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Claims  |
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What is claimed is:
1. A four phase to two phase correlator for processing a received RF signal
modulated by a transmitted PN code and a digital data signal, said
correlator being adaptable for use within a modem having means for
generating a channel 1 PN code signal, a channel 1 R code signal, a
channel 2 PN code signal, and a channel 2 R code signal, said modem also
including means for generating first and second unmodulated RF signals,
said correlator comprising in combination:
(a) first modulator means connected to receive said RF signal, and to
receive said channel 1 and channel 2 R code signals, for modulating said
received RF signal with said channel 1 R code signal and with said channel
2 R code signal to generate a first channel 1 RF signal and a first
channel 2 RF signal respectively, the first channel 1 RF signal being
modulated by the transmitted PN code signal, the channel 1 R code signal
and the digital data signal, and the first channel 2 RF signal being
modulated by the transmitted PN code signal, the channel 2 R code signal
and the digital data signal;
(b) second modulator means connected to receive said channel 1 and channel
2 R code signals, said first unmodulated RF signal, and to receive said
channel 1 and channel 2 PN code signals, said second modulator means
modulating said first unmodulated RF signal to generate a second channel 1
RF signal modulated by the channel 1 PN code signal and by the channel 1 R
code signal and a second channel 2 RF signal modulated by the channel 2 PN
code signal and by the channel 2 R code signal; and
(c) third modulator means connected to said first and second modulator
means to receive said first and second channel 1 RF signals and said first
and second channel 2 RF signals for combining said signals to generate a
third RF signal modulated only by the digital data signal.
2. The correlator set forth in claim 1 wherein said third modulator
includes:
(a) a first mixer connected to receive and combine said first channel 2 RF
signal and said second channel 2 RF signal to produce a first mixer output
signal;
(b) a second mixer connected to receive and combine said second channel 1
RF signal and said first channel 1 RF signal to produce a second mixer
output signal; and
(c) a power combiner connected to said first and second mixers for
receiving and combining said first and second mixer output signals.
3. The correlator set forth in claim 2 including a band pass filter
connected to said power combiner and having a predetermined pass band of
frequencies excluding the frequency of the RF signal received by said
correlator.
4. The correlator set forth in claim 1 wherein said first modulator means
includes a fourth modulator means connected to receive said channel 1 and
channel 2 R code signals and said second unmodulated RF signal from said
modem for producing channel 1 and channel 2 R code modulated RF signals
for use by said first modulator means to generate said first channel 1 RF
signal and said first means to generate said first channel 1 RF signal and
said first channel 2 RF signal.
5. The correlator according to claim 4 wherein said first modulator means
includes:
(a) first power divider means for equally dividing the received RF signal
into first and second received RF signal components;
(b) first mixer means for mixing the first received RF signal component
with the channel 1 R code modulated RF signal to generate the first
channel 1 RF signal; and
(c) second mixer means for combining the second received RF signal
component with the channel 2 R code modulated RF signal to generate the
first channel 2 RF signal.
6. The correlator according to claim 5 wherein said second modulator means
includes:
(a) first digital mixer means for combining the channel 1 PN code signal
with the channel 1 R code signal to generate a channel 1 PN plus R code
signal; and
(b) second digital mixer means for combining the channel 2 R code signal
with the channel 2 PN code signal to generate a channel 2 PN plus R code
signal, said channel 1 PN plus R code signal and channel 2 PN plus R code
signal modulating said first unmodulated RF signal to produce said second
channel 1 RF signal and said second channel 2 RF signal respectively.
7. The correlator according to claim 6 wherein said second modulator means
further includes isolator hybrid means for equally dividing the first
unmodulated RF signal into a first channel 1 unmodulated RF signal and a
first channel 2 unmodulated RF signal, the first channel 1 unmodulated RF
signal and the first channel 2 unmodulated RF signal being shifted in
phase 90.degree. with respect to one another.
