Differential receiver

A phase differential determining apparatus generates low frequency output signals from a multiple channel receiver and extracts relative phase angles of the received RF signals therefrom. The same local oscillator signals, which are phase locked to one of the received RF signals, are utilized for all channels thus preserving the phase difference of the RF signals during the heterodyning process so that phase angle differences between the low frequency output signals is the same as the phase angle differences between the received RF signals. The relative phase angles are determined at the low frequency output level by phase shifting a low frequency signals from the local oscillator that is phase locked to a reference channel and varying the phase shift applied thereto until the phase difference between this low frequency local oscillator signal and the low frequency output signal of the selected channel is zero. The phase shift introduced to the low frequency local oscillator signal is equal to the difference in phase between the two RF signals.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to relative phase measurements and more 
specifically to receivers for determining phase differentials of RF 
signals. 
2. Description of the Prior Art 
Various electronic navigation systems are well known in the art. Loran 
systems, for instance, have found wide acceptance and are proved to be a 
valuable aid to navigation. More recently, a Global Positioning System 
(GPS) has been proposed wherein a constellation of satellites will be 
positioned in orbit around the Earth. Each satellite transmits data by via 
signals operating at two frequencies which are modulated by dual 
psuedorandum noise codes in clear/acquisition (CA) and precision (P) codes 
peculiar to that satellite. The modulation is structured to establish a 
spread spectrum transmission that can be identified and processed by the 
user to determine his position. Receivers for utilization and such systems 
are described, for instance, in U.S. Pat. No. 4,457,006 issued to Reuben 
E. Maine on June 26, 1984 entitled "Global Positioning System Receiver" 
and in U.S. Pat. No. 4,453,165 issued to Reuben E. Maine on June 5, 1984 
entitled "Differential Doppler Receiver" both patents being assigned to 
the assignee of the instant invention. 
The receiver disclosed in the former U.S. Application utilizes a phase lock 
loop to derive local oscillators for a double heterodyne receiver which 
converts IF signals from the earth's orbital satellite to audio frequency 
output signals. Since both local oscillators are locked to a common 
reference oscillator, the phase variation of the audio signal contains all 
the information impressed on the RF signals. In the latter U.S. 
application, the receiver deisclosed retains the advantages of prior GPS 
receivers and additionally accurately measures the Doppler frequency of RF 
signals from a transmitter moving with respect to the receiver by using a 
second transmitter having a stable and precisely known frequency as the 
reference. Signals from the two sources are processed in a differential 
mode to eliminate the need for a precise timing clock in the receiver. 
The receiver of the present invention generates low frequency output 
signals from which the relative phase angles of an RF carrier signal 
received at separated antennas may be extracted. 
SUMMARY OF THE INVENTION 
The present invention relates to a receiver which determines the phase 
difference between signals indicent from a common source to antennas that 
are positioned at separated locations. The signals collected by each 
antenna are respectively coupled to heterodyning circuits, each of which 
may be of the double heterodyne type, for conversion to low frequency 
output signals. Included in each heterodyning circuit is circuitry for 
selecting a desired coded signal, from a plurality of coded signals, with 
the aid of a selection signal coupled thereto. A local oscillator, phase 
locked to one low frequency output signal and consequently to one of the 
RF signals, provides the local oscillator signals required for the 
heterodyning process. A signal representative of the phase locked local 
oscillator signals is phase shifted and coupled to a phase comparator with 
a low frequency output signal from a second heterodyning circuit, 
wherefrom a signal is provided that is representative of the phase 
difference between the phase shifted local oscillator representative 
signal and the low frequency output signal from the second heterodyning 
circuit. This phase representative signal is utilized to provide a control 
signal to vary the phase value of the phase shifter until the low 
frequency output signal from the second heterodyning circuit and the phase 
shifted local oscillator representative signals are in phase. The value of 
the phase shifter when the two signals are in phase is representative of 
the phase difference between the incident signals at the two antenna 
locations.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A plane wave signal instant to separated antennas 11 and 12 induces signals 
thereat that vary in phase in accordance with the angle of incidence and 
the separation of the antennas. In accordance with the invention, the 
signal from antenna 11 is coupled to a receiver 13, which may be of the 
type fully described in the aforementioned U.S. Pat. No. 4,457,006. 
Briefly, the signal from antenna 11 is coupled through a bandpass filter 
14 and an RF amplifier 15 to a mixer 16 through which a first local 
oscillator frequency is also coupled via line 17. The difference frequency 
signal from the mixer is amplified by wideband IF amplifier 21 and coupled 
to a modulator 22 wherein it is biphase modulated by a signal 
representative of the C/A code applied to the transmitted signals from the 
satellite. When the locally generated C/A code is in time synchronization 
with the transmitted code the effect of the transmitted code modulation is 
cancelled and the bandwidth of the signal is returned from the wide spread 
spectrum bandwidth to the original narrow bandwidth occupied by the 
satellite data stream. The selected C/A coded signal is coupled from 
modulator 22 through a narrowband IF amplifier 23 wherefrom it is coupled 
along with a second local oscillator signal via line 25 to a second mixer 
24 wherefrom a low frequency signal, which may be in the order of 1 KHz, 
is coupled to a low frequency amplifier 26. 
