This invention relates to radio frequency apparatus.
Optimized detector/modulator circuits are key components in passive
microwave links which use smart labels and tags for identification and tracking
of objects in shipping and transportation. The recent development of low-cost detector
diodes and miniature antennas made it possible to develop electronic labels for
the wireless supermarket and related applications requiring the use of inexpensive
tags or smart labels serving as terminals in a modulated backscatter microwave link.
EP-A-0 480 413 discloses a responder in a moveable-object identification
system which includes an interrogator for transmitting an interrogation signal and
a responder mounted on a moveable object for transmitting a reply signal in response
to the interrogation signal. The responder includes an antenna for receiving an
interrogation signal, and an input means connected to the antenna with the input
means including a receiving element for dividing the received electric power into
a first separation electric power and a second separation electric power corresponding
to a traveling wave and reflected wave respectively and with the receiving element
having an impedance. The responder also includes generating means for generating
predetermined identification information by the first separation electric power
and a modulating means connected with the input means and connected to the antenna
for varying the impedance of the receiving element in accordance with the generated
identification information, for modulating the second separation electric power
of the interrogation signal in accordance with the identification information to
generate the reply signal and for feeding the reply signal to the antenna.
EP-A-0 492 569 discloses a system and method for the non-contact transmission
of data between a station and a portable data carrier. The station includes a station
transmitter operating at a predetermined frequency for generating a first signal
and a receiver for receiving a second signal superimposed on the first signal. The
station further includes a matched station antenna coupled via a length of cable
to the station transmitter. The portable data carrier includes a data carrier modulator
for modulating the first signal with the second signal in response to the first
data generated by the data carrier by means of an inductive coupling, and thereby
enabling the first data to be transmitted from the data carrier to the station,
with the portable data carrier including a tuned antenna circuit inductively coupled
with the station antenna. The tuned antenna circuit may include a resonant circuit
tuned to the frequency of the first signal. The tuned antenna circuit may also include
a matched antenna circuit.
EP-A-0 254 954 relates to a transponder in a system for identifying
objects. The transponder includes a dipole antenna constructed to receive signals
from a reader at a suitable frequency, with an impedance matching section connected
to the dipole to match the impedance of the dipole to the impedance of remaining
US-A-5,119,099 discloses a microwave transponder whereby a diode which
is switched between ON and OFF states in response to a response signal is interposed
between two end portions of microstrip lines separated by a predetermined interval,
and a distance, between distal ends of the two linearly arranged microstrip lines,
which includes an impedance defined by the ON or OFF state of the diode is determined
to be one-half a wave length of the microwave.
US-A-5,319,802 discloses a device for the exchange of data by electromagnetic
waves, whereby a transistor works, under a first bias, as a detector of a wave transmitted
by a reader and demodulates this wave and then, under a second bias, it works as
an oscillator and modulates the response transmitted by the badge. A transistor,
mounted as a common source, has its gate connected by a matching network to the
input of a circuit (typically, a single transmission/reception antenna). Microstrip
lines are disclosed as impedance matching loads for coupling between the transistor
and the input in both types of operation, modulation and demodulation.
EP-A-0 344 885 discloses a beam powered antenna. The antenna includes
a folded planar dipole section having two separated oppositely disposed U-shaped
elements, each of the elements having a pair of ends, with a capacitor series coupled
between two oppositely disposed ends of the two elements and a diode having a high
impedance at a selected center frequency, series coupled between the other two oppositely
disposed ends of the two elements. The antenna may also have a matching section
of two conductive lines, with one end of each line coupled to each of the other
two oppositely disposed ends of the two elements, the diode being series coupled
between the other ends of the two conductive lines. The diode may be a Schottky
Our U.S. patent application, Serial No. 08/380277, filed January 30,
1995, and entitled "Wireless Electronic Module," teaches that the maximum sensitivity
for the downlink (base station to label module) communication is achieved if the
antenna port impedance is the complex conjugate of the diode impedance. While operating
the antenna at such an impedance does improve downlink communications, it does not
result in a maximum cross-section of a wireless label needed for efficient backscattering
of an incident CW carrier to provide uplink label to base station) communications.
