THEORY OF OPERATION
Power to the microphone is provided via the radio microphone connector. This +12
VDC source voltage is filtered by C3 and regulated to 5.0 volts by VR1.
Each time a key is pressed on the keypad, the normally low output of U2 pin 8 goes
high, forward biasing CR2 and causing C2 to charge. The rise in voltage on inputs of
U1C pins 8 and 9 causes the output pin 10 to go low. A low on either U1A pin 1 or U1B
pin 6 results in the output of U1A-3 and U1B-4 to latch to a high state. The high input
on the gate of Q1 in the drain output going to a low state and asserting a push-to-talk
output to the radio. The charge on C1 will keep PTT low for approximately 1.5 seconds
after each key is pressed. This delay allows for an adequate period of time for another
user-initiated key to be subsequently pressed before transmitter push-to-talk is allowed
to return to the unkeyed mode.
With jumper JP4 in-place, the DTMF tone generated will be a tone burst of
approximately 150 ms per digit, irregardless of how long the key is pressed. As
received from the factory, the jumper is removed and each DTMF tone is generated for
the entire duration of a pressed key.
If the Burst Mode is enabled (JP4 installed), the length of the DTMF tone burst may be
changed to accommodate a particular system requirement: Change R18 (270K) to a
higher value to increase, or to a lower value to decrease the time period for each DTMF
tone.
If the automatic push-to-talk feature is disabled by removal of resistor R2, the
microphone will have to be manually keyed (PTT depressed) to send the DTMF tones.
In this mode, when the PTT switch is depressed, an active high is placed on the gate of
Q1 causing the PTT output to go low enabling the transmitter and permitting the DTMF
tones to be transmitted.
The DTMF tone output is generated at U2 pin 16, and the DTMF output level is set with
RV1. The microphone element voice output is amplified by U3A, where the gain and
voice level is set by RV2 and then combined with the previously set DTMF audio level.
Both voice and DTMF tones are further amplified by U3B (the actual gain determined
by jumper JP1 setting: JP1 installed = low output, JP1 out = high output). The
combined DTMF tones and voice audio are coupled through a non-polarized capacitor
C11 to the microphone high input to the radio.
The DTMF tones are connected to the microphone element EM1 when the PTT button
is not depressed to provide audible sidetone of dialed digits. Sidetone is disabled if
resistor R2 is removed for the Manual PTT option.
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