Title: Transmit-receive switching circuit with audio muting

Description: BACKGROUND OF THE INVENTION

This invention relates to the field of two-way communications equipment, and more particularly to the capability of transmitter-receiver switching with minimum noise output, and channel selection with minimum noise during "receive" and power amplifier protection during transmit.

Most mobile two-way communications equipment utilizes the depression of a "push-to-talk" switch, which is typically located on a hand-held microphone, to initiate transmission. Depression of the PTT switch keys up the transmitter and cuts off the receiver; when the switch is released, the transmitter is turned off and the equipment returns to "receive" mode. Also, most such equipment has the capability of operating on any one of a number of channels. Since the equipment is likely to be used in some type of emergency situation, the latter capability usually makes it possible for the user to locate an unused channel as needed. However, both of these switching functions, as well as other switching circuitry, can cause objectionable noise in the audio output. Channel switching during "transmit" can also cause damage to the transmitter power amplifier and its related circuitry. Therefore, a need exists for a means of eliminating such noise and providing circuit protection without adding unnecessary cost or complexity to the equipment.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a means of switching a two-way communications apparatus between operating modes and between channels while, at the same time and with common circuitry, preventing switching noise in the audio output and circuit damage.

The above objectives are accomplished in the present invention by a transmitter switching circuit, a receiver switching circuit--the former controlling the latter-- two delay circuits, gating circuits and a one-shot multivibrator.

The gating circuits enable or disable the power amplifier of the transmitter and mute or unmute the audio portion of the receiver, and each gate has one input coupled to the one-shot which in turn is activated by the channel select switch or other circuits. A second input of the gate which enables the power amplifier in the transmitter is activated by the transmit-receive switching circuitry through a slow-react, fast-decay (SRFD) circuit. A second input of the gate which mutes the audio output of the apparatus is activated by the transmit-receive switching circuitry through a fast-react, slow-decay circuit (FRSD). Thus the audio is always muted when the power amplifier is switching.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a typical two-way communications apparatus embodying the present invention.

FIG. 2 is a block diagram of the circuit of the invention.

FIG. 3 is a portion of the diagram of FIG. 2 with an alternate embodiment.

FIG. 4 is a function chart showing the on-off relationships of parts of the apparatus.

FIG. 5 is a schematic of the transmitter and receiver switching circuits.

DESCRIPTION OF A PREFERRED EMBODIMENT

The structure and function of the present invention will be best understood in relation to the drawing of which FIG. 1 provides a block diagram of two-way mobile communications device. The device consists essentially of two main sections, a receiver 10 and a transmitter 11. The receiver 10 includes a tuner 12, a muting circuit 13, an audio amplifier 14 and a speaker 15. The transmitter 11 includes a power amplifier 16, an oscillator-modulator-amplifier circuit 17, and a microphone 20 which includes a push-to-talk switch 21. Functioning with both of the two main sections is an antenna 22, an antenna relay 23, a channel selector switch and associated circuitry 24, and the control circuit 25 according to the invention.

FIG. 2 shows the control circuit 25 broken down into a block diagram and also includes the PTT switch 21 circuit. This type of equipment operates normally in "receive" mode, with squelch when no signal is being received. When the PTT switch is depressed, the antenna 22 is switched from "receive" to "transmit" by the relay 23 which is connected to a 12 v. V supply. The receiver audio amplifier 14 is also biased off during transmission via terminal 26. The control circuit 25 includes a transmitter switching circuit 27 which is coupled to and actuates a receiver switching circuit 28. The transmitter is enabled through the terminal 30 and, through a fast-react, slow-decay circuit (FRSD) 31 and an OR gate 32, the audio muting circuit 13 of the receiver 10 is activated. The receiver switch 28 enables the receiver through a terminal 33 and, through a slow-react, fast-decay circuit (SRFD) 34 and a NOR gate 36, the power amplifier (PA) 16 of the transmitter 11 is disabled.

A one-shot multivibrator 37 whose input is coupled to the channel select switch 24, has its output coupled to second inputs of both the OR gate 32 and the NOR gate 36. The circuits and operation of the switches 27 and 28 will be described hereinafter in relation to FIG. 4.

FIG. 3 is another embodiment wherein the channel select switch 24 is coupled to the input of the one-shot 37 via a second OR gate 38. Other radio circuits, e.g. the noise blanker (not shown) could be coupled to the other inputs of the OR gate 38 for activating the one-shot 37.