According to one aspect of this invention there is provided radio
frequency apparatus as claimed in claim 1.
According to another aspect of this invention there is provided radio
frequency apparatus as claimed in claim 13.
Apparatus embodying the present, invention (e.g., a wireless label)
provides improved backscattering of an incident Radio Frequency (RF) signal by utilizing
a separate modulator diode, which connects across the antenna to modulate the backscattering
of the antenna. A separate detector diode connects across the antenna to detect
modulated signals received by the antenna, the detector diode connecting to the
antenna through a matching network such that the impedance of the detector diode
at a predetermined RF frequency conjugately matches an antenna port impedance transformed
through the matching network. The antenna may be dipole, microstrip patch or monopole.
In one embodiment, the detector diode connects to the antenna port
through an impedance matching network. In another embodiment, the modulator diode
is a tunnel diode which may be biased in a negative resistance portion of its current-voltage
characteristics when the apparatus is operated in a backscatter mode and biased
at its valley voltage when the apparatus is operated in a receive mode.
Brief Description of the Drawing
- FIG. 1 shows a first embodiment of the present invention;
- FIG. 2 shows a second embodiment of the present invention;
- FIG. 3 shows typical I-V characteristics of a tunnel diode;
- FIG. 4 shows typical I-V characteristics of a Schottky diode; and
- FIG. 5 shows an embodiment of the present invention using a folded monopole
The present invention describes a circuit for which the downlink communication
signal (base station to label) and the uplink communication signal (backscattering
from the label to base station) are optimized by means of separate detector and
switching (may also be referred to as modulator) diodes. Applicants have noted that,
in prior art wireless labels, the simultaneous optimization of the downlink and
uplink signals cannot be achieved using a single diode which is biased at two different
current levels. In such an arrangement, the diode acts as a detector when biased
at a first current level and acts as a low impedance when biased at a second current
level. However, such an arrangement results in a compromise or tradeoff in downlink
or uplink communication performance.
A wireless label including a dipole antenna 102, a detector 110 and
a modulator 120 is shown in FIG. 1. The dipole antenna 102 has a half-wavelength,
each of the two branches being a quarter-wavelength long. The dipole antenna 102
may also be implemented as a "microstrip patch" antenna.
The wireless label illustratively operates in one of three modes;
a sleep mode, a receiving mode, or a backscattering mode. The circuitry which performs
this function, as well as other functions which are part of the wireless label but
not necessary for an understanding of the present invention, is not further described
herein. Our previously referenced U.S. patent application illustratively describes
more details of the detector circuit operation.
Most of the time, the wireless label operates in the sleep mode so
as to extend the lifetime of its battery (not shown). The wireless label, using
well-known circuitry (not shown), periodically "wakes up" and enters the receiving
mode looking for any modulated carrier signals (downlink signals) being sent to
it. This downlink signal is typically an on/off keyed amplitude modulated carrier
signal. If no modulated carrier signal is detected, the wireless label returns to
the sleep mode. If a modulated carrier signal is received, it is detected and processed
by the wireless label. A predetermined period of time after the completion of the
receiving mode, the wireless label enters the backscatter or reflection mode. The
exact timing between the receiving mode and backscattering mode is determined by
label circuitry (not shown) which implements the predetermined communication protocol
established between the base station and the wireless label. During the backscatter
mode, the wireless label modulates a carrier wave signal received from the base
station with the information to be sent to the base station, thereby forming the
uplink signal. The uplink signal is modulated by switching modulator diode 121 on
and off using an information signal received from baseband modulator circuit 124,
as will be described in a later paragraph.
The detector circuit 110 includes matching network 113 that connects
detector diode 111, transmission line 118, capacitor 116, and baseband circuit 117
to port 101 of antenna 102. The modulator circuit 120 includes modulator diode 121,
transmission line 122, capacitor 123, and baseband modulator circuit 124.
