Transmit Audio Circuits - Motorola MCS 2000 Service Instructions Manual

Controller Section Theory of Operation: Audio and Signalling Circuits

Transmit Audio Circuits

(Refer to Table 7-2 for reference for the following sections)
(Refer to schematic page 10-27 for reference)
The radio supports 2 distinct microphone paths known as internal and external mic. The
microphones used for the radio require a DC biasing voltage provided by R0222 and R0223 for
internal, and R0231 and R0230 for external. These two microphone audio input paths enter the
ASFIC at U0200-A7 (external mic) and U0200-B8 (internal mic). Following the internal mic path; the
microphone is plugged into the radio control head and is connected to the controller board via J0405-
9.
From here the signal is routed to R0224. R0222 and R0223 provide the 9.3 VDC bias and R0224
provides input protection for the CMOS amplifier input. R0223 and C0219 provide a 560 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.
Filter capacitor C0224 provides lowpass filtering to eliminate frequency components above 3 kHz,
and C0218 serves as a DC blocking capacitor. C0427 bypasses RF interference which may couple
onto the line to ground. The audio signal at U0200-B8 should be approximately 80 mV to achieve
60% of maximum system deviation.
The ASFIC has an internal mic AGC that can control the gain in the mic audio path. The AGC can be
disabled/enabled in the radio codeplug using the RSS/CPS. Another feature that can be enabled or
disabled in the ASFIC is the VOX. This circuit, along with C0205, 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.
External Mic Path
The external microphone signal enters the radio on accessory connector J0403 pin 23. It is then
routed to the ASFIC through resistor R0229 and capacitors C0223 and C0221, with DC bias
provided by R0231/R0230.
PTT Sensing and TX Audio Processing
Mic PTT is sensed via SB9600. An external PTT can be generated by programming one of the digital
I/O lines on the accessory connector for PTT and grounding that pin. When microphone PTT is
sensed, the µP will always configure the ASFIC for the "internal" mic audio path, and external PTT
will result in the external mic audio path being selected.
68P81083C20-D
7-17
December 6, 2004