U.S. patent application number 09/777214 was filed with the patent office on 2001-10-11 for extension of dynamic range of emitter and detector circuits of spread spectrum-based antenna test range.
This patent application is currently assigned to Harris Corporation. Invention is credited to Boritzki, Daniel L., Killen, William D., Walley, George M., Zeitfuss, Michael P..
Application Number | 20010028323 09/777214 |
Document ID | / |
Family ID | 23135573 |
Filed Date | 2001-10-11 |
United States Patent
Application |
20010028323 |
Kind Code |
A1 |
Walley, George M. ; et
al. |
October 11, 2001 |
Extension of dynamic range of emitter and detector circuits of
spread spectrum-based antenna test range
Abstract
An antenna test range uses a direct spread-spectrum based test
signal to effectively electronically reject all unwanted signals
that may be present in the test range, and thereby allow both main
beam and off-axis performance of the antenna to be completely and
accurately measured. For increased dynamic range, the test signal
comprises a carrier signal that is sequentially modulated with low
rate, respectively different, direct spreading PN sequences applied
to a cascaded plurality of N mixer stages through successive ones
of which the carrier signal is coupled. The plurality of PN
spreading sequences are mutually offset in time by a fraction of a
chip, and thereby produce, at an output of an Nth mixer stage, a
direct sequence spread spectrum carrier signal having its energy
spread out over a bandwidth that is N times the spreading bandwidth
of an individual one of the PN spreading sequences.
Inventors: |
Walley, George M.; (San
Jose, CA) ; Boritzki, Daniel L.; (Palm Bay, FL)
; Killen, William D.; (Palm Bay, FL) ; Zeitfuss,
Michael P.; (Satellite Beach, FL) |
Correspondence
Address: |
Christopher F. Regan
Allen, Dyer Doppelt, Milbrath, Gilchrist, P.A.
P.O. Box 3791
Orlando
FL
32802-3791
US
|
Assignee: |
Harris Corporation
|
Family ID: |
23135573 |
Appl. No.: |
09/777214 |
Filed: |
February 5, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09777214 |
Feb 5, 2001 |
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09294940 |
Apr 20, 1999 |
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6184826 |
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Current U.S.
Class: |
342/360 |
Current CPC
Class: |
H04B 1/707 20130101;
H01Q 3/267 20130101 |
Class at
Publication: |
342/360 |
International
Class: |
H01Q 003/00 |
Claims
What is claimed:
1. A method for extending the dynamic range of a process for
testing one or more characteristics of an antenna, wherein a spread
spectrum carrier is emitted as a test signal from a test signal
source and is incident upon said antenna, and a replica of a
spreading sequence of said spread spectrum test signal is
correlated with a signal demodulated from energy received at said
antenna, so as to extract energy in said test signal and exclude
energy in unwanted signals that may be incident upon said antenna,
said extracted test signal energy being processed to derive a
measure of said one or more characteristics of said antenna, said
method comprising the steps of: (a) coupling said carrier signal
through successive ones of a cascaded arrangement of a plurality of
N mixer stages; (b) applying, to said successive ones of said
plurality of N mixer stages, respectively different, relatively low
rate, PN spreading sequences, that are mutually offset in time by a
fraction of a chip, thereby producing, at an output of an Nth mixer
stage of said plurality of N mixer stages, a direct sequence spread
spectrum carrier having its energy spread out over a bandwidth that
is N times the spreading bandwidth of an individual one of said PN
spreading sequences; and (c) emitting the spread spectrum carrier
produced in step (b) as said test signal from said test signal
source.
2. A method according to claim 1, wherein step (c) comprises
emitting said test signal from said test signal source at a
plurality of spaced apart signal source locations having
respectively different azimuth and elevation parameters relative to
the boresight of said antenna.
