Ptt Sensing And Tx Audio Processing - Motorola VHF2 Service Information

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7.1.2 Standard Microphone
Hook Pin is shorted to the hook mic inside the standard Mic, If the mic is out off hook, 3.3V is routed
to R429 via R458, D401, and it create 0.7V on MIC_SENSE (u.P U403-67) by Voltage Divider
R429/R430. U403 senses this voltage and sends command to ASFIC_CMP U504 to get GCB3 ='1'.
The audio signal is routed from C5045 via U509-3 (Z1), R5072, U507, R5026, C5091, R5014 via
C5046 to U504- 46 int mic (C5046 100nF create a159Hz pole with U504- 46 int mic impedance of
16Kohm). 9.3VDC is routed via R5077, R5075 to J2-15, It create 4.65V with Mic Impedance. C5010
supplies AC Ground to create AC impedance of 510 Ohms via R5075. and Filter 9.3V DC mic bias
supply.
Note: The audio signal at U504-pin 46 should be approximately 12mV for 1.5kHz or 3kHz of
deviation with 12.5kHz or 25kHz channel spacing.
The external microphone signal enters the radio on accessory connector P1 pin 2 and is routed via
line EXT MIC to R5054. R5078 and R5076 provide the 9.3Vdc bias. Resistive divider R5054/ R5070
divide the input signal by 5.5 and provide input protection for the CMOS amplifier input. R5076 and
C5009 provide a 510 ohm AC path to ground that sets the input impedance for the microphone and
determines the gain based on the emitter resistor in the microphone's amplifier circuit.
C5047 serves as a DC blocking capacitor. The audio signal at U504-pin 48 should be approximately
14mV for 1.5kHz or 3kHz of deviation with 12.5kHz or 25kHz channel spacing.
The FLAT TX AUDIO signal from accessory connector P1-pin 5 is fed to the ASFIC CMP (U504 pin
42 through U509 pin 2 to U509 pin 15 via U506 OP-AMP circuit and C5057.
The ASFIC has an internal AGC that can control the gain in the mic audio path. The AGC can be
disabled / enabled by the µP. Another feature that can be enabled or disabled in the ASFIC is the
VOX. This circuit, along with Capacitor C5023 at U504-pin 7, provides a DC voltage that can allow
the µP to detect microphone audio. The ASFIC can also be programmed to route the microphone
audio to the speaker for public address operation.
7.2

PTT Sensing and TX Audio Processing

Internal microphone PTT is sensed by µP U403 pin 71. Radio transmits when this pin is "0" and
selects inside the ASFIC_ CMP U504 internal Mic path. When the internal Mic PTT is "0" then
external Mic PTT is grounded via D402. External Mic PTT is sensed by U403 pin 72 via Q412
circuits. The radio transmits when this pin is "0" and selects inside the ASFIC _CMP U504 External
Mic path.
Inside the ASFIC CMP, the mic audio is filtered to eliminate frequency components outside the 300-
3000Hz voice band, and pre-emphasized if pre-emphasis is enabled. The signal is then limited to
prevent the transmitter from over deviating. The limited mic audio is then routed through a summer,
which is used to add in signaling data, and then to a splatter filter to eliminate high frequency
spectral components that could be generated by the limiter. The audio is then routed to an
attenuator, which is tuned in the factory or the field to set the proper amount of FM deviation. The
TX audio emerges from the ASFIC CMP at U504-pin 40 MOD IN, at which point it is routed to the
RF section.
THEORY OF OPERATION