Number:
PCT/US2015/024440
Publication Date:
October 13, 2016
Filing Date:
April 06, 2015
Export Citation:
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Assignee:
ENTROPIC COMMUNICATIONS INC (US)
International Classes:
H03H7/38
Foreign References:
US20120307852A12012-12-06
US20050184831A12005-08-25
US7020452B12006-03-28
US20100226292A12010-09-09
US6735418B12004-05-11
US6492956B12002-12-10
Attorney, Agent or Firm:
WINSLADE, Christopher C. (Held and Malloy Ltd.,500 W. Madison, 34th Floo, Chicago Illinois, US)
Download PDF:
Claims:
Claims

What is claimed is:

1. A radio frequency (RF) system for delivering power efficiently to a load, the system

comprising: a) an RF source (102) having source impedance (106), the RF source generating an RF signal to be transmitted from the RF source (102); b) an RF load (110) having a load impedance (108), the load impedance (108) being substantially different than the source impedance (106), the RF load (110) receiving the transmitted RF signal; and c) an impedance transformer (112) having an output coupled to the RF load (102) and an input coupled to the RF source (102), the impedance at the output (116) of the impedance transformer (112) being equal to the impedance (108) of the RF load (110) and the impedance at the input (114) to the impedance transformer (112) being between the impedance of the RF load (108) and the RF source (102); wherein the electrical distance between the RF source (102) and the input (114) to the impedance transformer (112) is established to ensure that the signal is delivered to the load (110) without excessive distortion.

2. The RF system of Claim 1, wherein the impedance transformer (112) is a filter having a first port (114) and a second port (116).

3. The RF system of Claim 2, further comprising a balun (402) having a balanced port and an unbalanced port, the balun (402) being coupled between the filter (414) and the RF source (401), the balanced port being coupled to the RF source (401) and the unbalanced port being coupled to the first port of the filter (414).

4. The RF system of Claim 3, wherein the filter (414) is an RF bandpass filter.

5. The RF system of Claim 3, wherein the balun (402) has a 1 : 1 ratio.

6. The RF system of Claim 3, wherein the balanced port of the balun (402) has a center tap (422).

7. The RF system of Claim 6, wherein the center tap (422) of the balun (402) is coupled to a voltage referenced to the RF source (401).

8. The RF system of Claim 6, wherein the center tap (422) is coupled to a ground potential.

9. The RF system of Claim 6, wherein the center tap (422) is coupled to a voltage source within the RF source (504).

10. The RF system of Claim 6, wherein a transmission line (418) couples the second port of the filter (414) to the RF load (420), the transmission line (418) having a characteristic impedance that is equal to the load impedance.

11. The RF system of Claim 1 , further including: a) an RF receiver (606) having a receiver input impedance; and b) a receive/transmit (RX/TX) switch (608) placed between the RF source (610, 604) and the RF load (612), the RX TX switch (608) being a double pole, double throw switch having a common port, a transmit port and a receive port, each port supporting a balanced signal, the common port being coupled to the RF load (612), the transmit port being coupled to the RF source (610, 604) and the receive port being coupled to the RF receiver (606), the RX/TX switch (608) having a transmit position in which the RX/TX switch (608) couples the RF source to the RF load (612) and a receive position in which the RX/TX switch (608) couples the RF load (612) to the RF receiver (606).

12. The RF system of Claim 11, further including a balun (622) coupled between the RX/TX switch (608) and the load (612), the balun (622) having a balanced port and an unbalanced port, the balanced port being coupled to the common port of the RX/TX switch (608) and the unbalanced port being coupled to the RF load (612).

13. The RF system of Claim 12, further including an RF filter (618), the RF filter (618) having a first filter port and a second filter port, the first filter port having a first filter port impedance and the second filter port having a second filter port impedance, the RF filter (618) being coupled between the balun (622) and the RF load (612), the first filter port being coupled to the unbalanced port of the balun (622) and the second filter port being coupled to the RF load (612), the first filter port impedance being substantially greater than the source impedance and substantially less than the load impedance, the second filter port impedance being equal to the load impedance, the electrical distance between the filter and the RF source being is established to ensure that the signal is delivered to the load without excessive distortion, and the electrical distance between the filter (618) and the RF load (612) being inconsequential.

