Push-to-talk interface circuit for a radio communication system

Details Push-to-talk interface circuit for a radio communication system Push-to-talk interface circuit for a radio communication system

1998-06-18

2001-04-03

To, Doris H. (Department: 2745)

Telecommunications

Transmitter and receiver at same station

With transmitter-receiver switching or interaction prevention

C455S073000, C455S083000, C327S427000

Reexamination Certificate

active

06212365

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to interface circuits for communication systems and, more particularly to an interface circuit for a push-to-talk radio having a single transistor and a minimal number of components for providing an indication to the radio to initiate transmission.
BACKGROUND OF THE INVENTION
Devices such as tactical radios often use an indicator signal to command the radio to begin transmission. The most common method of accomplishing this command is to provide a mechanical switch on the radio's handset. This switch provides an electrical signal to the radio to begin transmission. In other applications, this Push-to-Talk (PTT) signal is generated either electronically or electromechanically (via a relay).
The normal signaling protocol is for a closure to ground (zero volts) to command ‘transmit’. The command “don't transmit” is exemplified by an open circuit or via a high potential voltage or low voltage across the terminals of a circuit.
Due to the nature of how PTT signals are generated, with mechanical switches and electromechanical relays, there is often significant noise and high-voltage ringing present on PTT lines. In some applications, significant Lengths of cable are present on the PTT line providing additional opportunity for high voltage transients and noise to infiltrate the PTT interface.
Previous designs of PTT interfaces have used circuits with high cost components or large numbers of components.
FIG. 3
shows the traditional design used in the present production of a push-to-talk SINCGARS radio. As shown in
FIG. 3
, such design requires 2 transistors (Q
1
,Q
2
)and 13 total components including a power supply.
FIG. 4
shows another prior art implementation of a PTT interface. This design, while requiring only nine components, however, further includes a costly differential amplifier
100
and also requires both a positive supply V
2
and negative supply voltage V
3
to operate. The high component count, high cost, and the need for negative supply voltages associated with prior art PTT interface circuits make it extremely desirable to obtain a low cost, low power interface circuit which is operable with a minimal number of components.
As can be seen in
FIG. 1
, the design according to the present invention overcomes these shortcomings by the judicious use of a single, low cost field effect transistor. Current routing with diodes precludes the need for a negative supply voltage.
SUMMARY OF THE INVENTION
A circuit for selectively enabling communications by providing an output signal voltage of a first level indicative of a first transmission state and a second level corresponding to a second non-transmission state, the circuit comprising an FET having a gate, source, and drain, the source coupled to a first controlling means for providing a voltage Vs to the source, the drain coupled to an output terminal of the circuit for providing the output signal, and the gate coupled to a second control means for providing a voltage Vg to the gate, the FET operable in a first conducting mode when a gate to source voltage Vgs exceeds a predetermined threshold, indicative of a non-zero crossing amplitude region of an input voltage signal, to cause the output signal voltage to be at the second level corresponding to the non-transmission state, and in a second non-conducting mode, wherein the Vgs of the FET is less than the threshold, indicative of a zero-crossing of the input signal, for causing the output signal voltage to go to the first level, indicative of the transmission state, wherein the first and second control means responsive to the input voltage signal operate to vary the FET gate to source voltage and route current through the circuit to cause the FET to operate in the conducting and non-conducting modes.


REFERENCES:
patent: 5430401 (1995-07-01), Shtulman
patent: 5444400 (1995-08-01), Hall et al.
patent: 5475329 (1995-12-01), Jones et al.
patent: 5801550 (1998-09-01), Tanaka et al.
patent: 5818283 (1998-10-01), Tonami et al.
patent: 5959475 (1999-09-01), Zomorrodi

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