8. The correlator according to claim 7 wherein said second modulator means
further includes:
(a) third mixer means connected to receive said channel 1 PN plus R code
signal and said first channel 1 unmodulated RF signal to produce said
second channel 1 RF signal; and
(b) fourth mixer means connected to receive said channel 2 PN plus R code
signal and said first channel 2 unmodulated RF signal to produce said
second channel 2 RF signal.
9. The correlator according to claim 8 wherein said fourth modulator means
includes second power divider means connected to receive and equally
divide the second unmodulated RF signal into a second channel 1
unmodulated RF signal and a second channel 2 unmodulated RF signal.
10. The correlator according to claim 9 wherein said fourth modulator means
further includes:
(a) fifth mixer means connected to receive the channel 1 R code signal and
the second channel 1 unmodulated RF signal to modulate said second channel
1 unmodulated RF signal with said channel 1 R code signal to generate the
channel 1 R code modulated RF signal; and
(b) sixth mixer means connected to receive the channel 2 R code signal and
the second channel 2 unmodulated RF signal to modulate said second channel
2 unmodulated RF signal with said channel 2 R code signal to generate the
channel 2 R code modulated RF signal.
11. The correlator set forth in claim 10 wherein said third modulator
includes:
(a) a first mixer connected to receive and combine said first channel 2 RF
signal and said second channel 2 RF signal to produce a first mixer output
signal;
(b) a second mixer connected to receive and combine said second channel 1
RF signal and said first channel 1 RF signal to produce a second mixer
output signal; and
(c) a power combiner connected to said first and second mixers for
receiving and combining said first and second mixer output signals.
12. The correlator set forth in claim 11 including a band pass filter
connected to said power combiner and having a predetermined pass band of
frequencies excluding the frequency of the RF signal received by said
correlator.
13. A four phase to two phase correlator for use in the demodulator section
of a modem for processing a received spread spectrum RF signal, said modem
including means for generating channel 1 and channel 2 PN code signals,
channel 1 and channel 2 R code signals, and first and second unmodulated
RF signals, said correlator comprising:
(a) means for modulating said first unmodulated RF signal with said channel
1 R code signal to produce a channel 1 R code modulated RF signal;
(b) means for modulating said first unmodulated RF signal with said channel
2 R code signal to produce a channel 2 R code modulated RF signal;
(c) power divider means connected to receive said spread spectrum RF signal
and to divide said signal into first and second component signals;
(d) means connected to receive said first component signal and to receive
said channel 1 R code modulated RF signal to mix said signals and produce
a first channel 1 RF signal;
(e) means connected to receive said second component signal and said
channel 2 R code modulated RF signal to produce a first channel 2 RF
signal;
(f) a first digital mixer means connected to receive said channel 1 PN and
channel 1 R code signals to produce a channel 1 PN plus R code signal;
(g) a second digital mixer means connected to receive said channel 2 R code
and channel 2 PN code signals to produce a channel 2 PN plus R code
signal;
(h) means connected to receive said channel 1 PN plus R code signal and to
receive said second unmodulated RF signal to produce a second channel 1 RF
signal;
(i) means connected to receive said channel 2 PN plus R code signal and
said second unmodulated RF signal to produce a second channel 2 RF signal;
(j) mixer means connected to receive said first channel 1 RF signal and
said second channel 1 RF signal to produce a first mixer output signal;
(k) mixer means connected to receive said first channel 2 RF signal and
said second channel 2 RF signal and produce a second mixer output signal;
(l) power combiner means connected to receive said first and second mixer
output signals; and
(m) band pass filter means connected to receive an output signal from said
power combiner means, said filter having a center pass frequency different
than the frequencies of said first and second unmodulated RF signals and
different than the center frequency of said spread spectrum RF signal.