14. The RF system of Claim 13, wherein the RF load (612) is an antenna and the RF source (610, 604) is a transmitter.

15. The RF system of Claim 14, further including an integrated circuit (IC) (602), wherein the RF source (610, 604) and the RF receiver (606) are included within the IC (602).

16. The RF system of Claim 14, further including an integrated circuit (IC) (602), wherein the RF source (610, 604), the RF receiver (606) and the RX/TX switch (608) are included within the IC (602).

17. The RF system of Claim 16, wherein the IC (602) has an antenna port coupled to the common port of the RX/TX switch (608) and wherein the impedance plane of reference is at the antenna port of the IC (602).

18. The RF system of Claim 1, wherein the source (604) is a current source.

19. The RF system of Claim 11, wherein the RF load (612) is a current source when the RX/TX switch (608) is in the receive position.

Description:
METHOD AND APPARATUS FOR HIGH-EFFICIENCY CURRENT SOURCE AND

AMPLIFIER

Technical Field

[0001] The disclosed method and apparatus relates to electrical circuitry and more particularly to a system and method for efficiently transmitting power from an electrical source within and electronic device to an electrical load within the same or another electronic device.

BACKGROUND

[0002] It is common today for electrical engineers to strive to make electronic devices as power efficient as possible. Accordingly, efficiently conveying power from a power source within a first electronic device of a transmission system to a load is very desirable. For example, in electronic communication systems, it is desirable to transfer power efficiently from a transmitter to an antenna. Conventional wisdom within the electrical engineering discipline dictates that in order to maximize the power transfer, the impedance of the load should match the impedance of the source. In addition, each of the transmission lines that are used to couple signals among the components typically should have the same impedance as the source and load (unless an impedance matching network is used). This is commonly understood, since impedance mismatches cause inefficient transfer of power from source to load and also cause reflections that result in increased return loss and distortions in the signals that are conveyed between components.

[0003] The one area of electronics in which this general rule is not always adhered to is in audio circuit design. This is because the frequency of the signals handled by audio circuits is sufficient low that impedance mismatches do not result in distortion of the signals. In fact, impedance bridging circuits in which the load impedance is very high with respect to the source impedance are sometimes used in audio circuits. However, in circuits that are designed to handle relatively higher frequency signals, such as RF signals, such mismatches between the source and the load are intolerable, due to the reflections that cause distortion to the signal. This is especially true in communications systems in which data is modulated on the RF signals using quadrature amplitude modulation (QAM). In such communications systems, distortion of the RF signals causes errors in the modulated data which make it either very difficult or impossible to demodulate the data within a receiver.

[0004] Therefore, it is important in RF systems to match the impedance of the load, source and transmission lines. However, in addition to the positive aspects of matching the impedance of the source, load and transmission lines of a radio frequency RF circuit, which include reduced distortion and maximum power transfer (i.e., minimum return loss), there is a negative aspect to matching the impedance of all components. That is, half of the power generated to transmit a signal is dissipated in the transmitter itself. That is, only half the power generated by the transmitter is transmitted to the load. Power dissipated in the transmitter increases the thermal energy that needs to be handled and increases the size of transistors that need to be used, which in turn decreases the speed of the transistors.

[0005] Accordingly, there is currently a need for a method and apparatus for reducing the

amount of power dissipated in the transmitter with respect to the amount of power transmitted to the load.

SUMMARY

[0006] Various embodiments of a high efficiency current source and amplifier are disclosed. In accordance with one embodiment of the disclosed apparatus, the impedance of a source circuit in which a current source is used is designed to be substantially greater than the impedance of a load circuit. In addition, the impedance of any transmission lines between the source and the load may be substantially different from the source impedance. In an alternative embodiment, the impedance of a voltage source is made substantially lower than the impedance of a load to which voltage is supplied. In order to reduce the effect of reflections that would typically result from such a mismatch, the electrical length of a transmission line between the source and the load is made relatively short. In accordance with one embodiment, the electrical length of the transmission line is to be no longer than approximately one eighth of the wavelength of the highest frequency signal to be transmitted from the source to the load. Maintaining a short transmission line between the source and the load will reduce the amount of distortion that results from reflections that occur due to the mismatch between the impedance of the source impedance and the load.

