U.S. patent application number 12/111154 was filed with the patent office on 2008-10-30 for rf backscatter transmission with zero dc power consumption.
This patent application is currently assigned to AL TIERRE CORPORATION. Invention is credited to Anurag Goel, Mark Douglas McDonald, Sunit Saxena.
Application Number | 20080266022 12/111154 |
Document ID | / |
Family ID | 34714764 |
Filed Date | 2008-10-30 |
United States Patent
Application |
20080266022 |
Kind Code |
A1 |
McDonald; Mark Douglas ; et
al. |
October 30, 2008 |
RF BACKSCATTER TRANSMISSION WITH ZERO DC POWER CONSUMPTION
Abstract
A method for minimizing power consumption in a wireless device
which utilizes backscatter transmission in half-duplex mode,
wherein a switching device is interposed between an antenna and a
transmitter-receiver, and the switching device is capable of
causing the antenna load impedance characteristic to be either a
short, a value which substantially matches the antenna impedance,
or an open, depending on the portion of the half-duplex mode.
Inventors: |
McDonald; Mark Douglas;
(Campbell, CA) ; Saxena; Sunit; (Monte Sereno,
CA) ; Goel; Anurag; (Pleasanton, CA) |
Correspondence
Address: |
DLA PIPER US LLP
2000 UNIVERSITY AVENUE
E. PALO ALTO
CA
94303-2248
US
|
Assignee: |
AL TIERRE CORPORATION
San Jose
CA
|
Family ID: |
34714764 |
Appl. No.: |
12/111154 |
Filed: |
April 28, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
11019494 |
Dec 20, 2004 |
7369019 |
|
|
12111154 |
|
|
|
|
60530819 |
Dec 18, 2003 |
|
|
|
60530818 |
Dec 18, 2003 |
|
|
|
60530817 |
Dec 18, 2003 |
|
|
|
60530816 |
Dec 18, 2003 |
|
|
|
60530795 |
Dec 18, 2003 |
|
|
|
60530790 |
Dec 18, 2003 |
|
|
|
60530783 |
Dec 18, 2003 |
|
|
|
60530823 |
Dec 18, 2003 |
|
|
|
60530784 |
Dec 18, 2003 |
|
|
|
60530782 |
Dec 18, 2003 |
|
|
|
Current U.S.
Class: |
333/32 |
Current CPC
Class: |
G06K 19/0723 20130101;
G09G 2380/04 20130101; G06K 19/07703 20130101; G06K 19/0712
20130101; G06K 19/0707 20130101; G09G 3/001 20130101 |
Class at
Publication: |
333/32 |
International
Class: |
H03H 7/38 20060101
H03H007/38 |
Claims
1. In an transmitter-receiver circuit capable of half-duplex
communication wherein the transmitter-receiver is connected to an
antenna, a method of minimizing power consumption comprising
setting, during a listening stage of a half-duplex cycle, an
antenna load impedance characteristic to match the impedance of the
antenna, varying, during a transmitting stage of a half-duplex
cycle, the antenna load impedance characteristic between a short
impedance characteristic and a matching impedance
characteristic.
2. The method of claim 1 wherein the varying step comprises
switching impedances into and out of the circuit to vary the
impedance.
3. The method of claim 2 wherein the switching step involving
applying a voltage to the gate of a FET.
4. A transmitter-receiver circuit capable of half-duplex
communication comprising an antenna having an impedance, a
half-duplex transmitter-receiver, and switching logic connected
between the antenna and the transmitter-receiver, the switching
logic adapted to establish an antenna load impedance characteristic
which matches the antenna impedance during a listening portion of a
half-duplex cycle, and, during the transmit portion of the
half-duplex cycle, to vary the antenna load impedance
characteristic between a short and a match to the antenna
impedance.
5. A transmitter-receiver circuit capable of half-duplex
communication comprising an antenna, a transmitter-receiver capable
of operating in half-duplex mode at a baseband having a phase and a
magnitude, and logic for varying the magnitude and the phase of the
baseband as applied to the antenna.
6. The transmitter-receiver circuit of claim 5 further including
switching logic connected between the antenna and the
transmitter-receiver for varying the antenna load impedance
characteristic in accordance with the portion of the half-duplex
cycle.
Description
RELATED APPLICATIONS
[0001] The present invention is a continuation of Ser. No.