14. A four phase to two phase correlator for use in the demodulator section
of a modem for processing a received spread spectrum RF signal, said modem
including means for generating channel 1 and channel 2 PN signals, channel
1 and channel 2 R code signals, and first and second RF signals, said
correlator comprising:
(a) means for dividing said spread spectrum RF signal into first and second
channel signals;
(b) modulating means for modulating each said channel signals with a first
RF signal modulated by a channel R code signal to thereby produce a
channel RF signal for each of said channels;
(c) modulating means for modulating each of said channel RF signals with a
signal generated by modulating said second RF signal with the digitally
mixed PN and R codes for that channel to produce a second RF signal for
each channel;
(d) modulating means for modulating each channel RF signal with the second
RF signal for each respective channel to produce an RF output signal; and
(e) power combiner means for combining said RF output signals and applying
the combined output signal to a band pass filter. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to demodulators, and more particularly, to a spread
spectrum demodulator for demodulating a spread spectrum four phase PSK RF
signal.
In digital data transmission systems it is frequently desirable to provide
a system of modulation which will increase the immunity of the
transmission system to intentional jamming and to provide a highly secure
data encoding system which can only be demodulated by selected receiver
systems. One method of accomplishing these two goals is to provide a
digital communication system in which the bandwidth of the transmitter
signal is spread over a substantially greater bandwidth than the bandwidth
of the data signal. This is generally accomplished by mixing the digital
data signal with a wide band pseudo-random sequence of pulses. The
psuedo-random modulated data signal then phase modulates a reference
carrier signal.
This modulation method generates a wide band RF signal which is commonly
known as a spread spectrum signal. Spread spectrum signals are highly
secure in that only receivers capable of generating a pseudo-random
sequence of pulses identical to the transmitted sequence will be capable
of demodulating the digital data signal.
Prior art spread spectrum demodulators have incorporated four phase to two
phase correlator circuits which perform adequately but are structurally
relatively simple. As a result of the structural simplicity of these prior
art correlators, the maximum achievable processing gain of the correlator
is limited. Utilization of a minimum number of mixers in prior art
correlator circuits often results in a less than desirable degree of
isolation between the input of the correlator and its output. An
additional disadvantage of the relatively straightforward prior art
correlator circuit designs is that they provide only a minimal resistance
to jamming. In a hostile electromagnetic environment which includes active
electronic countermeasures systems, extremely high resistance to jamming
is an absolutely essential requirement for the successful operation of a
communications system.
Apparatus related to the present invention and which are designed to
demodulate a spread spectrum signal modulated by a pseudo-random code
sequency have been described in two commonly assigned pending United
States Patent Applications entitled, "Spread Spectrum Demodulator" (Ser.
No. 611,366) and "Improved Spread Spectrum Demodulator" (Ser. No. 611,367)
which were filed simultaneously on September 8, 1975. Each of these
pending patent applications contains background and disclosure material
which is relevant to the description of the present invention.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a four
phase to two phase correlator which provides a significantly increased
resistance to CW jamming signals by correlating a wide bandwidth
transmitted signal into a narrow bandwidth data output signal to
substantially decrease the energy of the jamming signal contained in the
data output signal.
Another object of the present invention is to provide a four phase to two
phase correlator which provides increased isolation between its input and
output stages.
Yet another object of the present invention is to provide a four phase to
two phase correlator which provides increased signal processing gain.
Briefly stated and in accordance with one embodiment of the invention, a
four phase to two phase correlator is incorporated within the demodulator
section of a spread spectrum modem to process a received RF signal
modulated by a transmitted PN code and a digital data signal. The
correlator correlates a four phase PN (pseudo noise) code which is
internally generated within the demodulator section of the modem with a
received four phase PSK (phase shift keyed) modulated carrier signal to
provide a two phase collapsed spectrum data output signal.
The demodulator section of the modem includes a circuit for generating a
channel 1 PN code signal and a channel 1 R (phase shifted PN code) code
signal and another circuit for generating a channel 2 PN code signal and a
channel 2 R code signal.
The correlator itself includes first modulator means for combining the
channel 1 and channel 2 R code signals with the received RF signal to
generate a first channel 1 RF signal and a first channel 2 RF signal. The
first channel 1 RF signal is modulated by the transmitted PN code signal,
the channel 1 R code signal and the digital data signal. The first channel
2 RF signal is modulated by the transmitted PN code signal, the channel 2
R code signal and the digital data signal.