[0007] In accordance with one embodiment of the disclosed apparatus, the particular ratio of the source to load impedance can be selected to achieve a desired balance (i.e., trade-off) between the amount of return loss and the amount of power savings that result from the mismatch between the impedance of the source and the load.

[0008] In one embodiment of the disclosed apparatus, a filter is placed at a short electrical

distance from the source. The filter is designed to mitigate the difference between the impedance of the load and the impedance of the source. The output of the filter is designed to have an impedance matched to the load. The input of the filter is designed to have an impedance that is at the geometric center of the source and load impedances. Accordingly, the amount of power that is dissipated in the source is reduced by virtue of the source having a higher impedance than the load coupled to the source (i.e,. the impedance presented by the input to the filter).

[0009] The electrical distance between the source and the input to the filter (including the

distance through a balun, if used) is made relatively short to reduce any distortion that resulting from the impedance mismatch between the input of the filter and the output of the source. In one such embodiment, the impedance at the output of the filter is matched to the load and to any transmission line that transmits signals from the filter to the load. Since the impedance of the transmission line and the load are matched, the transmission line can be as long as is practical. In this way, the source can be located a substantial distance from the load, while the amount of power dissipated in the source can still be reduced by mismatching the impedance of the source and load while limiting the amount of distortion to the signal transmitted from the source to the load.

[0010] In accordance with another embodiment of the disclosed apparatus, the transmission source has a differential output (i.e., balanced output). The load has a single-ended input (unbalanced input). A device commonly known as a balun is used to adapt the output to the input of the load. In such an embodiment, the electrical length from the output of the source through the balun and to the input of the load is held to a very short distance. In one such embodiment, the electrical distance is held to less than one eighth of the wavelength of the highest frequency to be transmitted from the source to the load.

[0011] In yet another embodiment of the disclosed apparatus in which the disclosed apparatus is implemented in a communications system, an integrated circuit (IC) transceiver includes an RX/TX switch to switch from transmit mode to receive mode. In one such embodiment, during transmit mode, the load is an antenna. In one such embodiment, the source is a transmitter within the IC transceiver. The RX/TX switch connects the IC transmitter to the antenna in transmit mode. In receive mode, the load is a receiver within the IC transceiver and the source is the antenna.

[0012] In receive mode, the RX/TX switch connects the antenna to the receiver (and disconnects the transmitter from the antenna). In one such embodiment, the transmitter has an impedance that is essentially equal to the impedance of the antenna. In contrast, the receiver has a relatively high input impedance with respect to the antenna. The impedance of both the receiver is substantially mismatched to the impedance of the antenna. A filter provides an intermediate impedance to reduce the effect of the mismatch. Alternatively, a balun is used to couple the balanced output of the transmitter in transmit mode to the unbalanced antenna circuit. In receive mode, the balun couples the receiver's balanced input to the antenna. In one such embodiment, the unbalanced port of the balun is coupled to the filter The electrical length of the transmission line from the IC transceiver to the filter is relatively short. The impedance at the side of the filter that is coupled to the IC transceiver is mismatched with both the receiver during receive mode and with the transmitter during transmit mode. The impedance at the side of the filter facing the antenna is designed to be matched to the antenna and to any transmission lines that transmit the signal from the filter to the antenna.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The disclosed method and apparatus, in accordance with one or more various

embodiments, is described with reference to the following figures. The drawings are provided for purposes of illustration only and merely depict examples of some embodiments of the disclosed method and apparatus. These drawings are provided to facilitate the reader's understanding of the disclosed method and apparatus. They should not be considered to limit the breadth, scope, or applicability of the claimed invention. It should be noted that for clarity and ease of illustration these drawings are not necessarily made to scale.

