Teledyne 440, Системное руководство, Страница: 221 / 256

A – Operating Theory
DPN 402196 Issue 4.1 © Teledyne TSS A- 3
A.2.1 Timing
Timing during each cycle of measurement is an important consideration:
Figure A-2: Timing relationship for a single cycle of measurement
Each measurement cycle starts at the beginning of the voltage drive pulse – see Figure A-2 . This pulse
lasts for 1500µs while the current flowing through the coil rises as shown in Figure A-1 (b).
At the end of the 1500µs pulse, the SEP removes drive voltage from the coil. As the magnetic field
collapses with the current, self-inductance causes a 250V spike to appear across the coil windings with
a polarity opposite from that of the original drive pulse.
The 440 System waits for a further 100µs to allow the voltage spike to decay and to allow the effects of
sea water to dissipate. The System then begins a 1200µs sampling period as shown at
Figure A-2 (c).
During this period the SEP makes 300 measurements at intervals of 4µs and digitises these to 12-bit
accuracy before storing them in a buffer memory.
Within the sampling period there are three regions, each of which is 100µs wide:
❐
Sample Region 1 – ‘Early’
This region begins 130µs after the beginning of the sample period and provides the ‘early’
level for the channel. The SEP calculates and stores the average value of the 25 individual
samples taken within this region
❐
Sample Region 2 – ‘Standard’
This region begins 300µs after the beginning of the sample period and provides the ‘standard’
level for the channel. The SEP calculates and stores the average value of the 25 individual
samples taken within this region.
❐
Sample Region 3 – ‘Zero’
This region begins 1000µs after the beginning of the sample period and provides the ‘zero ref-
erence’ level. The SEP calculates and stores the average value of the 25 individual samples
taken within this region.
At the end of the sampling period, there is an interval of 325µs while the SEP performs calculations on
its measurements. See
section A.2.2 "Derivation of Signal Voltage" on page A-4 for an explanation of
1500
1000µs
s
(a)
70µs
(b)
1200µs
(c)
325µs (d)
3125µs
Drive pulse
0V
Standard Zero Early
130µs
300µs
Each sample window is 100 µ s