The correlator also includes second modulator means for combining the
channel 1 and channel 2 PN code signals and the channel 1 and channel 2 R
code signals with a first unmodulated RF signal to generate a second
channel 1 RF signal which is modulated by the channel 1 PN code signal and
by the channel 1 R code signal and a second channel 2 RF signal which is
modulated by the channel 2 PN code signal and the channel 2 R code signal.
Third modulator means combines both channels of the first and second RF
signals to generate a third RF signal which is modulated only by the
digital data signal. Wideband noise and virtually all of any CW jamming
signal will have been eliminated from the third RF signal. The digital
data signal can readily be detected from the third RF signal and further
processed.
DESCRIPTION OF THE DRAWINGS
The invention is pointed out with particularity in the appended claims.
However, other objects and advantages, together with the operation of the
invention, may be better understood by reference to the following detailed
description taken in connection with the following illustrations wherein:
FIG. 1 is a generalized block diagram of a complete spread spectrum modem
which includes a four phase to two phase correlator of the present
invention.
FIG. 2 is a series of three graphs which further explain the operation of
the spread spectrum modem shown in FIG. 1.
FIG. 3 is a highly generalized block diagram of the four phase to two phase
correlator of the present invention.
FIG. 4 is an intermediate level block diagram of the four phase to two
phase correlator of the present invention.
FIG. 5 is a schematic representation of the four phase to two phase
correlator of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, correlator 10 of the present invention is typically
incorporated within a spread spectrum PSK modem similar to the one shown
in FIG. 1. The digital data input signal is mixed in digital mixer and
channelizer 14 with a pseudo-random (PN) sequence which is generated by a
PN generator 16. The modulation of the digital data signal with the
pseudo-random sequence spreads the digital data signal over the wide
bandwidth of the PN sequence. A PN clock oscillator 18 sets the bit rate
of PN generator 16 which will be high compared to the bit rate of the
digital data signal. The digitally mixed wideband signal is channelized as
required by digital mixer and channelizer 14. The model carrier oscillator
22 generates an output signal which is phase modulated by the wideband
signal from digital mixer 14 in phase modulator 20. This modulation
process produces a spread spectrum signal which is transmitted over a
communications channel by a transmitter 24. Alternatively, the spread
spectrum output signal may be achieved by first phase modulating the
carrier signal with the digital data system and then combining that
modulated signal with the PN sequence. In either case, the final bandwidth
of the output signal is dictated by the PN sequence bandwidth and the type
of modulation, such as biphase, four phase or staggered four phase
modulation.
The transmitted signal is received and amplified in receiver 26, which band
limits and converts to a higher frequency both the transmitted spread
spectrum PSK signal and any jamming signals accepted by the receiver
antenna. The output of receiver 27 is fed to both a PN timing loop and
synchronizer 28 and to the correlator 10, which is the subject of the
present disclosure.
PN timing loop and synchronizer 28 generates a local PN sequence identical
to that generated by PN generator 16 in the modulator section of the
modem. Upon receiving an input signal, PN timing loop and synchronizer 28
searches the received spread spectrum signal until the locally generated
PN sequence has the same phase relationship as the PN modulation of the
received spread spectrum signal. Synchronizer 28 then maintains the
locally generated PN sequence phase locked to the received PN modulation.
The locally generated PN sequence produced by PN timing loop and
synchronizer 28 under phase locked conditions is correlated by correlator
10 with the received spread spectrum signal. The output signal from
correlator 10 is a narrow band carrier signal which is phase modulated by
the digital data signal plus wideband noise and a spread CW jamming
signal.
An IF strip 30 in the demodulator section 12 of the modem amplifies and
filters the data output signal from correlator 10 removing most of the
undesired energy contained in the wideband noise and spread CW jamming
signal. The narrow band PSK output signal from IF strip 30 is coupled to a
carrier recovery loop 32 which generates a clean carrier signal and locks
it with one of the carrier phases of the signal from IF strip 30. The
output signal from carrier recovery loop 32 is then correlated with the
signal from IF strip 30 in a carrier correlator 34. The output of carrier
correlator 34 is baseband data plus some undesired signal components
caused by the jamming signal and noise.