[0014] FIGURE 1 is a simplified schematic diagram of an electrical system in which power is transferred from a source to a load.

[0015] FIGURE 2 is a graph of the savings in power verses the associated return loss.

[0016] FIGURE 3 is an embodiment of an electrical system in which the source is a voltage source.

[0017] FIGURE 4 is a simplified schematic of an electrical system in accordance with one

embodiment of the disclosed method and apparatus in which the source has a balanced output and the load has an unbalanced input.

[0018] FIGURE 5 is a simplified schematic of an electrical system in which a balun is coupled to a source that employs a voltage source.

[0019] FIGURE 6 is a simplified schematic of an electrical system, such as might be used in a MoCA node or a WiFi access point.

[0020] The figures are not intended to be exhaustive or to limit the claimed invention to the

precise form disclosed. It should be understood that the disclosed method and apparatus can be practiced with modification and alteration, and that the invention should be limited only by the claims and the equivalents thereof.

DETAILED DESCRIPTION

[0021 ] FIGURE 1 is a simplified schematic diagram of an electrical system 100 in which power is transferred from a source 102 to a load 1 10. The source 102 is modeled to include a current source 104 and a source impedance 106. The nature of a current source 104 is that the current source 104 will attempt to provide the same amount of current ¾ to the load regardless of the impedance presented to the source. The current ¾ that is provided by the source 102 is split between a load impedance R L 108 and the source impedance Rs 106. The source impedance 106 and the load impedance R L 108 are shown in FIGURE 1 as resistances for the sake of simplicity in the analysis. However, each may be a complex impedance that includes inductance or capacitance. [0022] A filter 112 is coupled between the source 102 and the load 110. The filter 112 has a first port 114 that is coupled to the source 102 and a second port 116 that is coupled to an F- connector 118. In one embodiment, the filter 112 is a bandpass filter. However, the particular transfer function of the filter 112 will depend upon several implementation related factors that are outside the scope of this disclosure. The particular transfer function of the filter 112 does not significantly alter the analysis of the disclosed apparatus. However, as will be discussed in further detail below, the impedance looking into the two ports 114, 116 of the filter 112 will have a significant impact on the analysis.

[0023] The general definition for return loss is:

10 Log (Pi/P r ) Eq. 1 where Pi is the incident power and P r is the reflected power.

[0024] Accordingly, the return loss for the signal transmitted from the source to the load 110 (assuming the filter 114 to have an impedance matched to the load 110) can be calculated as:

-20 Log[(R s - R L )/( RS + RL)] Eq. 2

[0025] The return loss shown in Eq. 2 is a measure of the ratio of the amount of power that is reflected (reflected power) with respect to the amount of power that is launched forward (incident power). Accordingly, it can be seen that for the conventional system in which the load and source impedances are matched, the return loss approaches infinity dB. That is, the ratio of power provided to the load with respect to power that is reflected back is infinite (note the negative sign in Eq. 2).

[0026] When the impedance of the source is equal to the impedance of the load all of the power is provided forward and no power is reflected back to the source. However, with the value of R L 108 equal to the value of Rs 106, half the power generated by the current source 104 is dissipated in the source impedance Rs 106. That is, the current is split equally between R L 108 and Rs 106, disregarding any power that is dissipated by the filter 112, the F-connector 118 and the transmission lines that couple the source 102 to the load 110.