3. An antenna test range comprising: a test signal source, spaced
apart from an antenna under test and being operative to emit a test
signal that is incident upon said antenna under test, said test
signal source including a cascaded arrangement of a plurality of N
mixer stages through successive ones of which a carrier signal is
coupled, and a direct spreading PN generator that is operative to
couple a plurality of different, relatively low rate, PN spreading
sequences, that are mutually offset in time by a fraction of a
chip, to respective ones of said plurality of N mixer stages, and
thereby produce, at an output of an Nth mixer stage of said
plurality of N mixer stages, a direct sequence spread spectrum
carrier signal as said test signal having its energy spread out
over a bandwidth that is N times the spreading bandwidth of an
individual one of said PN spreading sequences; a receiver coupled
to said antenna under test, and being operative to demodulate a
signal received by said antenna under test, and to correlate a
replica of said PN spreading sequences of said test signal with the
demodulated signal so as to extract energy in said test signal and
exclude energy in unwanted signals that may be incident upon said
antenna under test; and a signal processor, coupled to said
receiver, and being operative to process the test signal energy
extracted by said receiver and derive a measure of one or more
characteristics of said antenna under test.
4. An antenna test range according to claim 3, wherein said test
signal source is operative to emit said test signal from a
plurality of spaced apart signal source locations having
respectively different azimuth and elevation parameters relative to
the boresight of said antenna under test.
5. A method of spreading a carrier signal with a direct spreading
PN sequence comprising the steps of: (a) coupling said carrier
signal through successive ones of a cascaded arrangement of a
plurality of N mixer stages; and (b) applying, to said successive
ones of said plurality of N mixer stages, respectively different,
relatively low rate, PN spreading sequences, that are mutually
offset in time by a fraction of a chip, thereby producing, at an
output of an Nth mixer stage of said plurality of N mixer stages, a
direct sequence spread spectrum carrier having its energy spread
out over a bandwidth that is N times the spreading bandwidth of an
individual one of said PN spreading sequences.
6. A method according to claim 5, further including the steps of:
(c) emitting said direct sequence spread spectrum carrier from an
antenna range test signal source located at a plurality of spaced
apart signal source locations having respectively different azimuth
and elevation parameters relative to the boresight of an antenna;
and (d) receiving and demodulating signals received by said
antenna; (e) correlating a replica of said test signal with signals
received and demodulated in step (d), so as to extract energy in
said test signal and exclude energy in unwanted signals that may be
incident upon said antenna; and (f) processing test signal energy
extracted in step (e) to derive a measure of said one or more
characteristics of said antenna.
7. An apparatus for spreading a carrier signal with a direct
spreading PN sequence comprising a cascaded plurality of N mixer
stages through successive ones of which a carrier signal is
coupled, and a direct spreading PN generator that is operative to
couple a plurality of different, relatively low rate, PN spreading
sequences, that are mutually offset in time by a fraction of a
chip, to respective ones of said plurality of N mixer stages, and
thereby produce, at an output of an Nth mixer stage of said
plurality of N mixer stages, a direct sequence spread spectrum
carrier signal having its energy spread out over a bandwidth that
is N times the spreading bandwidth of an individual one of said PN
spreading sequences.
8. An apparatus according to claim 7, further comprising an antenna
range test signal source, spaced apart from an antenna under test
and being operative to emit, as a test signal that is incident upon
said antenna under test, said direct sequence spread spectrum
carrier signal, a receiver coupled to said antenna under test and
being operative to demodulate a signal received by said antenna
under test, and to correlate a replica of said PN spreading
sequences of said test signal with the demodulated signal so as to
extract energy in said test signal and exclude energy in unwanted
signals that may be incident upon said antenna under test, and a
signal processor, coupled to said receiver, and being operative to
process the test signal energy extracted by said receiver and
derive a measure of one or more characteristics of said antenna
under test.
9. An apparatus according to claim 8, wherein said test signal
source is operative to emit said test signal from a plurality of
spaced apart signal source locations having respectively different
azimuth and elevation parameters relative to the boresight of said
antenna under test.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The subject matter of the present invention relates to that
disclosed in co-pending U.S. patent application Ser. No. ______
filed coincident herewith, entitled: "Mitigation of Antenna Test
Range Impairments Caused by Presence of Undesirable Emitters," by
M. Walley et al (hereinafter referred to as the '______
application), assigned to the assignee of the present application,
and the disclosure of which is incorporated herein.