The output signal from carrier correlator 34 is coupled to a bit timing
loop 36 and to a data processor 38. Bit timing loop 36 generates a clean
bit timing signal at the data clock rate and phase locks in with the
proper phase relationship to the baseband data. Data processor 38 match
filters the baseband data, samples every bit time at the correct bit
phase, holds the sampled data between samples, and converts the sampled
data into the appropriate data output levels.
As illustrated in FIG. 2, the increased resistance to jamming signals is
obtained by spreading the transmitted signal over a wide bandwidth (FIG.
2A) and then correlating the received spread spectrum transmitted signal
into a narrow bandwidth digital data signal (FIG. 2C) from which virtually
all jamming signal strength has been removed. In the process of
establishing this narrow bandwidth digital data signal, the correlator 10
which is situated within the demodulator section 12 of the modem spreads
an incoming CW jamming signal over a wide bandwidth (FIG. 2B) thus
decreasing the jamming energy contained within the narrow digital data
bandwidth.
Channel 1 (0-180.degree.) PN and R codes and channel 2 (90-270.degree.) PN
and R codes are locally generated in the demodulator section of the modem
shown in FIG. 1 by PN timing loop and synchronizer 28 in a manner well
known to those skilled in the art. The PN and R codes of each channel are
identical digital signals which have been shifted in phase by a
predetermined number of bits with respect to each other by PN timing loop
and synchronizer 28.
Referring now to FIG. 3, a highly generalized block diagram of correlator
10 of FIG. 1 will be discussed. The received spread spectrum RF input
signal to correlator 10 has a carrier frequency F.sub.1 modulated by the
transmitted PN code and the digital data signal. This received spread
spectrum RF signal is coupled to the input of first modulator means 40
which combines it with the channel 1 and channel 2 R code signals (locally
generated in the modem) to generate a first channel 1 RF signal and a
first channel 2 RF signal. The first channel 1 RF signal is modulated by
the transmitted PN code signal, the channel 1 R code signal and the
digital data signal. The first channel 2 RF signal is modulated by the
transmitted PN code signal, the channel 2 R code signal and the digital
data signal. The channel 1 and channel 2 output signals from first
modulator means 40 are coupled to third modulator means 42.
Second modulator means 44 combines the channel 1 and channel 2 PN code
signals (locally generated in the modem) and the channel 1 and channel 2 R
code signals with a first unmodulated RF signal having a frequency F.sub.2
to generate a second channel 1 RF signal. The first unmodulated RF signal
of frequency F.sub.2 is generated in the modem and is applied to the
correlator of the present invention. The second channel 1 RF signal is
modulated by the channel 1 PN code signal and by the channel 1 R code
signal. Second modulator means 44 also generates a second channel 2 RF
signal which is modulated by the channel 2 PN code signal and by the
channel 2 R code signal. The channel 1 and channel 2 output signals from
second modulator means 44 are also coupled to inputs of third modulator
means 42.
Third modulator means 42 combines both channels of the first and second RF
signals generated by first modulator means 40 and second modulator means
44 to generate a third RF signal which is modulated only by the digital
data signal.
Referring now to FIG. 4, the elements of correlator 10 and the signal flow
path thereof will be discussed in greater detail.
First modulator means 40 also includes fourth modulator means 46. Fourth
modulator means 46 includes second power divider means 54, fifth mixer
means 56 and sixth mixer means 58. Second power divider means 54 equally
divides the second unmodulated RF signal having a frequency F.sub.3 into a
second channel 1 unmodulated RF signal and a second channel 2 unmodulated
RF signal. The second unmodulated RF signal, of frequency F.sub.3, is
generated in the modem; the relationship of the frequencies F.sub.2,
F.sub.3 and the received signal center frequency F.sub.1 will be described
hereinafter. Fifth mixer means 56 combines the channel 1 R code signal
with the second channel 1 unmodulated RF signal to generate the channel 1
R code modulated RF signal. Sixth mixer means 58 combines the channel 2 R
code signal with the second channel 2 unmodulated RF signal to generate
the channel 2 R code modulated RF signal.
