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D – ALTIMETER

DPN 402197 © TSS (International) Ltd Page 9 of 18

The dc-dc converter is also controlled by an automatic switch which puts the circuit in

SHDN mode if the DC input is higher than 15.7VDC.

The transmitter voltage is regulated to 12.9VDC to allow operation of the transmitter

driver chip which requires at least 12VDC to operate, (the driver output will go open

circuit if the voltage falls below this).

The digital 5Vsupply is fed from a normal linear regulator. This supply inhibits the dc-

dc converter if it falls below approximately 4.5VDC.

Smoothing reservoir capacitors are used at the DC input and also at the +5VDC line

to eliminate any noise that is passed from the power supply.

D.3.2.2 Transmitter

The microcontroller generates a TTL signal pulse at TP7 which determines operating

frequency and pulse length, both these parameters are programmed into the micro-

controller’s flash memory and can be altered if necessary, by the manufacturer, using

the programming input header J4.

The transmitter power section is interfaced to the microcontroller signal level by a

power driver which is designed to switch high current signals via the two MOSFET

transistors IC15 & IC17 through the step up transformer T1 or optionally T2.

The secondary inductance of the transformer and the capacitance of the transducer

components form a tuned circuit at the operating frequency thus forming a high ampli-

tude sine wave. Fine tuning of the transmitter output is achieved by adding capacitors

to C43 and C47, (working voltage of the capacitors are 1000VDC).

The transmitter is inhibited if the +5VDC supply falls below 4.5VDC.

The transformer secondary inductance and tuning capacitors are kept out of the

receiver path by steering diodes. A damping resistor R45 reduces ringing from the

transducer when the transmitter pulse is removed.

D.3.2.3 Receiver

The same transducer is used to receive and transmit therefore protection diodes in

series with a resistor protect the sensitive receiver circuit when transmission occurs.

TVG is applied to the signal before being fed to a band pass filter set to the operating

frequency. The signal is then demodulated and fed to a threshold detector.

Gain control lines GAIN1-8 are fed from the microcontroller and provide TVG control

of the receiver; this enables received signals varying over a wide dynamic range to be

received. The initial sensitivity of the system is controlled by the microcontroller which

switches an attenuator into circuit reducing the amplitude of the signal before reach-

ing the receiver.

Signal BLANK, from the microcontroller controls the attenuator for the initial reception

period which is set at a nominal 1 metre during which period high amplitude signals

from side lobes and near field objects are attenuated.

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