Zonge GDP-32 - Timing Section

GDP-32
II
DESIGN
Section 16, Page 19 May 2002
TIMING SECTION
The primary function of the timing section is the Frequency Generator that generates
precise timing signals required for the digitization and synchronous detection at 24 binary
frequencies (2
-10
≤ f ≤ 2
13
Hz). There is also a Real-Time Clock that provides a more
accurate time base than the MPU clock.
Frequency Generator
The frequency generator divides the 4.980736 MHz
1
frequency from the master oscillator
down into binary frequencies representing the fundamental detection frequencies for the
GDP-32
II
receiver. Corresponding to each of the 24 selectable frequencies, there are 3
relevant synchronous pulse rates:
a) Period - A 100% duty cycle pulse rate at the selected frequency.
b) Duty Cycle - A 100% duty cycle pulse rate at twice the selected frequency. This
frequency is used for the current modulation in time-domain drive and to indicate
current ON and current OFF timing for time-domain data acquisition.
c) Sample - This pulse frequency provides the ADC convert command that is used for
data acquisition at the selected base frequency.
Under control of the MPU, the appropriate frequencies are selected to drive the analog
boards, and the transmitter I/O.
Real-Time Clock
A second ASIC has been added to BD244 and BD288, providing a real time clock driven
by the 2.490368 MHz frequency. This clock is set by the GDP-32
II
firmware and
thereafter, data are time-stamped with the time kept by BD244/288 rather than the less
accurate time kept by the MPU board (running under ROM-DOS). This real-time clock
was added to provide an absolute time reference to facilitate obtaining synchronous time-
series MT data with multiple receivers, and to enable synchronizing with the GPS system
(BD288).
GPS Receiver
A PCI-4 based GPS receiver board can be installed to provide discipline of the 4.980736
MHz crystal. It can also provide for automatic syncing of multiple GDP-32
II
’s, as well as
position information.
1
The GDP-16 and its predecessor, the GDP-12 were manufactured with 5.0 MHz oscillators, a
frequency that cannot be used to generate exact binary frequencies using integer division. Thus,
while the frequencies generated are nominally the same, for the purposes of synchronous
detection they are different. Because of this the GDP/XMT-32 instruments cannot be used
together with earlier generations of the GDP/XMT instrumentation without changing the earlier
equipment crystal frequencies from 5.0 MHz to 4.980736 MHz. The crystal oscillator frequency
was changed to achieve a significant improvement in noise rejection at the desired detection
frequencies.