First modulator means 40 includes first power divider means 48 which
equally divides the received RF signal into first and second received RF
signal components. The first received RF signal component is coupled to
first mixer means 50 which mixes that signal with the channel 1 R code
modulated RF signal to generate the first channel 1 RF signal. Second
mixer means 52 combines the second received RF signal component with the
channel 2 R code modulated RF signal to generate the first channel 2 RF
signal.
Second modulator means 44 includes a quadrature hybrid or isolator hybrid
means 60, third mixer means 62, fourth mixer means 64, first digital mixer
means 66 and second digital mixer means 68.
First digital mixer means 66 combines the channel 1 PN code signal with the
channel 1 R code signal to generate a channel 1 PN plus R code signals.
Second digital mixer means 68 combines the channel 2 R code signal with
the channel 2 PN code signal to generate a channel 2 PN plus R code
signal.
Isolator hybrid means 60 equally divides a first unmodulated RF signal
having a frequency F.sub.2 into a first channel 1 unmodulated RF signal
and a first channel 2 unmodulated RF signal. Isolator hybrid means 60 is a
90.degree. hybrid which alters the phase angle between the first channel 1
unmodulated RF signal and the first channel 2 unmodulated RF signal so
that they are phase shifted 90.degree. with respect to one another.
Third mixer means 62 combines the channel 1 PN plus R code signal with the
first channel 1 unmodulated RF signal to generate the second channel 1 RF
signal. Fourth mixer means 64 combines the channel 2 PN plus R code signal
with the first channel 2 unmodulated RF signal to generate the second
channel 2 RF signal.
Third modulator means 42 receives the first channel 1 RF signal and the
first channel 2 RF signal from first modulator means 40 and the second
channel 1 RF signal and the second channel 2 RF signal from second
modulator means 44. Third modulator means 42 processes these four separate
input signals in a manner to be described to generate the third RF signal
which is modulated solely by the digital data signal plus very low
amplitude noise and jamming signals.
Referring now to FIG. 5, a detailed schematic representation of the four
phase to two phase correlator 10 of the present invention will now be
described. The channel 1 and 2 PN code signals and channel 1 and 2 R code
signals are provided by the demodulator section of the modem. These
signals are synchronized and locked to the incoming RF signal by the PN
loop and synchronizer of the demodulator. The received spread spectrum and
jamming signals having a frequency centered about F.sub.1 are equally
split in phase by first power divider means 48 to form an input signal for
each of the two identical signal processing channels of correlator 10.
Although the two processing channels are identical in implementation, the
code inputs and phase of the RF inputs to first mixer means 50 and second
mixer means 52 are different.
Mixer means 50 and 52 include mixers 70 and 72, band pass filters 74 and
76, and amplifiers 78 and 80, respectively. Mixer means 56 and 58 include
flip flops 82 and 84, drivers 86 and 88, balanced mixers 90 and 92, and
band pass filters 94 and 96, respectively. Flip flops 82 and 84 serve
merely to reclock the channel 1 and channel 2 R code signals to ensure
time synchronization between these R code signals and the other digital
signals in correlator 10. Drivers 86 and 88 increase the output amplitude
of the digital signals from flip flops 82 and 84 to a level sufficient to
drive balanced mixers 90 and 92. Power divider 54 operates as previously
discussed to equally divide its input signal into a pair of signals to
drive the second inputs of balanced mixers 90 and 92. The output signals
from mixers 90 and 92 are R code modulated signals having a frequency
centered about F.sub.3. These two output signals have been referred to
above as the channel 1 and channel 2 R code modulated RF signals. Band
pass filter and amplifiers 94 and 96 decrease the switching signal
amplitude from drivers 86 and 88 within the pass band of filters 74 and
76. The isolation provided by mixers 70 and 72 reduces the filter
requirements for filters 94 and 96. Filters 94 and 96 isolate the
switching signals from drivers 86 and 88 from the data signal path through
first and second mixer means 50 and 52.