FIELD OF THE INVENTION
[0002] The present invention pertains in general to communication
systems, and is particularly directed to a improved PN sequence
multiplier--generator and its use as a mechanism for extending the
dynamic range of test signal emitter/detector components of an
antenna test range, that uses direct spread-spectrum test signals
to mitigate against measurement impairments, such as those caused
by multipath and or the presence of one or more interfering
emitters or to prevent interference of signals in a licensed
frequency band.
BACKGROUND OF THE INVENTION
[0003] As described in the above-referenced '______ application,
historically the design and testing of radio wave antennas has been
principally concerned with antenna gain along its boresight (main
beam axis). For this purpose, as shown diagrammatically in FIG. 1,
an antenna 10 the performance of which is to be measured may be
mounted within an indoor compact test range 12, such as an
EMI-shielded anechoic chamber, that is configured to eliminate
reflections and interference from unwanted sources of
electromagnetic radiation.
[0004] Testing of any antenna typically involves directing radio
wave emissions from a test signal source 14 toward the antenna, and
measuring the antenna's response by a range receiver 16, the output
of which may be displayed or recorded via an associated test and
measurement workstation 18. Varying the primary axis of the antenna
10 and test signal source 14 (for example, by varying the
orientation in orthogonal principal planes of either the antenna or
the test source), enables both boresight and off-axis flexibility
of performance parameters including gain, polarization, etc., of
the antenna to be measured.
[0005] Unfortunately, at relatively low frequencies (e.g., UHF),
the size of the indoor test range needed to test the antenna
becomes physically and cost-wise prohibitive, making it necessary
to test the antenna outdoors. While finding an `open air` location
to set up an antenna test range that is free of interferers may not
have been particularly difficult several decades ago, it has
recently become a significant problem, principally as a result of
the proliferation of wireless commercial products, such as cellular
phones and citizen band radios, as well as specular reflections
from buildings and the like. Moreover, not only should the test
range be free of interference from outside sources, but it is
desired that the test range emissions themselves not interfere with
other `off-range` communication equipment. This interference and
reflection free test range problem is compounded by the fact that,
in addition to measuring main lobe performance, antenna designers
are interested in the antenna's off-axis or sidelobe
characteristics, that will allow placement of nulls on one or more
interferers, such as a cellular radio transmission tower.
[0006] Advantageously, the invention described in the '______
application is designed to effectively alleviate this test range
impairment problem by employing a spread spectrum signal as the
test signal. Because a spread spectrum signal has high
autocorrelation properties with itself and high cross-correlation
properties with other signals including interferers, as well as
time delayed versions of itself due to specular reflection from
multipath, it provides a means for enabling only the intended
receiver that processes the energy received by the antenna under
test to electronically reject all other signals that may be present
in the test range, and thereby allows both main beam and sidelobe,
off-axis performance of the antenna to be accurately measured,
while also preventing interference with other communication
equipment.
[0007] Now even though spread spectrum signal processing provides
an effective means of achieving many dB of processing gain, by
spreading out over a wide bandwidth and thereby substantially
reducing the influence of energy from unwanted test range
interferers, the degree of improvement may be influenced by
operational conditions of the test range and circuit parameters of
the test range equipment.
[0008] For example, as diagrammatically shown in FIG. 2, where the
test signal source 14 is positioned at an off-axis location 15 for
the purpose of conducting a sidelobe measurement, the presence of a
strong interferer 21 in the antenna's main beam 11 (which typically
has a substantially larger gain than a sidelobe), may diminish the
ability to resolve the sidelobe.
[0009] To overcome this problem it is necessary to increase the
spreading processing gain--namely substantially increase the chip
rate of the spreading sequence of the test signal. While this can
be achieved using very high speed electronic components, doing so
may add a substantial cost to both the test signal emitter and the
receiver processing equipment. A second problem is the fact that
reasonably priced RF mixer circuits that are used to modulate the
RF carrier with the spreading signal, suffer some degree of leakage
of the local oscillator signal (e.g., as a 30 dB down spur). While
this carrier spur leakage problem can also be reduced by using more
complex mixer circuitry (which usually requires very fine tuning),
such circuitry would also add further expense to the test signal
generator and receiver processing equipment.