[0027] By altering the ratio of the source impedance 106 with respect to the load impedance 108, the amount of power that is provided to the load 110 can be increased at the expense of an increase in the return loss and at the expense of distortion to signals that are transmitted at radio frequencies (RF). For example, in one embodiment of the disclosed apparatus, the source impedance Rs 106 is set to a value of 200 Ohms and R L is set to 75 Ohms. The return loss looking from the source 102 into the load in this case is:

-20 Log[(200 - 75)/(200 + 75)] ~ 6.9 dB Eq. 2

[0028] Therefore, a significant amount of the power provided to the load 108 will be reflected back due to the impedance mismatch between R L 108 and Rs 106. Yet, the ratio of current in the load impedance and in the source impedance is (VS/R L )/(VS/RS) = RS R L = 200/75 = 2.7. Therefore, about 73% of the current will be delivered to load and only 27% of the current will flow through the source impedance R L 106. Therefore, the ratio of power that is dissipated in the load verses the power that is dissipated in the source is equal to the ratio of RS/R L - Accordingly, for a cost of a relatively high return loss, there is a savings in the amount of power that is dissipated in the source impedance. Reducing the amount of power that is dissipated in the source 102 has significant advantages in terms of the amount of heat that is dissipated in the source 102 and the size of the source 102 (i.e., the transistors used to construct the source 102). Reducing the size of the transistors required to generate the source current Is reduces the cost of the source 102, but potentially at least as important, smaller transistors have lower parasitics, and thus can switch faster allowing operation at higher frequencies.

[0029] FIGURE 2 is a graph of the savings in power versus the associated return loss. It can be seen from line 201 and line 203 that for a return loss of approximately 17 dB, the power savings is approximately 25%>. As the amount of power dissipated in the source impedance decreases, the return loss viewed from the load side will degrade; that is, the reflected power viewed from the load side will become greater. In one embodiment in which the source is providing a MoCA (Multimedia over Coax Alliance) signal to a MoCA network, the return loss is permitted to be 5 dB. By setting the return loss to a level of approximately 8 dB (providing a design margin of 3 dB) for a MoCA node transmitting MoCA signals to the network, the requirement can be meet and a reduction in the power dissipated in the source of approximately 50% can be achieved. For a WiFi access point, the load is the antenna.

Typically, a return loss of approximately 6 dB can be tolerated. Accordingly, a power savings of 65 %> can be realized. [0030] By plotting the chart provided in FIGURE 2, a designer can select the tradeoff between return loss and power savings. However, it is critical in high frequency communications systems to address the resulting distortion. The distortion due to reflections and standing waves can be minimized by designing the system to have an electrical length between the source 102 and the load 110 that is as short as possible. By having an electrical length that is less than ¼ the length of the wavelength of the highest frequency (and thus less than a ¼ the wavelength of all other frequencies), the reflections and related distortion to the signals can be reduced. The shorter the length, the less reflection will occur and so the less distortion to the signals. A trade off can be made between the amount of distortion that is acceptable, the amount of power to be saved (i.e., the reduction of power dissipated in the source 102) and the length of the connection between the source 102 and the filter 112. In one embodiment, the amount of distortion that can be tolerated is determined based on the error rate that can be accepted when demodulating data at the receiver. In addition, the filter 112 is designed to perform an impedance transformation from the impedance of the load on the one side to a higher impedance on the other side. In accordance with one embodiment, the impedance on the current source side of the filter is designed to be at the geometric center of the source impedance and the load impedance.

[0031] For a system in which the load impedance is 75 Ohms and the source impedance is 200

1/2

Ohms, the geometric center is 122.5 = (200 x 75) . Therefore, the filter is designed to have an input impedance of 122.5 Ohms and an output impedance of 75 Ohms. If the filter were terminated in 122.5 Ohms at the input, the return loss looking in from the F-connector 118 would be relatively low. However, because the filter is terminated in the source impedance of 200 Ohms, the return loss looking into the F-connector is degraded. Nonetheless, the impedance transformation of the filter 112 from 75 Ohms to 122.5 Ohms provides a compromise between improved return loss and improved power dissipation in the source.

[0032] In one embodiment, the electrical length is less than 0.1 λ (i.e., one tenth the wavelength of the highest frequency to be transmitted). In accordance with one embodiment of the disclosed method and apparatus, the filter 112 has an input port 114. The difference between the impedance at the input to the filter and the source impedance Rs 106 establishes the amount of current that will flow through the source impedance Rs 106. The impedance of the output port 116 of the filter 112 is matched to the impedance of transmission lines 120, 122 that connect the filter 112 to the load 110 through the F-type connector 118. Since the impedance of the transmission lines 120, 122 and the load are matched, the length of the transmission lines 120, 122 will not significantly affect the amount of distortion due to reflections. Therefore, the load 110 can be placed at a distance from the source 102.