FIG. 4 is a function chart showing the on-off relationships 40, 41, 42, 43 of the transmitter 11, receiver 10, PA 16 enable and audio mute 13, respectively. At T.sub.0, the receiver is on, the transmitter, PA enable and audio mute are off. At T.sub.1, the PTT button has been depressed and the receiver 10 will go off and the transmitter 11 on. Due to the SRFD circuit 34, the PA 16 of the transmitter 11 is slow to come on while the audio mute 13, due to the FRSD circuit 31, is quickly activated. The audio bias function, mentioned hereinbefore in relation to terminal 26, is too slow to provide this capability. Noises due to transmitter turn on are thus prevented from being heard through the audio amplifier 14 and speaker 15. At T.sub.2, the PTT switch 21 is released to switch from transmit to receive. Now the PA 16 is disabled quickly through the SRFD circuit 34 before the audio mute 13 is disabled through the FRSD circit 31. T.sub.3 is a repeat of T.sub.1, and T.sub.4 is a repeat of T.sub.2.

At T.sub.5 and T.sub.6 the one-shot has been activated by the channel select switch or other circuitry. At T.sub.5 the PA 16 of the transmitter is on and could be damaged by pulses caused by the channel select switch (which may involve a number of relays). Therefore, the PA is briefly disabled through the NOR gate 36. The audio mute 13 is already enabled at this time. At T.sub.6, the receiver is on, the PA 16 is already off and the audio mute 13 is disabled. Thus it is only necessary for the channel select switch to enable the audio mute at T.sub.6.

FIG. 5 is a schematic diagram of the transmitter switch 27 and receiver switch 28 which form the major part of the switching circuit 25. The switch 27 comprises two transistors 44, 45 in series across a regulated 9 v. supply. The transmitter bias terminal 30 and FRSD terminal 31 connect to the mid-point 46 of the transistors 44, 45. If the transistor 44 is conducting and the transistor 45 is not conducting, the mid-point 46 will be "high", the transmitter will be "on" and the receiver audio mute will be enabled through the FRSD 31. If, on the other hand, transistor 44 is not conducting and transistor 45 is conducting, the mid-point 46 will be "low", and the transmitter will be "off" and the audio not muted.

When the PTT switch 21 is depressed the terminal 47 is grounded, putting approximately 9 v. between the emitter of the transistor 44 and the terminal 47. The transistor 44 will go into saturation and the terminal 30 is high or approximately 9 v. Conversely, when the PTT switch is released, the terminal 47 goes high, transistor 44 is cut off and transistor 45 conducts.

These two transistors control the transmitter, however if both were to conduct at the same time, they would be destroyed. Therefore, the remaining components, resistors 50, 51, 52, 53, zener diodes 54, 55, and diode 56, provide the appropriate voltages with a "dead" zone where neither transistor 44 or transistor 45 will conduct. The zener diodes 54, 55 are chosen to have voltage drops of more than half the voltage of the supply, so that as the voltage on the terminal 47 goes from 0 to 12 v., or from 12 to 0 v., a "dead" zone is created extending from about 3.5 to 5.5 v. where neither transistor 44 or transistor 45 can conduct. Thus, if the PTT switch should arc over because of dust or dirt, or the operator should depress or release the PTT switch slowly, transistors 44, 45 would not be damaged. The diode 56 provides circuit protection for transistor 56 in the event of a voltage rise at the terminal 47.

Since the midpoint 46 of the transmitter switch is also a terminal for the receiver switch 28 (analogous to terminal 47), the same theory of operation applies to the receiver switch. The transistors 57, 58 are connected across the supply voltage with their mid-point 59 supplying voltage for receiver bias 33 and the SRFD 34. When the mid-point 46 is high, the mid-point 59 is low, the receiver is biased off and the power amplifier for the transmitter is actuated through SRFD 34. When the mid-point 46 is low, the mid-point 59 is high, the receiver is biased on, and the power amplifier of the transmitter is disabled. As in the transmitter switch, the resistors 60, 61, 62, 63 and zener diodes 64, 65 provide the appropriate operating voltages and the "dead" zone for the transistors 57, 58. The extra diode is not needed in the receiver switching circuit.

There has been provided for a two-way communications apparatus an improved means of switching from "receive" to "transmit" and from channel to channel with minimal noise and adequate circuit protection from switching pulses. Although specific embodiments are given by way of illustration, it is evident that other modifications and variations would be apparent to those skilled in the art. It is intended to include all such variations as fall within the spirit and scope of the appended claims.