1998 Product of the Year Award Winner

Two-chip GPS receiver promises
location-fixing in consumer products

The Global Positioning System has become an indispensable utility for
marine and aircraft navigation. Its inroads into more cost-sensitive markets
will doubtless be sped by the STB5600 and ST20GP6, a clever two-chip implementation
of a 12-channel GPS receiver from STMicroelectronics. Its already moderate introductory
price will plummet as production ramps up for a Japanese consumer-electronics giant.

The low-cost two-chip ST20GP6/STB-5600 GPS receiver
uses numerical processing to correct its local oscillator,
deriving its precision from the satellite clocks themselves.

A significant contribution to its low cost is the set's ability to use
an imprecise crystal oscillator and correct its internal frequency in the
process of locking the satellites. Since the signals coming from satellites
are Doppler shifted by the motion of the satellite, the receiver must have
the flexibility to correlate and lock the coarse acquisition code over
a range of frequencies.

With correlators that can accomodate both Doppler and local clock error,
the chip set leaves correction to the very end, and corrects the combined
frequency error in one operation.

The system uses two-stage downconversion, with the second mixer, local
oscillator, and filter all handled in the digital chip. A single-transistor
reference oscillator generates an 82-MHz fundamental frequency at the emitter
and 18th through 20th harmonics at the collector. A simple crystal without
temperature compensation provides adequate stability for initial acquisition.

When the satellites are locked, local signals are multiplied with the
correlated signals to produce a nominal zero frequency. The local numerical
oscillator is then adjusted to make the residual signal actually zero,
and its frequency is then exact. (See Electronic Products , June 1998, p. 91
for more explanation.)

The digital chip has room for masked ROM, enabling a high-volume application
to fit on the chipset entirely. Lower-volume applications will need a program memory.
(STB-5600, $4.50 ea/100,000; ST20GP6, $21 ea/100,000–available now.)

STMicroelectronics
Lexington, MA
Mark Hopkins 408-451-8169