11/019,494, filed on Dec. 20, 2004, which in turn claims the
benefit of priority from the following United States provisional
applications: U.S. patent Ser. No. 60/530,819 filed Dec. 18, 2003
entitled "Wireless Display Tag (WDT) Using Amplified Backscatter";
U.S. patent Ser. No. 60/530,818 filed Dec. 18, 2003 entitled
`Wireless Display Tag (WDT) Using an Active Transmitter"; U.S.
patent Ser. No. 60/530,817 filed Dec. 18, 2003 entitled "Wireless
Display Tag (WDT) Using an Active Receiver; U.S. patent Ser. No.
601530,816 filed Dec. 18, 2003 entitled "Wireless Display Tag (WDT)
Using an Active Transmitter and Diode Receiver"; U.S. patent Ser.
No. 60/530,795 filed Dec. 18, 2003 entitled "Wireless Display Tag
(WDT) Using Active and Backscatter Transceivers"; U.S. patent Ser.
No. 60/530,790 filed Dec. 18, 2003 entitled "Wireless Display Tag
(WDT) Unit": U.S. patent Ser. No. 60/530,783 filed Dec. 18, 2003
entitled "RF Backscatter Transmission with Zero DC-Power
Consumption"; U.S. patent Ser. No. 60/530,823 filed Dec. 18, 2003
entitled "Wireless Display Tag (WDT) Initialization; U.S. patent
Ser. No. 60/530,784 filed Dec. 18, 2003 entitled "Wireless Display
Tag (WDT) with Environmental Sensors": U.S. patent Ser. No.
60/530,782 filed Dec. 18, 2003 entitled "High Readability Display
for a Wireless Display Tag (WDT)" This application is also related
to the following US utility applications filed simultaneously
herewith: U.S. patent Ser. No. 11/019,660 Filed Dec. 20, 2004
entitled "Error Free Method for Wireless Display Tag (WDT)
Initialization"; U.S. patent Ser. No. 11/019,976 filed Dec. 20,
2004 entitled "Wireless Display Tag (WDT) Using Active Backscatter
and Transceivers"; U.S. patent Ser. No. 11/019,978 filed Dec. 20,
2004 entitled "Wireless Display Tag (WDT) Unit"; U.S. patent Ser.
No. 11/019,916 filed Dec. 20, 2004 entitled "Multi User Wireless
Display Tag (WDT) Infrastructure and Methods"; and U.S. patent Ser.
No. 11/019,705 filed Dec. 20, 2004 entitled "Low Power Wireless
Display Tag (WDT) Systems and Methods".
BACKGROUND OF THE INVENTION
[0002] Backscatter transmission is a radio technique whereby
signals are sent with typically lower power consumption than
comparative techniques. The system requires a Radio Frequency (RF)
source, an antenna, a receiver, and a transmitter. Most radio
systems include a transmitter and a receiver, both of which are
coupled to a logic circuit. The source sends a radio wave over the
air using the transmitter. The radio wave propagates from the
transmitter's antenna to the receiver's antenna. The impedance
terminating the receiver/transmitter's antenna can be in one of
three general states: open, short, or the same impedance as the
antenna's characteristic impedance. When the impedance
characteristic of the antenna matches the characteristic input
impedance of the antenna load, then the impedance is considered to
be "the same" as the terms is used herein.
[0003] Referring now to FIG. 1, an antenna 10 is shown having a
termination impedance characteristic that is representative of an
open circuit or high impedance. Accordingly, the signal, having a
specific electromagnetic wave property, propagates without
change.
[0004] Referring now to FIG. 2, the antenna 10 is shown employed in
a system 20 that has a characteristic termination impedance equal
to the characteristic impedance of the antenna 10. Accordingly, the
power reflected from the antenna is equal to the power absorbed.
The characteristic impedance is created electronically by allowing
a controlled current to flow through a diode 22. The impedance is
then set to the desired value in response to the amount of direct
current. Z.sub.o, the characteristic impedance, is set by the diode
current as set forth in equation (1):
Z o = 1 g m = KT q I DC ##EQU00001## [0005] K=Boltzman's constant
[0006] T=temperature in degrees K [0007] q=electronic charge
[0008] Referring now to FIG. 3, the antenna 10 is employed in a
system 30 having a characteristic terminating impedance
representative of a short or low impedance. Accordingly, the power
reflected from the antenna 10 is approximately four times the
reflected power value when connected to a system having a
characteristic impedance that is the same as the antenna's
characteristic impedance. The short is created with a significant
amount of current flowing from IDC through the diode 32. The exact
value of the short can be described and determined using equation
(1) above.