SUMMARY OF THE INVENTION
[0010] In accordance with the present invention, these potential
problems are successfully remedied by configuring the test signal
emitter to include a cascaded arrangement of relatively inexpensive
(leaky) mixer stages through which the RF carrier is successively
conveyed. Each successive mixer stage of the local oscillator's
cascaded transport path is fed with a respectively different,
relatively low rate, PN spreading sequences, that is offset in time
by a fraction of a chip from the sequence applied to an adjacent
mixer.
[0011] Sequentially cascading the PN sequence by
carrier-multiplying mixer stages in this manner produces an output
carrier the energy in which is now spread out over the very wide
bandwidth of the resultant PN sequence, whose chip rate corresponds
to that of an individual one of the respective PN sequences times
the number of cascaded stages. This not only allows the use of
relatively low chip rate (and therefore inexpensive) PN generator
components to substantially enhance spreading processing gain, but
significantly reduces the net leakage of the local oscillator
carrier spur output at the downstream end port of the cascaded
mixers.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] FIG. 1 diagrammatically illustrates a compact, indoor
antenna test range;
[0013] FIG. 2 diagrammatically illustrates an outdoor antenna test
range, in which a test signal source is positioned in an off-axis
location and an interferer is located in the antenna's main
beam;
[0014] FIG. 3 diagrammatically illustrates an embodiment of an
antenna test range of the type described in the above referenced
'______ application, that employs a spreading sequence based test
signal source to mitigate against the presence of test range
impairments;
[0015] FIG. 4 diagrammatically illustrates a direct spread-spectrum
signal based test signal source for use in the antenna test range
of FIG. 3;
[0016] FIG. 5 diagrammatically illustrates the configuration of
range receiver equipment for the antenna test range of FIG. 3;
[0017] FIG. 6 diagrammatically illustrates a XN multiplied PN
sequence spreading signal generator in accordance with the present
invention;
[0018] FIG. 7 diagrammatically illustrates a despreader for
despreading the XN multiplied PN sequence spreading signal produced
by the generator of FIG. 6; and
[0019] FIGS. 8 and 9 show the general architecture and operation of
a high performance PN tracking circuit.
DETAILED DESCRIPTION
[0020] Before describing in detail the new and improved PN sequence
multiplier--generator in accordance with the present invention, and
its use in extending the dynamic range of spread spectrum-based
test signal emitter/detector components of an antenna test range,
it should be observed that the invention resides primarily in a
prescribed arrangement of conventional communication circuits and
associated digital signal processing components and attendant
supervisory control circuitry therefor, that controls the
operations of such circuits and components.
[0021] Consequently, the configuration of such circuits components
and the manner in which they are interfaced with other antenna test
range equipment have, for the most part, been illustrated in the
drawings by readily understandable block diagrams, which show only
those specific details that are pertinent to the present invention,
so as not to obscure the disclosure with details which will be
readily apparent to those skilled in the art having the benefit of
the description herein. Thus, the block diagram illustrations are
primarily intended to show the major components of the PN sequence
multiplier--generator and its use in an antenna test range in a
convenient functional grouping, whereby the present invention may
be more readily understood.
[0022] FIG. 3 diagrammatically illustrates an embodiment of an
antenna test range of the type described in the above referenced
'______ application, that employs a spreading sequence based test
signal source to mitigate against the presence of test range
impairments, such as but not limited to specular reflections or
signals emitted from one or more `interference` sources that may be
incident on an antenna whose performance is to be measured. The
antenna 30 may be fixedly mounted at a prescribed location at which
measurements are to be conducted by way of associated range
receiver equipment 35 connected to the antenna 30. Radio wave
emissions in the band of operation of the antenna are directed from
a test signal source 37 toward the antenna 30, and the response of
the antenna 30 is measured by means of the range receiver equipment
35. To measure the antenna's performance gain parameters (including
gain, polarization, etc.) for variations in the principal planes,
the antenna's response may be monitored as the antenna's boresight
axis is moved in the principal planes relative to the test range
signal source. As detailed in the '______ application, the
potential impairing influence of reflections, such as those from a
building 34 and/or emissions from `interference` sources such as a
cellular radio 33, are readily mitigated by using a direct
spread-spectrum signal as the test signal waveform.
