Gating 



The gating circuit (Fig 6) is entirely digital for precise repeat- 

 ability and stability. The gating ranges are in fixed incremental steps, 

 identified by an accurate calibration dial. The circuitry operates on a 

 constant time basis; for example, it will allow keying for 5 seconds and 

 recording for 5 seconds. As a result, when the recorder is switched from 

 the UOO to the 100-fathom scale, the times for gating and keying remain 

 the same. This allows expanded scale operation on the 100-fathom scale 

 at depths where gating is needed, without adjustments when scales are 

 switched . 



The gating cycle is controlled by a timing pulse which is generated 

 in the timing pulse generator gearbox. This pulse normally occurs once 

 per scan when the stylus is precisely at the middle of the chart. When 

 the gate circuit causes the recorder to change from key to record or 

 vice versa, the change-over point occurs at the center of the chart. 

 The timing pulse is divided by a string of binary, integrated circuit 

 flip-flops. On the UOO-fathom scale, the once-per-second pulse is fed 

 to FF3, and FF3 through FF6 divide the pulse by an integer ratio. The 

 Gate Range switch selects how often that the Gate FF is triggered. The 

 Gate FF provides a binary control signal for the key or record Interval. 



When the recorder is on the 100-fathom scale, FFl and FF2 divide the 

 timing pulse (now at h per second) by a factor of k, such that the input 

 to FF3 is again at one pulse per second. The gate operation now is the 

 same as on UOO fathoms, with the exception that i| keys per second are now 

 made. This assures the same sampling density, in time, regardless of 

 scale. Gating is disabled by grounding the Gate FF output, thus allowing 

 simultaneous keying and recording for identification of scattering layer 

 structures . 



Key Circuit 



The key circuit (Fig 7) provides an accurate, fixed-length contact 

 closure simulation to an external sonar transceiver. The time of occur- 

 rence of the pulse is also controlled by the timing pulse generator, which 

 generates a highly stable and accurate pulse to trigger the keying 

 circuitry. The jitter of this pulse is in the order of +1 sec. Since 

 the key timing pulse occurs at the zero edge of the chart, the gate and 

 key timing pulses can be interchanged to provide center of chart keying. 

 'Both pulses can be adjusted with relation to the paper with a micrometer- 

 type screwdriver adjustment, allowing the leading edge of the key pulse 

 to be adjusted easily with respect to the chart. 



The pulse from the timing generator is fed to the "key" miltivibrator 

 which generates the length of the keying interval. This length is varied 

 in two ranges by a front panel control from 0.2 to 20 ms . The output of 

 the multivibrator is fed to the Key Gate which has as a second input the 



190 



