Electronic Design

The following information has been excerpted from Brown, W. P. and E. G. Kerut. 1978. Air droppable RAMS (ADRAMS) buoy. AIDJEX Bulletin 40:21-29.

The oscillator is a temperature compensated crystal oscillator (TCXO) at a frequency of 3.134375 MHz. A higher frequency crystal could have been used, with less post-multiplication needed, but the 3 MHz crystal is inherently more stable and easier to compensate over the required wide temperature range. The 3 MHz oscillator signal is multiplied to 50 MHz in a X16 multiplier and then to the final 401.2 MHz carrier frequency with a X8 multiplier. The carrier signal is then phase-modulated by the 3-state phase modulator. The phase modulator uses a hybrid coupler for ease of impedance matching and uses pin diode switched transmission lines. Individual 10 volt logic levels from the COSMOS digital logic selects the phase state as 0 degrees, 60 degrees or -60 degrees. The phase modulator drives the power amp, which is a single chip hybrid circuit capable of up to 2.4 watts output. The digital encoder and timer generates the bit sync, frame sync and platform ID as MANCHESTER CODED 100 Hz data which is applied to the +/- 60 degree phase modulation inputs. It also generates a 0 degree phase signal at the beginning of the transmit cycle for 350 msec. The timer portion of the digital module contains a crystal oscillator and countdown circuit to generate the basic transmit timing cycle of one second per 62 seconds. The one second transmit gate is applied to the power switching and regulator circuit which provides a switched regulated voltage to the multiplier and power amplifier stages.

The battery supply provides continuous power to the TCXO and digital encoder and timer. These two units have an average drain of 12.7 milliamps. The remaining units have an average drain of 7.3 milliamps with their 1/62 duty cycle. The total average drain is 20 milliamps. Five parallel banks of four inorganic lithium cells in series are used as the battery supply. These cells are a relatively new development by General Telephone and Electronics Laboratories. They are hermetically sealed and therefore do not have the SO2 corrosion problem of the more common organic lithium cells. They provide very high energy on a weight and volume basis with reliable operation well below the required -50 degrees C. The cells weigh 6.5 oz, have a terminal voltage of 3.6 volts, a flat discharge characteristic, and a capacity of 22 amp hours. The twenty-cell battery pack has a voltage of 14.4 volts and a capacity of 110 amp hours. A single additional cell is used to provide switch and regulator bias. Expected life from the above pack is 7.5 months.

The antenna is a modified canted turnstile antenna which provides right hand circular polarization and good hemispherical coverage. It consists of four quarter wave vertical stubs canted 45 degrees tangentially around a half wave diameter circle. Each of the stubs is fed 90 degrees out of phase with the preceding one, progressing around the circle.

The original ADRAMS was designed for tracking only. The IABP project introduced a requirement for a data capability. Several units were modified to incorporate a precision barometer and an internal temperature sensor. Both sensor outputs were converted to 10 bit digital words and interfaced to the digital encoder. The temperature sensor range is +30 degrees to -50 degrees C with better than .1 degree C resolution. The barometer pressure range is 950 to 1050 millibars with .1 millibar resolution. Since the buoy had to be water tight, a special Teflon membrane was used for the pressure port which allowed barometric pressure changes to enter the buoy but kept moisture out.