Band pass filters 74 and 76 are designed to select the sum output frequency
from mixers 70 and 72 to provide an upward frequency translation. Without
this upward frequency translation at mixers 70 and 72, only the isolation
provided by mixers 70 and 72 would control the switching signal level
within the pass band of filters 74 and 76. Although band pass filters 74
and 76 do provide increased protection against pulsed or swept jamming
signals, their primary function is to attenuate the undesired mixer
products generated by mixers 70 and 72. Amplifiers 78 and 80 are required
to reduce the input noise figure of correlator 10 and to compensate for
the signal losses induced by first power divider 48, mixers 70 and 72, and
filters 74 and 76.
In second modulator means 44 first and second digital mixer means 66 and 68
include exclusive or gates or digital mixers 91 and 93 and flip flops 95
and 97, respectively. Third and fourth mixer means 62 and 64 include
drivers 98 and 100, mixers 102 and 104, band pass filters 106 and 108, and
amplifiers 110 and 112, respectively.
Digital mixers 91 and 93 mix the channel 1 PN code with the channel 1 R
code and the channel 2 PN code with the channel 2 R code. Flip flops 95
and 97 serve strictly to reclock the output signals from mixers 91 and 93
to ensure proper time synchronization with the other digital signals of
correlator 10. Drivers 98 and 100 increase the output amplitude of the
digital signals from flip flops 95 and 97 to provide a sufficient drive
level to balanced mixers 102 and 104.
The RF local oscillator signal F.sub.2 is split by isolator hybrid means 60
(a quadrature hybrid) into two equal but 90.degree. out-of-phase signals.
Each of these output signals from isolator hybrid means 60 is then phase
modulated in mixers 102 and 104 by the respective digitally mixed PN plus
R code signals. The 90.degree. hybrid is provided to convert the
0.degree.-180.degree. phase shift of mixer 102 to a phase shift of
90.degree.-270.degree.. A similar phase shifting or phase map rotation
could be accomplished by interchanging isolator hybrid means 60 with power
divider means 54 without changing the bandwidth requirements of the
90.degree. hybrid.
The outputs of mixers 102 and 104 are bandwidth limited by filters 106 and
108, respectively, to attenuate both the switching and modulation
frequencies which would otherwise lie within the desired data bandwidth
centered at F.sub.4. Without the attenuation of these two filters,
undesired signal components would be amplified by amplifiers 110 and 112
to a level such that the isolation of mixers 114 and 116 in third
modulator means 42 would be insufficient to prevent severe limiting of the
input sensitivity of correlator 10. Mixers 70 and 72 are bandwidth limited
by filters 74 and 76 for similar reasons. Amplifiers 110 and 112 are
required to obtain sufficient drive power for mixers 114 and 116 to
minimize conversion loss and intermodulation products. In a similar manner
amplifier sections are required in band pass filters 94 and 96 to provide
the appropriate drive levels to mixers 70 and 72.
The first channel 1 RF signal and the first channel 2 RF signal are mixed
with the second channel 1 and channel 2 RF signals by mixers 114 and 116
and the resulting mixer output signals are added vectorially by a power
combiner 118. This vector addition results in a 3dB signal loss. Provided
that F.sub.4 .noteq. F.sub.1, F.sub.1 + F.sub.3 .noteq. F.sub.4, and
F.sub.2 - F.sub.1 .noteq. F.sub.4, the frequency translations accomplished
by mixers 70, 72, 114, and 16 will prevent the isolation of these mixers
from limiting the maximum achievable processing gain or resistance to a CW
jamming signal. The specific frequencies used are usually dictated by
considerations outside the present correlator; for example, transmission
characteristics, circuit capabilities and the like, will affect the choice
of frequencies. However, it is important to select frequencies that are
sufficiently and properly spaced relative to each other to minimize
intermodulation.