[0023] For this purpose, as shown in FIG. 4, a carrier-spreading
pseudo-random chip sequence is produced by a pseudo random noise
(PN) generator 40, the output of which is a `spread` or `chipped`
data stream having a prescribed number of chips per baud. The chip
sequence is coupled to the test source's RF section 42, which may
comprise an RF mixer and bandpass filter, as a non-limiting
example. The resulting spread RF test carrier produced by the RF
section 42 is then transmitted via a test source antenna 44 along a
prescribed transmission axis toward the antenna under test.
[0024] A non-limiting example of range receiver equipment, to which
the output of the antenna under test is coupled, is shown
diagrammatically in FIG. 5, as comprising an RF receiver-despreader
section 50, which receives the spread test signal emitted by the
test signal source and despread-correlation processes the received
signal to recover the earliest line-of-sight emission from the test
source. For this purpose, the receiver section 50 may include a
mixer 51 to which the output of a local oscillator 52 is applied,
to provide a baseband spread signal that is coupled through a
bandpass filter 53 to a correlation processor 54. The correlation
processor is coupled to receive a spread-spectrum reference signal
pattern produced by a pseudo random noise (PN) generator 55. The PN
generator 55 is operative to generate the same direct spreading PN
sequence employed by the test signal source of FIG. 4, described
above.
[0025] Impairments due to multipath are readily avoided by
selecting the earliest-in-time correlator output signal whose
energy content exceeds a prescribed threshold to identify the
first-to-arrive (line-of-sight) test signal of interest. RF
emissions other than those sourced from the test signal source are
avoided, since the energy in the correlator output for such other
emissions is highly cross-correlated (rather than highly
auto-correlated) with the reference PN sequence, and therefore
effectively nulled out. The energy in the highly autocorrelated
(first-to-arrive) output of the correlator processor 54 is
digitized and processed by way of the antenna performance
measurement algorithm executed by a workstation 56.
[0026] As pointed out above, the degree of impairment rejection
provided by such use of spread spectrum signal processing may be
influenced by operational conditions of the antenna test range
(such as the presence of an interferer in the main lobe, which
might overwhelm a test signal from the direction of the side lobe),
and performance parameters of its circuit components (e.g., carrier
spur leakage through RF mixer circuitry). To overcome the effect of
the relatively large ratio of main lobe gain to sidelobe gain, it
is necessary to increase the spreading processing gain--namely,
substantially increase the chip rate of the spreading test
signal.
[0027] In accordance with the present invention, this performance
improvement is readily accomplished by means of a spreading signal
generator implementation, diagrammatically illustrated in FIG. 6,
that also suppresses carrier leakage using low cost PN generator
and carrier mixer components. More particularly, FIG. 6 shows the
test signal generator of FIG. 4 implemented in accordance with the
present invention as a cascaded arrangement of N, relatively
inexpensive (leaky) mixers 60-1, 60-2, . . . , 60-N. Since
cascading the mixers 60 has the effect of significantly attenuating
local oscillator leakage at each stage (e.g., by 30 dB per stage),
over a series of N stages, the total RF carrier spur leakage
realized at the downstream end of the cascaded mixer arrangement of
some plurality of N mixer stages will be well suppressed.
[0028] In the cascaded PN generator--mixer arrangement of FIG. 6,
first inputs 61-1, 61-2, . . . , 61-N of the mixers are coupled to
PN generators 70-1, 70-2, . . . , 70N. These PN generators produce
N respectively different, relatively low rate, PN spreading
sequences, which are mutually offset in time by a fraction of chip,
via delay units 72-1, 72-2, . . . . The cascaded carrier path has
the second input 62-1 of mixer 60-1 coupled to receive a carrier
frequency signal generated by a local oscillator 80. The output
63-1 of mixer 60-1 is coupled to the second input 62-2 of mixer
60-2; the remaining mixers 60-3 . . . 60-N have their second inputs
similarly cascaded with outputs of successively upstream mixers, as
shown.