[0009] A radio that uses the current art of backscatter requires
that direct current be used to create the characteristic impedance
and the short circuit. Such systems use power that shortens the
battery life and generated a great deal of heat, which becomes a
problem in design trends that dictate smaller and more compact
components. Compact designs typically call for smaller batteries
and reduced heat generation. Thus, what is needed is a system and
method that minimizes, or even eliminates, current consumption in
order to maximize battery life and reduce heat generation.
SUMMARY OF THE INVENTION
[0010] Accordingly, a system and method are disclosed that minimize
and even eliminate direct current demands and consumption in order
to maximize battery life and reduce heat generation. This invention
varies the load impedance on the antenna by electronically
connecting either fixed impedances or impedances created using a
FET. This is in contrast to the prior art where the impedance was
created by changing current value in a device.
[0011] An advantage of the present invention is that the system has
low power consumption and, hence, low heat generation. Thus, the
system is capable of operating with minimum drain on the system
battery.
DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 shows a prior art figure of an antenna with an open
or high termination impedance characteristic;
[0013] FIG. 2 shows a prior art figure of an antenna with a
matching or characteristic termination impedance;
[0014] FIG. 3 shows a prior art figure of an antenna with a short
or low termination impedance characteristic;
[0015] FIG. 4 shows a radio communication system in accordance with
the present invention;
[0016] FIG. 5a shows a block diagram representation of a radio
transceiver in accordance with the present invention;
[0017] FIG. 5b shows a graph of radio transmission vs. time in
accordance with the present invention;
[0018] FIG. 5c is a flow chart for a radio communication system in
accordance with the present invention;
[0019] FIG. 6 shows a transmitter portion of the radio transceiver
of FIG. 5 with an open or a high, or low impedance characteristic
in accordance with the present invention;
[0020] FIG. 7 shows a transmitter portion of the radio transceiver
of FIG. 5 with a matching characteristic or open impedance in
accordance with the present invention;
[0021] FIG. 8 shows a transmitter portion of the radio transceiver
of FIG. 5 with a matching impedance characteristic or open
impedance in accordance with the present invention;
[0022] FIG. 9 shows a transmitter portion of the radio transceiver
of FIG. 5 with a combined implementation of characteristic
impedance, short or open impedance in accordance with the present
invention;
[0023] FIG. 10 shows a transmitter portion of the radio transceiver
of FIG. 5 with a combined implementation of characteristic
impedance, short, or open impedance in accordance with the present
invention;
[0024] FIG. 11 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with a short or low
impedance characteristic or open impedance in accordance with the
present invention;
[0025] FIG. 12 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with a short or low
impedance characteristic or open impedance in accordance with the
present invention;
[0026] FIG. 13 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with an open or matched
characteristic impedance characteristic in accordance with the
present invention;
[0027] FIG. 14 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with an open or matched
characteristic impedance characteristic in accordance with the
present invention;
[0028] FIG. 15 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with an open, shorted, or
matched characteristic impedance characteristic in accordance with
the present invention;
[0029] FIG. 16 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement-mode CMOS with an open, shorted, or
matched characteristic impedance characteristic in accordance with
the present invention;
[0030] FIG. 17 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement and depletion mode CMOS with a
short or low impedance characteristic, or open impedance in
accordance with the present invention;
[0031] FIG. 18 shows a transmitter portion of the radio transceiver
of FIG. 5 having a enhancement and depletion mode CMOS with a
matching or open impedance characteristic in accordance with the
present invention;
[0032] FIG. 19 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement and depletion mode CMOS with a
short or low impedance characteristic, or open impedance in
accordance with the present invention; and
[0033] FIG. 20 shows a transmitter portion of the radio transceiver
of FIG. 5 having an enhancement and depletion mode CMOS with a
matching or open impedance characteristic, or open impedance in
accordance with the present invention.
DESCRIPTION OF THE INVENTION
[0034] Referring now to FIGS. 4 and 5a, a system 40 is shown with
radio communication occurring between radio 42, which in some
embodiments may be a wireless device adapted to fit within the
C-channel of a shelf display, and an access point or wireless
terminal 50 in accordance with the teachings of the present
invention. Each radio 42 includes a receiver 52 and a transmitter
54, as shown in FIG. 5. As disclosed in U.S. patent Ser. No.