The low frequency signal from the low frequency amplifier 26 and a signal 
from local oscillator (LO) 27, representative of the local oscillator 
frequencies are respectively coupled to the "D" and clock terminals of a 
"D" type flip-flop 28 which is included in biphase demodulator 32, which 
may be of the type described in U.S. Pat. No. 4,344,041 entitled "Biphase 
Detector", issued to Reuben E. Maine on Aug. 10, 1982 and assigned to the 
assignee of the present invention. Flip-flop 28 functions as a phase 
detector, detecting the polarity of the low frequency signal when clocked 
by the LO representative signal. The signal at the clock terminal is 
additionally coupled to an exclusive OR gate 33, to which the "Q" output 
of flip-flop 28 is also coupled. Exclusive OR gate 33 operates to invert 
the clock pulse when the low frequency signal from amplifier 26 lags the 
clock signal and maintains the clock phase when the low frequency signal 
leads the clock signal. Output signals from OR gate 33 are coupled via 
line 34 to one input terminal of a second exclusive OR gate 35, having a 
second input terminal coupled, via line 36, to the output terminal of the 
low frequency amplifier 26. Signals at the output terminals of OR gate 35 
are coupled via a lowpass filter (LPF)37 to a d.c. amplifier 38, wherefrom 
signals representative of the phase difference between the clock signal to 
flip-flop 28 and the low frequency signal from amplifier 26 are coupled to 
a signal from the comparator circuit 41, the output terminal of which may 
be coupled to a frequency increment command unit 42. After each system 
clock pulse for which the filtered and amplified signal at the output 
terminal of exclusive OR gate 35 exceeds a threshold signal coupled to the 
comparator 41, the frequency increment command unit 42 provides a signal 
via line 43, that is representative of a coarse frequency increment of the 
local oscillator 27, to a sum circuit 44. The signal at the output 
terminal of amplifier 38, representative of a fine frequency adjustment, 
is also coupled, via line 45, to the sum circuit 44. The sum of the 
signals coupled to the sum circuit 44 is coupled via line 46 to a voltage 
controlled crystal oscillator 48 which is the basic unit of the local 
oscillator 27. Signals from the voltage controlled crystal oscillator 48 
are coupled via line 52 to a frequency multiplier 53, wherefrom the 
previously mentioned first and second local oscillator frequencies are 
respectively coupled to the mixers 16 and 17. The output signal of the 
voltage controlled crystal oscillator 48 is also coupled via line 54 to 
divide by N circuit 55, wherefrom a signal at a frequency that is 1/N 
times the frequency of the signal at the output terminals of the voltage 
controlled crystal oscillator 48 is coupled via line 56 to the clock 
terminal of flip-flop 28. 
Similarly, signals received at antenna 12 are coupled via bandpass filter 
61 and RF amplifier 62 to a mixer 63 to which the first local oscillator 
signal is also coupled. An IF signal from mixer 63 is coupled via a 
wideband amplifier 64 to coded signal modulator 65 wherefrom the selected 
signal is amplified by narrowband IF amplifier 66 and coupled to a second 
mixer 67 in the channel, to which the second local oscillator signal is 
also coupled. The low frequency signal from the second local oscillator is 
coupled via amplifier 68 to an input terminal of an exclusive OR gate 72 
included in a second biphase demodulator 70. One input terminal of 
exclusive OR gate 72 is coupled to the output terminal of a second 
exclusive OR gate 73 in biphase demodulator 70. Exclusive OR gate 73 has a 
first input terminal coupled via phase shifter 74 to the output terminal 
of the divide by N circuit 55 and a second input terminal coupled to the 
"Q" terminal of the flip-flop 28 in the biphase demodulator 32. 
When the clock signal to flip-flop 28 leads the signal at the "D" input 
terminal thereof, and the signal at the "Q" terminal is high, OR gate 73 
inverts the phase shifted clock signal prior to coupling to the second 
input terminal of exclusive OR gate 72, while a non-inverted signal is 
coupled when the clock signal lags the signal at the "D" terminal. Thus, 
the biphase demodulator 70 is slaved to the biphase demodulator 32. 
Exclusive OR gate 72 provides a phase comparison between the signal at the 
output terminal of exclusive OR gate 73 and the low frequency signal 
coupled from amplifier 68, which is of a phase substantially equal to 
phase of the received signal at antenna 12. The signal at the output 
terminals of exclusive OR gate 72 is coupled via lowpass filter 75 and a 
d.c. amplifier 76 to an amplitude comparator 77, wherefrom a signal is 
coupled to a phase shift incrementing signal generator 78 whenever the 
signal from d.c. amplifier 76 exceeds a threshold signal coupled to 
comparator 77. For each system clock pulse, during which a signal is 
coupled to the incrementing phase shift signal generator 78, a signal to 
increment the phase shift, by a predetermined differential phase angle, is 
coupled to a phase shifter 74. This process continues until the signal 
from the d.c. amplifier 76 does not exceed the threshold signal coupled to 
comparator 77. At this time, the total phase angle of the phase shifter 74 
as read out of "read out device" 79 is substantially the phase difference 
between the signal at antenna 11 and the signal at antenna 12. 
While the invention has been described in its preferred embodiments, it is 
to be understood that the words which have been used are words of 
description rather than limitation and that changes may be made within the 
purview of the appended claims without departing from the true scope and 
spirit of the invention in its broader aspects.