Digital direct sequence spread spectrum receiver

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS FIG.
1 shows a block diagram of a direct sequence spread spectrum receiver according to the present invention having an RF signal converter 1, a digitizing IF signal sampler 2, a code demodulating signal extractor 3, and a data processing signal tracking section 4.
A particular embodiment will be described, by way of illustration, for receiving GPS navigational positioning signals.
In this embodiment, the receiver receives an L.
sub.
1 band signal at 1575.
42 MHz and an L.
sub.
2 band signal at 1227.
6 MHz.
The signal converting stage 1 performs IF down conversion and includes an amplifier 6 followed by a first local oscillator section 5 where the amplified received signal is mixed with the local oscillator signal at a frequency of 1401.
51 MHz to provide a signal at a common IF frequency of 173.
91 MHz for both the L.
sub.
1 and L.
sub.
2 satellite signals.
This IF signal is filtered by filter 7, and amplified by amplifier 8.
It then passes through a second local oscillator stage 9 where it is mixed with a local oscillator signal at 161.
41 MHz to form an IF signal centered at 12.
5 MHz.
The second local oscillator frequency is selected to obtain an IF signal at a frequency slightly greater then the 10 MHz GPS code frequency offset by the maximum expected carrier Doppler.
At any given time, the actual IF frequency thus varies directly with the Doppler shift.
This IF frequency is denoted f.
sub.
o.
The signal then passes through a 20 MHz low pass filter 10 as an amplified IF output signal on line 11.
The IF signal at frequency f.
sub.
o is then sampled and digitized at a sampling frequency f.
sub.
s by A/D converter 2 to form a received N-bit digital IF signal.
For purposes of a commercial GPS receiver of moderate accuracy, useful for example in general aviation and small boat navigation, as few as 4-bit signals (3 amplitude bits and 1 sign bit) may be used, although preferably higher-accuracy 6- or 8-bit A/D conversion is employed.
The N-bit digitized signals are sent to first and second signal discrimination stages 3a, 3b of the signal extractor section 3 to develop in phase and quadrature tracking component signals