[0033] In accordance with one embodiment of the disclosed method and apparatus, the current source 104 is programmable. Accordingly, the current source 104 can be programmed to optimize for the conditions, including the load impedance, the amount of power to be dissipated in the source, the amount of distortion that can be accepted due to reflections, etc. In one embodiment, the current source 104 is programmed dynamically on-line.

Alternatively, the current source 104 is configured prior to be put into service. For example, the amount of current output by the current source 104, the source impedance 106, etc.

[0034] FIGURE 3 is an embodiment of an electrical system 300 in which the source 302 is a voltage source 304 rather than a current source (i.e., the source is voltage-swing limited), the same general principles apply regarding creating an imbalance in the source and load impedance. The source impedance 306 is modeled to be in series with the voltage source 304. Therefore, making the source impedance Rs 306 lower than the load impedance R L 308 will reduce the voltage drop across the source. Having a short electrical length between the source 302 and the filter 112 will reduce the effects of reflections similar to the case discussed above with regard to the embodiment of FIGURE 1.

[0035] FIGURE 4 is a simplified schematic of an electrical system 400 in accordance with one embodiment of the disclosed method and apparatus in which the source 401 has a balanced output (differential output) and the load has an unbalanced input (single-ended input). A balanced to unbalanced impedance adapter 402 (commonly referred to as a "balun") is coupled between a filter 414 and the source 401. In one embodiment of the disclosed method and apparatus, the balun has a 1 : 1 impedance transformation ratio. The source impedance 410 is greater than the load impedance R L 420 to reduce the amount of current drawn through the source impedance and thus allow a greater percentage of the current to flow through the load. The impedance of a transmission line 418 and the load 420 are equal. The electrical length from the source 401 to the filter input 413 is made sufficiently short to reduce the distortion due to reflections of the signal. As noted above, any length that is less than ¼ wavelength will reduce the amount of reflection. In accordance with one embodiment of the disclosed method and apparatus, an electrical length of 0.1 λ is used to provide an acceptable amount of reflection and resulting distortion. However, as noted above with respect to the embodiment of FIGURE 1, the length can be set based on the amount of distortion that is acceptable and the amount of the imbalance in the impedance of the source 401 with respect to the load 420.

[0036] In one embodiment of the disclosed method and apparatus, the input to the balun 402 is center tapped to allow voltage swings that can exceed the voltage of the power supply within the source 401. In an ideal center tapped balun, the voltage at the output can be twice the voltage provided from the power supply (i.e., the rail voltage). However, in practical circuits, the voltage at the output of the balun will more typically be approximately 1.6 to 1.8 times greater than the rail voltage.

[0037] In an alternative embodiment of the disclosed method and apparatus, the balun can either step up the source impedance or step down the source impedance, depending upon whether the source is a voltage source or current source. When the source 401 employs a current source 408, the source impedance Rs is made larger to ensure that more current flows to the load 420 and less current is dissipated in the source 401. By using a balun that steps up the impedance at the balanced input 404/406, the impedance presented to the source 401 can be increased while maintaining a relatively lower impedance at the load 420. In accordance with one embodiment of the disclosed method and apparatus, the balun presents an impedance to the source that is at the geometric center of the source impedance and the load impedance. For example, if the source impedance is 200 Ohms and the load impedance is 75 Ohms, the balanced input to the balun is set to 122.5 Ohm (i.e., the geometric center between 200 and 75). The impedance at the unbalanced output 412 of the balun 402 is matched to the impedance of the filter 414, the transmission line 418 and the load 420.