11/019,660 Filed Dec. 20, 2004 entitled "Error Free Method for
Wireless Display Tag (WDT) Initialization"; U.S. patent Ser. No.
11/019,976 filed Dec. 20, 2004 entitled "Wireless Display Tag (WDT)
Using Active Backscatter and Transceivers"; U.S. patent Ser. No.
11/019,978 filed Dec. 20, 2004 entitled "Wireless Display Tag (WDT)
Unit"; U.S. patent Ser. No. 11/019,916 filed Dec. 20, 2004 entitled
"Multi User Wireless Display Tag (WDT) Infrastructure and Methods";
and U.S. patent Ser. No. 11/019,705 filed Dec. 20, 2004 entitled
"Low Power Wireless Display Tag (WDT) Systems and Methods"; all of
which are incorporated herein by reference, the radio can include
an active transceiver and coupled with a backscatter
transceiver.
[0035] In a half-duplex environment, with respect to the operation
of the radio 42, during the listening stage of the communication
cycle, receiver 52 takes the incoming radio information from an
antenna 56 and processes the information in a manner that a digital
logic unit 58 can utilize. During the transmission stage, as
discussed in detail below, the transmitter 54 varies the
characteristic impedance of the antenna load that is coupled to the
antenna 56 in correspondence to the information that is being
transmitted from the radio 42.
[0036] Referring now to FIG. 5b, during the listening stage of the
communication, labeled t1, the wireless terminal 50 transmits data
to the radio 42. The radio 42 sets the antenna load impedance
characteristic to match the impedance of the antenna 56. During the
transmission stage, labeled t2, the radio 42 transmits data by
varying the antenna load impedance characteristic between a short
impedance characteristic and matching impedance characteristic.
[0037] Under ideal conditions, there is no DC current flow into the
gate or control node of the FET. In order to simulate a digital
transmission the load impedance is switched between short and
matching load impedance. On the other hand, in order to operate in
an analog environment, then the load impedance can vary in the
range between short impedance, matching, and open impedance. In an
alternative embodiment, the phase and magnitude of the baseband can
be altered instead of or in addition to alteration of the antenna
load impedance characteristic.
[0038] Thus, as detailed above, the transmitter 54 takes data or
information from the digital logic unit 58 and processes the
information so that the information can be sent wirelessly via the
antenna 56 using radio waves. The receiver 52 and transmitter 54
are made primarily with analog circuits. In contrast, the digital
logic unit 58 is made with digital circuits.
[0039] In the various embodiments that follow, N-channel
enhancement mode devices are shown due to the popularity of their
use; however, in alternative embodiments, N-channel, P-channel,
enhancement, or depletion mode Field Effect Transistors (FETs) can
be used. Additionally, CMOS FETs are shown due to their popularity.
However, other types of FETs or IgFETs can be used, such as
MOSFETs, JFETs, and other types. Different FET technologies can be
used besides Silicon, such as GaAs, InGaAs, SOI, plastic
transistors, and others.
[0040] In order to achieve the desired impedance levels various
systems and methods can be utilized. For example, in one
embodiment, the FETs are used as low-impedance switches to switch
in and out the desired impedances. In another embodiment, the FET's
channel impedance is designed to be the desired impedance in order
to eliminate the resistor.
[0041] Furthermore, in another embodiment, at least one FET can be
used as low-impedance switches to switch in and out the desired
impedances along with another FET, wherein the channel impedance is
designed to be the desired impedance, which would eliminate the
resistor. This embodiment can produce either a short or an open
characteristic impedance, as desired, by appropriately turning on
or off the FET.
[0042] An enhancement-mode NMOS FET is turned on by raising the
gate or control voltage above the source voltage by at least
v.sub.t, which is the threshold voltage for the particular FET. On
the other hand, the enhancement-mode NMOS FET is turned off when
the voltage difference between the gate and source is less than
v.sub.t. The same is true for a depletion-mode PMOS. The reverse is
true for both depletion-mode NMOS and enhancement-mode PMOS.