Separate diodes 111 and 121 are utilized in the detector 110 and modulator
120 circuits, respectively, to maximize the receiving of a modulated carrier signal
and reflecting (backscattering) of a received carrier signal.
The detector diode 111 and modulator diode 121 are connected across
the antenna 102 in opposite polarities. Such a connection arrangement insures that,
when the modulator diode 121 is modulated into the forward-biased region during
the backscatter mode, it does not forward-bias the detector diode 111. Similarly,
during the receive mode, when the detector diode 111 is forward-biased, the modulator
diode 121 is reverse-biased.
If the diode 121 shown in FIG. 1 connected across the antenna port
is a tunnel diode, a bypass capacitor must be added in series with the inductor
115 shown in FIG. 1. The bypass capacitor is needed because diode 121, being a tunnel
diode, is conducting current for both negative and positive voltages. The bypass
capacitor prevents current from flowing through the detector diode 111 for any voltage
applied through the baseband modulator circuit 124.
In detector circuit 110, during a receiving mode, the detector diode
111, illustratively a zero DC current biased Schottky detector diode 111, detects
or rectifies a received modulated carrier signal. As shown in FIG. 4, the detector
diode 111, when operated at a zero biased current, has essentially a capacitive
impedance. Returning to FIG. 1, the capacitive impedance of detector diode 111,
at the received modulated carrier frequency, is conjugately matched to the inductive
impedance Z1 of the matching network 113 terminated by port impedance
of dipole antenna 102. This matching network 113 illustratively includes a capacitor
114 and inductor 115 which may be implemented by means of a simple, lumped or hybrid
integrated LC matching circuit.
This matching network 113 transforms the capacitive junction impedance
of detector diode 111 into an impedance Z2 which matches the real
port impedance of dipole antenna 102. The modulator diode 121, a tunnel diode shown
in FIG. 3, during the receiving mode is forward-biased in the valley 401 region
of the I-V characteristics curve, creating a small parasitic capacitance across
the port 101 of antenna 102. The capacitance of modulator diode 121 acts together
with capacitor 114 of matching network 113.
The detected output of detector diode 111 is inputted to baseband
detector circuit 117 via a quarter-wave transmission line 118 and a capacitor chip
116 which, at the received modulated carrier frequency, presents an open-circuit
impedance to detector diode 111 to prevent RF currents from flowing into the baseband
circuit 117. Similarly, during the detection mode, the quarter-wave transmission
line 122 and capacitor 123 also present an open-circuit impedance to prevent RF
currents at port 101 of antenna 102 from flowing back into baseband modulator circuit
During the reflecting or backscattering mode, modulator diode 121
is forward-biased by an on-off modulation signal received from baseband modulator
circuit 124. The forward-biasing current in diode 121 causes diode 121 to present
an RF short-circuit impedance across antenna port 101. When a continuous wave (CW)
signal at a resonant predetermined frequency is received by antenna 102, it generates
high RF currents (+i,-i) in antenna 102 which maximize the backscatter energy re-radiated
from antenna 102. Consequently, the incoming CW signal is backscatter modulated
when the impedance of modulator diode 121 is modulated by turning it on and off
using the signal from baseband modulator circuit 124.
Backscattering is enhanced if modulator diode 121 is a tunnel diode
which is biased in the negative resistance region, shown as 402 in FIG. 3, when
it is turned on.
When the modulator diode 121 is turned off during the backscattering
mode, the quarter-wave transmission line 122 and capacitor 123 present an open-circuit
impedance to prevent RF antenna currents from the incoming CW signal from flowing
into baseband modulator circuit 124. The quarter-wave transmission line 118 and
capacitor 116 also present an open-circuit impedance to prevent any of the RF antenna
currents from flowing into baseband circuit 117 of detector circuit 110.
The intensity of the backscattered wave re-radiated from antenna 102
is the product of the incident CW signal field intensity times the backscattering
cross-section of the antenna. This cross-section is a function of the antenna geometry
and the port impedance of the antenna as outlined below.