[0038] In yet another alternative embodiment, the impedance of the input port 413 of the filter, the output port 415 of the filter and the impedance transformation ratio of the balun 402 can be coordinated to provide the desired trade-off between current dissipated in the source 401, distortion to the signal due to reflections caused by the mismatch between the source impedance and either the impedance of the balun 402 or filter 414 and the desired load impedance. That is, the impedance of the output port 415 of the filter 414 will be matched to the impedance of the load 420. However, depending upon the impedance of the source and the impedance transformation ratio of the balun 402, the impedance at the input port 413 of the filter can be set to provide the desired amount of imbalance with the source impedance 410. In accordance with one embodiment of the disclosed method and apparatus, the impedance presented to the source 401 is equal to the geometric center between the impedance of the load 420 and the source impedance 410.

[0039] In accordance with one embodiment of the disclosed method and apparatus, the

transmission line 418 can be a coaxial cable, strip-line or micro-strip conductor printed on a printed circuit board or any other transmission line with a characteristic impedance that is matched to the load impedance 420. Therefore, the filter 414 can be at a distance from the load 420 with minimal additional distortion to the signal.

[0040] FIGURE 5 is a simplified schematic of an electrical system 500 in which a balun 506 is coupled to a source 502 that employs a voltage source 504. In this case, the source impedance 508 will be lower than the load impedance 510 to allow the voltage drop across the source impedance to be smaller than the voltage delivered to the load 510. The impedance of the filter 510, and the impedance transformation ratio of the balun 506 will be set to provide the desired imbalance between the source impedance 508 and the load impedance 510. In accordance with one embodiment, the impedance presented to the output of the source 401 is the square root of the product of the load impedance 420 and source impedance 410 (i.e., the geometric center). It should be noted that while not shown in FIGURE 5, the balun 506 can be center tapped in the same manner as shown in FIGURE 4.

[0041] FIGURE 6 is a simplified schematic of an electrical system 600, such as might be used in a MoCA node or a WiFi access point. In accordance with one embodiment of the disclosed method and apparatus, the system 600 has an integrated circuit 602 that includes a current source 604 with an associated source impedance 610 modeled as a resistance for the sake of simplicity. However, the impedance could be complex, as noted with regard to the source impedance of the embodiment of FIGURE 1. In accordance with one embodiment of the disclosed method and apparatus, the current source is a power digital to analog converter (PDAC). The integrated circuit 602 further includes a receiver, such as a low noise amplifier (LNA) 606. The current source 604 and the LNA 606 are coupled to a balun through a transmit/receive (Rx/Tx) switch 608. The Rx/Tx switch 608 is a double pole, double throw (DPDT) switch that allows the system 600 to operate in a transmit mode when the switch 608 is in a transmit position. A common port is connected to the balanced port of a balun 622. The common port is connected through an antenna port on the integrated circuit 602. A transmit port of the switch 608 is coupled to the current source 604. A receive port of the switch 608 is coupled to the input to the LNA 606. In transmit position, the current source 604 is connected to the balanced port of the balun 622 by the switch 608. Alternatively, the system operates in a receive mode when the switch 608 is in a receive position. In the receive position, the switch 608 connects the balanced input of the LNA 606 to the balanced port of the balun 622.

[0042] In accordance with one embodiment of the disclosed method and apparatus, the

impedance of the current source 604 is greater than a load impedance R L 612 to which the current source 604 is providing signals during transmit mode. In one embodiment in which the system 600 is a WiFi access node, the load 612 is an antenna. The load 612 is coupled to a filter 618 through a transmission line 614. In one such embodiment, the impedance of the antenna side port 620 of the filter 618 is designed to be equal to the characteristic impedance of the transmission line 614 and the load impedance 612 of the antenna. The filter port 621 on the integrated circuit side of the filter 618 has an impedance that, when transformed by the impedance transformation ratio of the balun 622, will be at the geometric center between the impedance of the input of the LNA 606 and the source impedance 610. For example, in one embodiment, the source impedance 610 is 75 Ohms and the load impedance 612 is 200

1/2

Ohms. The geometric center between 75 and 200 is (75 x 200) ~ 122.5. By setting the impedance that is presented to the integrated circuit at 122.5 Ohms, during transmit mode, there will be a mismatch between the source impedance 610 and the load presented to the source. Looking from the F-connector, the return loss will be approximately 12.4 dB (as can be seen by applying Eq.l : -201og(122.5-75)/(122.5 + 75) ~ 12.4 dB). A resulting power savings of approximately 40% will be realized.