[0029] With this relatively simple cascaded mixer--PN generator
architecture, the output 63-N of mixer 60-N yields a carrier
frequency whose energy is spread out over the very wide bandwidth
(that of the resultant PN sequence, having a chip rate that
corresponds to that of an individual one of the respective PN
sequences times the number of cascaded mixer stages). As described
above, this not only allows the use of relatively low chip rate
(and therefore inexpensive) PN generator components to achieve the
desired enhanced spreading processing gain, but significantly
reduces the net leakage of the local oscillator carrier spur output
at the downstream end port of the cascaded mixers.
[0030] The manner in which the correlation and tracking of the
spread spectrum receiver section of the test range receiver
equipment of FIG. 5 is implemented and operated to despread the
very wide bandwidth PN sequence produced by the test signal
generator of FIG. 6 will now be described with reference to FIGS.
7-10. In order to understand the detailed implementation of the
correlation and tracking of the spread spectrum receiver, it is
useful to review some basic aspects of spread spectrum signal
processing.
[0031] In a basic spread spectrum communication system as
diagrammatically illustrated in FIG. 7, a narrow band RF signal 101
is used as the local oscillator for modulation in a mixer 103 by a
pseudo noise (PN) sequence 105 at a high rate. This has the effect
of spreading the energy of the originally narrow band signal 101
into a much broader band or `spread` signal, shown at 107, for
transmission over the communication link 109. The area encompassed
by the narrow band RF signal 101 is the same as the area under the
wide band spread signal 107. Therefore, the apparent power in a
particular signal bandwidth decreases by sacrificing bandwidth.
[0032] On the receive side of the communication link 109, if the
received spread RF signal is again modulated in a mixer 113 by a PN
sequence 115 that is identical to the PN sequence 105 used at the
transmitter, and at the same time and phase as the transmitter's PN
sequence 105, the effects of spreading the received signal is
reversed and the spread spectrum signal 107 collapses back to the
original narrow band RF signal 101.
[0033] Before describing the general architecture and operation of
the high performance PN tracking circuit of FIGS. 8 and 9, it is
useful to define terms that are employed to describe a spread
spectrum system. A `chip` for the purpose of the present discussion
is defined as a binary 1 or 0, that is generated by the pseudo
noise (PN) generator used to spread or de-spread the narrow band RF
carrier. In the test signal generator described above, the PN
sequence is comprised of a combination multiple PN generators. The
period of a single chip is considered a chip-time, having a
reciprocal that is defined as the chip rate and is sometimes
referred to as the PN rate.
[0034] In an antenna test range, since the receive side of the
system is on the other or geographically `far` side of the test
range from the modulator-source, it is necessary to determine the
proper PN timing needed to perform the de-spreading function.
First, the de-spreader must determine the proper PN time, and then
it must track changes in PN rate due to any system induced offsets.
There are several methods commonly used in communications
technology to achieve PN tracking. The most common are Early-Late
gate and tau-dither, which may be implemented by a variety of
approaches. The preferred approach is to provide the transmitter
and receiver with a common reference, so that no tracking is
required, since the PN chip rate of the transmitters and that of
the receiver can be phase-locked to that common reference.
[0035] FIGS. 8 and 9 diagrammatically illustrate an Early-Late gate
scheme that combines discrete hardware with a digital signal
processor (DSP) to aid in the acquisition and tracking of the PN
sequence. The PN tracking loop architecture of FIGS. 8 and 9 has a
number of refinements over the simplistic approach, described
above, but implements an Early-Late gate tracking loop. The
tracking operation will be describe first, as it is less
complicated than acquisition and many of the parameters used in its
analysis are useful in explaining the acquisition process.
[0036] Three PN sequences are generated--termed Early (E), On-Time
(OT), and Late (L). These PN sequences are delayed in time relative
to each other by 1/2 chip. When the loop is tracking, the On-Time
(OT) PN sequence is perfectly aligned in time with and thereby
fully correlated with the transmitted PN sequence as it arrives at
the demodulator.