The effective area of a small isotropic antenna can be readily derived
from the basic antenna equations. If the antenna aperture, D0,
is small with respect to one wavelength
D0 < λ0
one obtains for the radius, r0, which defines the boundary between
the radiative and the reactive region,
r0 = λ0 / (2π).
The resulting effective area, which is the cross-section of the virtual cavity
formed by the antenna, is for a small isotropic antenna
Aeff = πr20 = λ20 / (4π).
Various detailed derivations of this basic equation have been reviewed recently
by D. C. Hoff in the article entitled "Fun with the Friie Free-Space Transmission,"
IEEE Antennas and Propagation Magazine, Vol. 35, August 1993, pp. 33-35.
The cross-section for backscattering is identical to the effective
area if the antenna port is terminated with an impedance which is equal to the port
impedance. For the case of a dipole antenna with a total length of λ02,
with a load impedance of 73 ohms terminating the antenna port, the cross-section
is that shown by equation (3).
This cross-section is enhanced by a factor of four if the port is
short-circuited and if the antenna is resonant at the frequency of the incident
field. The increase of the scattering cross-section by 6 dB occurs because the currents
+i and -i flowing in and out of the port are doubled, similar to the doubling of
the current in a shorted transmission line as opposed to a current flowing into
the matched load terminating a transmission line. For the case of an open antenna
port, the antenna is split into two scatterers which resonate at the second harmonic
frequency 2 f0 . At the fundamental frequency f0
, the scattering cross-section drops to nearly zero because the currents flowing
in the two conducting parts are very small.
Consequently, for our FIG. 1 embodiment, by switching modulator diode
121 between an open- and a short-circuit impedance, the load impedanceZ2
presented across antenna port 101 switches between 72 ohms (Z2)
and zero ohms. This results in a modulated backscatter wave that varies by a factor
of 4, or 6 dB in power.
With reference to FIG. 2, there is shown a wireless label identical
to FIG. 1 except that it includes a quarter-wavelength monopole antenna 201 rather
than the dipole antenna 102 of FIG. 1. One such monopole may be implemented using
an inverted-F structure as shown in FIG. 5. Such a monopole antenna 501 is typically
utilized with a ground shield 502 mounted on ground plane 503 to shield the detector
110 and modulator 120 circuits as well as other circuits of the wireless label from
antenna 501. The antenna 501 illustratively is implemented as a unitary L-shaped
microstrip conductor 510 having two support legs or strips 511 and 512, thereby
forming the monopole inverted-F antenna. These support strips 511 and 512 maintain
the antenna 501 a predetermined height above ground plane 503. The first support
strip 511 is electrically connected or shorted to ground plane 503 which is formed
by a deposited metal surface on the top and bottom of printed circuit board 510.
The second strip 512 is isolated from ground plane 503 by a thin dielectric material
which is deposited over the ground plane 503. The dielectric material may be, illustratively,
FR-4, a low-cost circuit board material. The bottom part 513 of the second strip
512 forms an antenna port 513 for antenna 501, which means that a signal incident
on antenna 501 generates an RF voltage between the bottom of the second strip 512
(antenna port 513) and the ground plane 503. This RF voltage is connected to the
detector 110 and modulator 120 circuits of FIG. 2, illustrated for convenience as
circuit blocks in FIG. 5.
The antenna has a total length (522 + 523) of about 3λ08
which is about 5.0 cm at an operating modulated carrier frequency of 2.45 GHz. The
height (511) of the support strips 511 and 512 is about 0.8 cm. The antenna 501
illustratively may be fabricated from a stainless steel sheet by cutting an essentially
L-shaped geometry (formed by segments 523 and 511, 522, in addition to the second
strip 512 extending perpendicularly to 522) using a well-known computer-controlled
wire Electron Discharge Machining (wire EDM). The resulting L-shaped metal piece
is then appropriately bent to obtain the inverted-F shape of antenna 501.