[0043] In the receive direction, the LNA impedance may be optimized based on a different criteria, for example to optimize the Noise Figure, and as such may be different than the transmit mode impedance. In one embodiment of the disclosed method and apparatus the input impedance of the LNA 606 is matched to the impedance at the IC side 621 of the filter 618. However, due to design considerations associated with the LNA 606, it may not be possible to match the input impedance to the filter impedance. Some of the considerations that might affect the input impedance of the LNA include the amount of bias current required within the LNA 606, the amount of feedback required to achieve the gain required, the required linearity of the LNA 606 and whether the LNA is a purchased component.

[0044] In accordance with one embodiment of the disclosed method and apparatus, transmission lines with a characteristic impedance matched to the current source 604 are used to conduct the signal from the current source 604 to the antenna port of the IC. Using a matched transmission line places the plane of reference for the current source 604 at the antenna port of the IC. Therefore, when determining the electrical length from the current source 604 to the filter 621, the measurement is taken from the antenna port of the IC.

[0045] In accordance with one embodiment of the disclosed method and apparatus in which the signal being transmitted is modulated using orthogonal frequency division multiplexing (OFDM), the levels of individual OFDM subcarriers are adjusted to flatten the response at the output. That is, the power of an OFDM modulated signal is divided in to a plurality of OFDM channels. The amount of power used in each channel can be adjusted to achieve a flatter frequency response for the overall signal.

[0046] Although the disclosed method and apparatus is described above in terms of various examples of embodiments and implementations, it should be understood that the particular features, aspects and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described. Thus, the breadth and scope of the claimed invention should not be limited by any of the examples provided in describing the above disclosed embodiments.

[0047] Terms and phrases used in this document, and variations thereof, unless otherwise

expressly stated, should be construed as open ended as opposed to limiting. As examples of the foregoing: the term "including" should be read as meaning "including, without limitation" or the like; the term "example" is used to provide examples of instances of the item in discussion, not an exhaustive or limiting list thereof; the terms "a" or "an" should be read as meaning "at least one," "one or more" or the like; and adjectives such as "conventional," "traditional," "normal," "standard," "known" and terms of similar meaning should not be construed as limiting the item described to a given time period or to an item available as of a given time, but instead should be read to encompass conventional, traditional, normal, or standard technologies that may be available or known now or at any time in the future. Likewise, where this document refers to technologies that would be apparent or known to one of ordinary skill in the art, such technologies encompass those apparent or known to the skilled artisan now or at any time in the future.

[0048] A group of items linked with the conjunction "and" should not be read as requiring that each and every one of those items be present in the grouping, but rather should be read as "and/or" unless expressly stated otherwise. Similarly, a group of items linked with the conjunction "or" should not be read as requiring mutual exclusivity among that group, but rather should also be read as "and/or" unless expressly stated otherwise. Furthermore, although items, elements or components of the disclosed method and apparatus may be described or claimed in the singular, the plural is contemplated to be within the scope thereof unless limitation to the singular is explicitly stated.

[0049] The presence of broadening words and phrases such as "one or more," "at least," "but not limited to" or other like phrases in some instances shall not be read to mean that the narrower case is intended or required in instances where such broadening phrases may be absent. The use of the term "module" does not imply that the components or functionality described or claimed as part of the module are all configured in a common package. Indeed, any or all of the various components of a module, whether control logic or other components, can be combined in a single package or separately maintained and can further be distributed in multiple groupings or packages or across multiple locations.

[0050] Additionally, the various embodiments set forth herein are described with the aid of block diagrams, flow charts and other illustrations. As will become apparent to one of ordinary skill in the art after reading this document, the illustrated embodiments and their various alternatives can be implemented without confinement to the illustrated examples. For example, block diagrams and their accompanying description should not be construed as mandating a particular architecture or configuration.