[0043] In alternative embodiments, the FET characteristics are
different if the device is operated in triode (linear) mode or
saturated mode. In an embodiment where the device is operated in a
saturated mode, then the ideal device would have constant-current
characteristics.
[0044] Referring now to FIG. 5c, the process of determining the
communication mode between the radio and terminal begins at step
500. At step 502, communication between the radio and the terminal
is initiated. At step 504, if the terminal initiated the
communication, then the terminal sends an indicator signal to the
radio at step 506; if not, then the process moves to step 510, as
discussed below. At step 508 it is determined if the indicator
signal transmitted to the radio from the terminal is an indicator
to communicate in backscatter mode. If the indicator signal is an
indication to communicated in back scatter mode, then at step 510
it is determined if the radio can transmit using backscatter; if
not, then the radio selects active mode transmission at step 516,
as discussed below.
[0045] If the radio can transmit using backscatter mode, then at
step 512 the radio selects to transmit in backscatter mode. At step
514, the radio uses backscatter mode to transmit or send
information to a nearby device, such as the terminal. At step 520,
if the transmission from the radio is complete, then the process
ends at step 522; otherwise the process returns to step 510 to
determine if the radio can continue to transmit using backscatter.
If at step 510 it is determined that the radio can not transmit in
backscatter, then at step 516 the radio selects active mode and at
step 518 the radio uses active transmission to send information to
the terminal.
[0046] With respect to FIGS. 6, 7, 8, 9, and 10 that follow, the
embodiments contemplate systems deployed in environments wherein
the signal has low voltage or small radio signals are present.
Thus, the system is operating in the triode mode region of the
current-voltage (I-V) characteristics of inherently small-signal
operation. In this mode, the channel resistance, which is the
small-signal resistance between the source and the drain of the FET
is approximately linear. The operation is over two
diagonally-opposed quadrants of operation that is defined by a
near-linear I-V characteristic response.
[0047] With respect to FIGS. 11, 12, 13, 14, 15, 16, 17, 18, 19 and
20, alternative embodiments are shown with the system operating in
an environment wherein the signal has high voltage. Thus, if the
FET device is large enough with relatively low resistance, this
mode approximates a low impedance characteristic or a short circuit
and this is large-signal operation. The operation of the FET and
its I-V characteristic curve is non-linear and operates in one
quadrant of the I-V characteristic.
[0048] Referring now to FIG. 6, a system 60 is an embodiment
wherein the antenna load characteristic impedance, which is
measured relative to the impedance characteristic of an antenna 62,
can be varied or switched from short to matching to open using a
Field Effect Transistor (FET) 64. In the system 60, the antenna 62
is coupled to the FET 64. The FET 64 is coupled to and controlled
by control signals from a control unit 66. When an open impedance
characteristic is desired, the control signal is connected to
ground, turning off the FET 64. When a short or low impedance
characteristic is desired, then the control signal is set high
turning on the FET 64, thereby shorting the antenna 62 to
ground.
[0049] Referring now to FIG. 7, a system 70 is shown wherein the
characteristic impedance is created with a FET 72 and a resistor
74. The FET 72 is designed to have a low source-to-drain impedance.
The resistor 74 is connected between the source of the FET 72 and
ground. The value of the resistor 74 is equal to the characteristic
impedance of an antenna 76. When the characteristic impedance,
which is the load characteristic impedance that matches the
characteristic impedance of the antenna 76, is desired the control
signal voltage from the control unit 78 is set to high voltage.
Otherwise, the control is set to low voltage.
[0050] Referring now to FIG. 8, a system 80 is shown with an FET 82
coupled to an antenna 86 and a control unit 88 for generating
control signals. The FET's characteristic impedance can be chosen
to be equal to the desired characteristic impedance, which is the
same as the impedance of the antenna 86. Accordingly, when the
characteristic impedance is desired, the control signal from a
control unit 88 is set high. Otherwise, the control signal from the
control unit 88 is set low.
[0051] Referring now to FIG. 9, a system 90 is shown with an
antenna 92 coupled to an FET 94 and an FET 96. The FET 94 is
coupled to a control unit 95 and the FET 96 is coupled to a control
unit 97. When an open or high impedance characteristic is desired,
the control signals from the control units 95 and 97 are low.