Frequency translations at mixer pairs 70-72 and 114-116 are required to
prevent switching signals from limiting the input sensitivity of
correlator 10. The R code effectively increases the maximum achievable
processing gain with respect to a synchronous CW jamming signal from that
of the isolation of a single mixer, to that of two mixers in series. That
is, the isolation of a single mixer such as 102 or 104 is effectively
increased to that of two mixers in series such as mixer 90 in series with
mixer 102, or mixer 92 in series with mixer 104.
Band pass filter 120 receives the output of power combiner means 118 and
removes most of the undesired energy contained in the wideband noise and
spread CW jamming signals. The filter bandwidth required depends on
whether the following IF strip 30 (FIG. 1) includes a smaller bandwidth
filter with the maximum bandwidth being set by the data bandwidth.
Four possible phase map rotation combinations exist which will result in a
data biphase modulated output signal from correlator 10. The output of the
90.degree. hybrid 60 can be inverted to couple its 90.degree. port to
mixer 104 and the 0.degree. port to mixer 102. Also, the
90.degree.-270.degree. channel PN code input to correlator 10 from the PN
loop and synchronizer 28, can be inverted or the code channel signal
between mixer 92 and mixer 104 can be inverted to generate an appropriate
data output from correlator 10. With the isolator hybrid means coupled as
shown in FIG. 5, inverting both the 0.degree.-180.degree. and the
90.degree.-270.degree. code inputs provides the correct data output. With
isolator hybrid means 60 inverted, only the 0.degree.-180.degree. channel
PN code input need be inverted. Map rotations of this nature are well
known to those skilled in the art.
The quadrature hybrid or isolator hybrid means 60, as well as the mixers,
power dividers and combiners, are circuits which are well known and are
readily available to those skilled in the art; for example, catalogs such
as the Mini-Circuits Laboratory Catalog, 1975, or the Anzac Catalog, 1975,
provide those skilled in the art with a selection of circuits that may be
used in the correlator of the present invention.
In operation, the received spread spectrum RF signal is equally divided by
the power divider 48 to form two channels. Although the two channels are
identical and incorporate identical elements, the code inputs and phases
of the RF inputs of the respective channels are different. The two outputs
from the power divider 48 are mixed in mixers 70 and 72 with their
respective R code channel signals, the latter generated by the phase
modulations of the internally generated unmodulated RF frequency F.sub.3
in mixers 90 and 92 respectively. The resulting first channel 1 and first
channel 2 RF signals are applied to mixers 114 and 116 to be combined with
the second channel 1 and second channel 2 RF signals. Each code channel
has its PN and R codes digitally mixed by digital mixers or exclusive OR
circuits 91 and 93; the locally generated unmodulated RF signal F.sub.2 is
split by the hybrid means 60 into two equal but 90.degree. out-of-phase
signals to be modulated in mixers 102 and 104 by its respective digitally
mixed channel codes derived from the mixers 91 and 93. The outputs of the
mixers 102 and 104 are each bandwidth limited by band pass filters 106 and
108 to derive the second channel 1 and second channel 2 RF signals. The
outputs of the mixers 114 and 116 are added vectorially by the power
combiner 118 and applied to band pass filter 120 to remove the undesired
energy contained in the wide band noise and spread CW jamming signals
existing outside of the band pass of the filter 120. The center frequency
F.sub.4 of band pass filter 120 is related to the RF input carrier
frequency and the locally generated unmodulated frequencies F.sub.2 and
F.sub.3 in the manner shown in FIG. 5. It may be noted, however, that
while the relationship of the frequencies (F.sub.4 = F.sub.1 + F.sub.3 -
F.sub.2) is inherent in the correlator of the present invention, the
selection of the locally generated frequencies F.sub.2 and F.sub.3 is
limited only by the frequency capabilities of the circuits used and the
need to eliminate intermodulation among the signals. The output signal
from the band pass filter 120 is thus a two-phase nonspread PSK output
signal which is available for further processing in the modem demodulator.
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