[0037] There is a partial correlation of the received RF signal
applied to respective Early and Late mixers 120E and 120L for the
Early and Late channels, since each Early and Late channel is
offset in time by 1/2 chip from the fully correlated On-Time
channel. Essentially, if the Early and Late correlated signals are
equal, then the On-time channel will be perfectly centered between
them. The resulting IF signals produced by the despread operations
on the Early and Late channels are applied to a sum and difference
circuit 122, wherein they are summed with and subtracted from each
other to produce respective E+L and E-L signals.
[0038] These signals are filtered in respective filters 124S and
124D and the resulting IF carrier is digitized in respective
analog-to-digital converters (ADCs) 126S and 126D, and then
digitally quadrature down-converted by way of down-converters 128S
and 128D to eliminate any DC offsets created by the mixing or A/D
conversion. The resulting I and Q signals for each of the sum and
difference channels are then filtered and decimated in respective
I&Q units 130S and 130Q, to reduce the detection bandwidth.
[0039] The (I and Q) E+L channels are multiplied by the (I and Q)
E-L channels, in respective multipliers 132I and 132Q and the
products summed in adder 134 to produce an output E.sup.2-L.sup.2
tracking error signal, with the E+L channel being conjugated to
eliminate the effects of small residual frequency offsets in the
channel. In other words, the carrier power is detected without the
use of a carrier tracking loop. This E.sup.2-L.sup.2 tracking error
signal is the phase detector error signal of a digital phase locked
loop 136 that tracks the PN rate.
[0040] When the transmit PN sequence is aligned with the Early
correlator, all the signal power is in the Early channel, with none
in the Late channel; therefore, the result of E.sup.2-L.sup.2
tracking error is equal to +P. When the transmit PN sequence is
aligned with the Late correlator, all the signal power (P) is in
the Late channel and therefore the result of E.sup.2-L.sup.2
tracking error is equal to -P. When the correlation is aligned with
the On-Time correlator, an equal amount of power will be in each of
the Early and Late channels, and therefore the tracking error
(E.sup.2-L.sup.2) will go to zero. This also eliminates the
possibility of offsets in the tracking error signal being caused by
variations in gain of the E+L channel versus the E-L channel.
[0041] Multiplying respective loop gains G1 and G2 by the sum and
difference channels as G1(E+L).times.G2(E-L) yields
G1.times.G2.times.(E.sup.2-L.sup.2). When E.sup.2-L.sup.2 goes to
zero, the tracking error goes to zero, even if the two IF channels
are not perfectly matched. The DSP, shown by way of a functional
block diagram 140 in FIG. 9, performs the second order loop filter
function of the PN tracking loop.
[0042] The normalizer section 138 keeps the gain of the tracking
loop constant over variations in signal strength and signal to
noise ratio. Maintaining a constant loop gain allows the loop
bandwidth to remain constant under these changing conditions. The
loop filter sets the tracking loop bandwidth. The correlators 115
provide the tracking phase error signal that is then filtered by
the DSP 140 and the result controls the frequency of a numerically
controlled oscillator (NCO) 142, which alters the correlation point
of the PN sequence produced by PN generator unit 143 until the
tracking error goes to zero.
[0043] The acquisition process consists of several sequential steps
that are executed by a state machine within the software of the DSP
140. At the start of the acquisition routine, the length of one of
the chips in the PN sequence by 1/2 chip is extended by causing the
logic in the field programmable gate array (FPGA) to absorb one
2.times.PN rate clock cycle. To trigger this event, a dummy
write-to-port within the FPGA may be used. This elongation of the
PN sequence on demand appears as an instantaneous movement, or a
step in PN time by 1/2 chip. These 1/2 chip steps are used to
search for the proper alignment of the PN sequence. The PN sequence
correlation process can occur even when a small residual carrier
offset is present. The total amount of uncertainty is broken up
into discrete frequency slices or bins.
[0044] These bins are searched until the limits are reached, or
lock is detected by a lock detector 144. If lock is not detected,
the bin search is repeated. It should be noted that bin searching
becomes necessary only if the initial frequency uncertainty is
outside the lock detection bandwidth. Lock detection is based on
detecting the presence of carrier energy in a particular bin when
the PN sequence is correlated. In the case of an antenna range,
stepping through the frequency bins is unnecessary.