Alternatively, when a short or low impedance characteristic is
desired, the control signal from the control unit 97 is set to high
voltage and the control signal from the control unit 95 is set to
low voltage. If a characteristic impedance is desired, other than
an open or short, high or low characteristic impedance
respectively, then the control signal from the control unit 97 is
set to low voltage and the control signal from the control unit 95
is set to high voltage. In an alternative embodiment, a digital
logic circuit can be implemented if desired using a similar
approach.
[0052] Referring now to FIG. 10, a system 100 is shown with an
antenna 102 coupled to an FET 104 and an FET 106. The FET 104 and
the FET 106 receive control signals from the control units 105 and
107, respectively. When an open or high impedance characteristic is
desired, the control signals from the both the control units 105
and 107 are low. When a short or low impedance characteristic is
desired, the control signal from the control unit 107 is high, and
the control signal from the control unit 105 is low. On the other
hand, when a characteristic impedance is desired, the control
signal from the control unit 107 is low, and the control signal
from the control unit 105 is high. In an alternative embodiment, a
digital logic circuit can be implemented if desired.
[0053] The previous circuits are less effective with large RF
signals when the DC voltage on the antenna is zero volts. The
reason is because the MOS current-voltage characteristics change
when the devices are "reverse biased" by the antenna voltage going
negative. If the RF voltages are small, then there is little
undesired effect. However, if the RF signal at the antenna is
large, then the undesired effect is noticeable.
[0054] In alternative embodiments, the system includes using
negative voltages at the antenna. The alternative circuits are
shown and discussed in detail below. The circuits use enhancement
mode FETs. However, circuits are also shown that use the
enhancement/depletion mode devices.
[0055] Referring now to FIGS. 11 and 12, a system 110 includes an
antenna 112 coupled to a device 114 and a device 116. In one
embodiment the devices 114 and 116 are standard enhancement-mode
devices. When the control signal from a control unit 118 is low, an
open impedance characteristic is presented to the antenna 112. When
the control signal from the control unit 118 is high, a short is
presented to the antenna 112.
[0056] When the control signal is low, both the device 114 and the
device 116 are off, so that virtually no current flows between the
drain and the source of the FET. With a high control signal, device
114 turns on and shorts the antenna 112 to ground; likewise, device
116 turns on. However, a capacitor 115 prevents direct current flow
from the drain side of the device 116 to the antenna 112. In one
embodiment, the capacitors is shown in one instance connected
between the antenna 112 and the drain of the device 116; in an
alternative embodiment the capacitor 115 is shown connected between
the antenna 112 and the drain of the device 114. In FIG. 11, the
antenna is at 0 V.sub.DC, while in FIG. 12 the antenna is at
approximately V.sub.DD.
[0057] Even though direct current (DC) can not flow through the
capacitor 115, current that results from the radio frequency can
flow through capacitor 115. Accordingly, the capacitance of the
capacitor 115 is selected so that the capacitor 115 presents a
low-impedance at the operating radio frequency.
[0058] In an alternative embodiment, the system 110 can be used to
terminate an antenna coupled to the devices 114 and 116 at the
characteristic impedance by sizing the device 114 and the device
116.
[0059] Referring now to FIGS. 13 and 14, a system 130 includes the
device 114 and the device 116, wherein the devices 114 and 116
function as open or short circuits depending on the control signals
from the control unit 118 while the resistors 120 and 122 set the
characteristic impedance. Alternative embodiments are possible
wherein the capacitor 115 is switched from the drain of the device
114 to the drain of the device 116.
[0060] Referring now to FIGS. 15 and 16, the characteristic
impedance of the antenna 112 of the system 150 is matched by the
correct sizing of the device 114 and the device 116. As indicated,
alternative embodiments are possible wherein the capacitor 115 is
switched from the drain of the device 114 to the drain of the
device 116.
[0061] Referring now to FIGS. 17, 18, 19, and 20, if enhancement
and depletion-mode devices are available, then alternative circuits
can be used. As indicated above, in a depletion mode device, as the
control signal voltage is increased, the depletion mode device gets
closer to proximating as open or high impedance characteristic.
Thus, the embodiments disclosed herein are similar to those using
enhancement mode devices and includes a voltage inverter 170 for
inverting the control signal that is sent to the depletion mode
device.
[0062] Having fully described various embodiment and various
alternatives, those skilled in the art will recognize, given the
teachings herein that numerous alternatives and variations exist
that do not depart from the invention and it is therefore intended
that the invention not be limited by the forgoing description.
* * * * *