Digital demodulator-correlator
First Claim
1. Apparatus for demodulation of a code modulated signal having a predetermined frequency and for correlation of the demodulated signal with a reference code, comprisingmeans for sampling said code modulated signal at a frequency four times said predetermined frequency and for converting each sample to digital form, thereby to produce four equally spaced successive samples A1, B1, A2 and B2 in digital form for each cycle of said modulated signal,four separate means for accumulating multiplication products of M1 and M2 with said samples in digital form thereby to produce four sums according to the following equations:
- ##EQU17## where M1 is a reference signal at said predetermined frequency multiplied by said reference code, and M2 is said reference signal multiplied by said reference code delayed one quarter of a cycle,means for computing inphase and quadrature correlation functions I and Q, respectively, from said sums in accordance with the following equations;
space="preserve" listing-type="equation">I = S1.sub.I cos α
- S2.sub.I sin α
space="preserve" listing-type="equation">Q = S1.sub.Q cos α
- S2.sub.Q sin α
where sin α and
cos α
are determined at the start of demodulation operations according to the following equations;
##EQU18## while both multipliers M1 and M2 are made to be equal to said reference signal, andmeans for making both multipliers M1 and M2 equal to said reference by setting said reference code and said reference code delayed one quarter of a cycle equal to 1 at the start of demodulation and correlation operations while sin α and
cos α
are being determined.
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Abstract
Apparatus for demodulation and correlation of a code modulated 10 MHz signal is comprised of a sample and hold analog-to-digital (A/D) converter synchronized by a frequency coherent 40 MHz pulse to obtain four evenly spaced samples A1,B1,A2 and B2 of each cycle of the signal, and means for adding, or subtracting, each sample to, or from, one of four accumulators to form the sums: ##EQU1## where M1 = 10 MHz Reference · Receiver Code ∠0°
M2 = 10 mhz Reference · Receiver Code ∠90°
The correlation functions that are used for the range measurements are then computed from the following equations:
I = S1.sub.I cos α - S2.sub.I sin α
Q = S1.sub.Q cos α - S2.sub.Q sin α
where sin α and cos α are determined at the start of each range acquisition by forcing the receiver code for both M1 and M2 to be equal to 1 and applying these equations: ##EQU2##
46 Citations
12 Claims
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1. Apparatus for demodulation of a code modulated signal having a predetermined frequency and for correlation of the demodulated signal with a reference code, comprising
means for sampling said code modulated signal at a frequency four times said predetermined frequency and for converting each sample to digital form, thereby to produce four equally spaced successive samples A1, B1, A2 and B2 in digital form for each cycle of said modulated signal, four separate means for accumulating multiplication products of M1 and M2 with said samples in digital form thereby to produce four sums according to the following equations: - ##EQU17## where M1 is a reference signal at said predetermined frequency multiplied by said reference code, and M2 is said reference signal multiplied by said reference code delayed one quarter of a cycle,
means for computing inphase and quadrature correlation functions I and Q, respectively, from said sums in accordance with the following equations;
space="preserve" listing-type="equation">I = S1.sub.I cos α
- S2.sub.I sin α
space="preserve" listing-type="equation">Q = S1.sub.Q cos α
- S2.sub.Q sin αwhere sin α and
cos α
are determined at the start of demodulation operations according to the following equations;
##EQU18## while both multipliers M1 and M2 are made to be equal to said reference signal, andmeans for making both multipliers M1 and M2 equal to said reference by setting said reference code and said reference code delayed one quarter of a cycle equal to 1 at the start of demodulation and correlation operations while sin α and
cos α
are being determined. - View Dependent Claims (2, 3, 4)
- ##EQU17## where M1 is a reference signal at said predetermined frequency multiplied by said reference code, and M2 is said reference signal multiplied by said reference code delayed one quarter of a cycle,
-
5. Apparatus for demodulation of a code modulated signal having a predetermined frequency, and for correlation of the demodulated signal with a reference code, comprising
means for sampling said code modulated signal at a frequency four times said predetermined frequency in response to a sampling control signal, said sampling means including means for converting each sample to digital form, means responsive to a reference signal at said predetermined frequency for producing said sampling control signal so that four equally spaced samples A1, B1, A2 and B2 all occur at the same phases of successive cycles of said code modulated signal, and means for accumulating multiplication products of M1 and M2 with said samples in digital form in two pairs to form the difference of samples A1 and A2, and the difference of samples B1 and B2 in each of two channels by adding or subtracting each successive sample in accordance with a first multiplier M1 comprised of the product of a reference code and a squarewave signal at said predetermined frequency, and a second multiplier M2 comprised of the product of said reference code delayed one quarter of a cycle of said squarewave signal to form four sums according to the following equations: - ##EQU19## and means for computing inphase and quadrature correlation functions I and Q, respectively, from said sums in accordance with the following equations;
space="preserve" listing-type="equation">I = S1.sub.I cos α
- S2.sub.I sin α
space="preserve" listing-type="equation">Q = S1.sub.Q cos α
- S2.sub.Q sin αwhere sin α and
cos α
are determined at the start of demodulation and correlation operations according to the following equations;
##EQU20## while said reference code and said reference code delayed are both set equal to one. - View Dependent Claims (6)
- ##EQU19## and means for computing inphase and quadrature correlation functions I and Q, respectively, from said sums in accordance with the following equations;
-
7. Apparatus for demodulation and correlation of a binary code modulated signal with a reference binary code, said signal being at a predetermined frequency greater than the pulse rate of said binary code, comprising
means for sampling said code modulated signal at four equally spaced intervals of each cycle of said code modulated signal, and for converting each sample to digital form, whereby four samples A1, B1, A2 and B2 in digital form are obtained at the same phases of said code modulated signal during successive cycles, means for accumulating products of M1 and M2 with said samples to form four separate sums S1I, S2I, S1Q and S2Q according to the following equations: - ##EQU21## where M1 is the product of a reference signal at the same frequency as said code modulated signal and said reference code, and M2 is the product of said reference signal and of said reference code delayed one quarter of a cycle of said reference signal, and
means for computing inphase and quadrature correlation functions I and Q in accordance with the following equations;
space="preserve" listing-type="equation">I = S1.sub.I cos α
- S2.sub.I cos α
space="preserve" listing-type="equation">Q = S1.sub.Q cos α
- S2.sub.Q sin αwhere sin α and
cos α
are determined in advance by forcing said reference code to 1 for both multipliers M1 and M2 while producing said sums and computing ##EQU22## - View Dependent Claims (8, 9, 10, 11, 12)
- ##EQU21## where M1 is the product of a reference signal at the same frequency as said code modulated signal and said reference code, and M2 is the product of said reference signal and of said reference code delayed one quarter of a cycle of said reference signal, and
Specification