[0045] When lock is detected, the lock detector 144 measures the
level of the normalized sum of the absolute value of the tracking
error (.SIGMA.ABS (E.sup.2-L.sup.2)). When pure noise (N) or
un-correlated signal plus noise (S+N) are all that is present at
the input to the lock detector 144, a normalizer 146 holds the
average value of this noisy signal at a constant predetermined
level. Once PN correlation occurs, the output of lock detector 144
will drop by an amount proportional to the square root of the SNR.
The longer the output of the lock detector is integrated, the lower
will be the variance of the output of the lock detector around its
average value.
[0046] The tracking error signal (E.sup.2-L.sup.2) is also
examined. An integration of this parameter will go to zero when
there is only noise, no correlation, or if PN correlation has been
achieved with no rate offset. If the energy E.sup.2 in the Early
signal or the energy L.sup.2 in the Late signal is larger than a
given value, it is used as a secondary indicator that the PN is
near the proper correlation point. A tracking loop filter 151 will
attempt to pull the PN sequence into lock. The length of the
integration of the lock detector output is the dwell time at a
particular 1/2 chip correlation interval, typically referred to as
a PN step. After each dwell time, if the output of the lock
detector 144 is not below an empirically derived threshold 153, or
the absolute value of the integration of the E.sup.2-L.sup.2 is
less than a secondary threshold, then another step of the PN is
initiated and the above process is repeated.
[0047] In the verify state, the tracking loop switches to second
order and allows the tracking loop to pull the PN loop very close
to perfect correlation. Only the output of the lock detector 144 is
considered against a third and more difficult threshold and a much
longer dwell time. Variance of the detection signal is extremely
low after this long period of integration; after passing of this
threshold, the state machine declares lock, and moves into the
tracking state. If the lock detector fails the verify threshold
operation, the state machine steps the PN and returns to the
acquisition state described above.
[0048] In tracking mode, the loop bandwidth is narrowed. A long
integration time is used against a relatively high threshold, such
that only the variance caused by pure noise into the lock detector
will eventually cause failure of the test indicating loss of lock.
Preferably, the tracking loop is a second order phase locked loop
using a lead/lag type loop filter. The state machine remains in
this tracking state, until a loss of lock occurs. It then
transitions to the acquisition state, described above, without
changing the frequency bin. Under normal conditions, the despreader
will remain in the tracking state for as long as the signal is
present or until commanded to disable.
[0049] While in the tracking state, the DSP 140 sets the number of
bins to be searched to only one bin, regardless of the initial
configuration. In all other respects, reacquisition is the same as
normal acquisition, described above. In order to resume a full
acquisition frequency bin search, it is necessary to disable,
reconfigure and enable the PN despreader.
[0050] As will be appreciated from the foregoing description, the
considerable costs associated with using very high speed electronic
components to increase spreading processing gain, and using complex
mixer circuitry to minimize local oscillator spur leakage are
effectively obviated in accordance with the invention by
configuring the test signal emitter as a cascaded arrangement of
relatively inexpensive (leaky) mixer stages through which the RF
carrier is successively conveyed. By sequentially cascading the PN
sequence through carrier-multiplying mixer stages that are fed with
respectively different, relatively low rate, PN spreading
sequences, offset in time by a fraction of a chip from the sequence
applied to an adjacent mixer, the energy in the output carrier is
spread out over the very wide bandwidth of the resultant PN
sequence. This not only allows the use of relatively low chip rate
and inexpensive PN generator components, but reduces the net
leakage of the local oscillator carrier spur at the downstream end
of the cascaded mixers.
[0051] While we have shown and described an embodiment in
accordance with the present invention, it is to be understood that
the same is not limited thereto but is susceptible to numerous
changes and modifications as known to a person skilled in the art,
and we therefore do not wish to be limited to the details shown and
described herein but intend to cover all such changes and
modifications as are obvious to one of ordinary skill in the
art.
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