it was chosen not to do so to ensure good reliable 
contacts, The signal circuit impedance is about 
500 ohms so that small conductance cable leaks 
will not adversely affect performance. 
TEMPERATURE TRANSDUCER 
The transducer pressure case contains 
thermistors imbedded in the lower end cap. 
See Figures 5, 6, and 7. The transistorized 
electronic oscillator and the Zener voltage 
regulator diode are attached to the upper end 
cap. The buoy's 24v batteries are used to 
power the temperature transducer through a 
dropping resistor. In the event that a short 
circuit occurs in the cable, only about 0.1 
amps will flow through the resistor. This is 
small compared to the current consumed by 
the rest of the transmitter. 
The transducer oscillator is a Wien Bridge 
type well known for its amplitude and frequency 
stability, This bridge is shown in Figure 8. 
The active thermistors R1 and R2 in the 
right hand arms are the frequency determining 
resistors along with capacitors Cl and C2. The 
frequency of oscillation is determ‘ned by the 
relation 
1 
i or Ray Cis (1) 
The thermistors Rl and R2 are located in the 
lower end cap. The capacitances Cl and C2 are 
the Mylar dielectric type chosen for a degree of 
temperature stability. They are attached to the 
back of the circuit board, Their temperature 
coefficient is such that their capacity at 0°C is 
2% less than that at room temperature. This is 
better than paper capacitors but not as good as 
the polyethylene type which has a nearly zero 
temperature coefficient. With the latter units, 
the response time for good accuracy can be 
reduced to that time required for the thermistors 
to come to equilibrium with their surroundings. 
In the drift buoy application the transducer will 
remain continuously in its environment and 
hence it is acceptable to calibrate the entire 
unit in a constant temperature bath. 
A square wave is generated by a squaring 
amplifier. It is fed by the divided outputs of the 
push-pull oscillator. The square wave is fed 
through an emitter follower for additional 
isolation and low impedance. The signal output 
is a square wave from 0 to +13 volts for a 
positive supply and 0 to -13 for a negative 
supply. The frequency of this square wave 
ranges from about 2 to 10 cps or from 100 to 
500 milliseconds per cycle. The latter time 
period expression is commonly used in this 
report. The transmitter keying relay can run 
at the highest repetition rate encountered and is 
used unmodified. Conductance switching can be 
used if higher rates are ever required. The 
buoy transmitter keying is achieved by opening 
the internal keying circuit and allowing the 
temperature transducer output signal to do the 
keying. See Figure 4, 
CALIBRATION DATA 
Figure 9 is a reproduction of the calibration 
curve from which the integer temperature read- 
ings were made. A table was prepared which 
gave interpolation values to 0.1°C, Further 
interpolation to 0.01°C could be made with the 
listing of proportional parts. The raw data is 
good to t+ .01°C, the manual curve fit to perhaps 
+ .03°C, For accurate calibration to within 
.03°C a least-square-curve fit should be made 
with the raw data. 
RECEIVING STATION ASHORE 
The buoy transmits ona frequency of 2398 KC 
with a power output of approximately 20 watts. 
The radiated power is on the order of 2 watts due 
to antenna inefficiency. A downward shift of the 
R.F. carrier frequency of 140 cps occurs when 
the keying contacts close. 
The receiving station is shown schematically 
in Figure 10. The carrier is converted to an 
audio tone by operating the receiver in the BFO- 
ON position, The carrier shift of 140 cps down- 
ward produces an audio tone shift of 140 cps 
downward when the BFO frequency is below the 
carrier frequency and 140 cps upward when the BFO 
frequency is above the carrier frequency. See 
Figure 11, 
In the case of temperature measurement it is 
inconsequential whether the BFO is set above or 
below the carrier since the datum comprises the 
total time period for one complete cycle of freq- 
uency shift. To convert the FSK audio signal 
into a signal suitable for triggering a timing cir- 
cuit, the signal is fed through a standard IRIG 
subcarrier discriminator centered at 1700 cps. 
This discriminator as a linear output over a 
+ 7.5% range. The discriminator response 
limits the bandwidth to that frequency occupied by 
its ''S'' curve. A narrow pass frequency occurs 
for lower center frequency units. The 960 cps 
center frequency (IRIG Channel No, 4) has a pass 
band of slightly more than 144 cps. The reason 
for choosing a low oscillator frequency is so that 
a very narrow shift hence a narrow discriminator 
response can be used and still have a high modu- 
lation index. The narrow shift allows a narrow 
band pass, off the shelf, discriminator to be used 
to advantage. Keying of the transmitter is 
simplified to merely "pulling" the crystal R.F. 
oscillator with series capacitance. Also the FSK 
mode of transmission has good antifade qualities. 
This is not to say that a wider shift, or extremely 
wide shift, say 6 KC, couldn't be used to ad- 
vantage, to allow frequency diversity reception 
techniques. This requires some special filters, 
keyers, etc. and comprises a new field of develop- 
ment effort. 
