Handset Audio; Filtered Audio; Rx Secure Audio (Optional) - Motorola GM-950 Manual

Receive Audio Circuits
The audio PA is enabled via AUDIO PA ENABLE signal from the ASFIC (U0201-B5). When the base
of Q0401 is low, the transistor is off and U0401-8 is high, using pull up resistor R0406, and the Audio
PA is ON. The voltage at U0401-8 must be above 8.5VDC to properly enable the device. If the
voltage is between 3.3 and 6.4V, the device will be active but has its input (U0401-1/9) off. This is a
mute condition which is not employed in this radio design. R0404 ensures that the base of Q0401 is
high on power up. Otherwise there may be an audio pop due to R0406 pulling U0401-8 high before
the software can switch on Q0401.
The SPK+ and SPK- outputs of the audio PA have a DC bias which varies proportionately with FLT
A+ (U0401-7). FLT A+ of 11V yields a DC offset of 5V, and FLT A+ of 17V yields a DC offset of 8.5V.
If either of these lines is shorted to ground, it is possible that the audio PA will be damaged. SPK+
and SPK- are routed to the accessory connector (J400-16 and 1) and to the control head (connector
J0101-1 and 2).
6.4

Handset Audio

Certain hand held accessories have a speaker within them which require a different voltage level
than that provided by U0401. For those devices HANDSET AUDIO is available at J0101-14.
The received audio from the output of the ASFIC's digital volume attenuator is also routed to U0202-
4 pin 9 where it is amplified 15 dB; this is set by the 10k/68k combination of R0233 and R0232. This
signal is routed from the output of the op amp U202-4 pin 8 to J0101-14. The control head sends this
signal directly out to the microphone jack. The maximum value of this output is 6.6Vp-p.
6.5

Filtered Audio

The ASFIC has an audio whose output at U0201-B2 has been filtered and de-emphasized, but has
not gone through the digital volume attenuator. From ASFIC U0201-B2 the signal is AC coupled to
U0202-2 by capacitor C0230. R0224 and R0225 determine the gain of op-amp U0202-2. The output
of U0202-2 is the routed to J0400-11.Note that any volume adjustment of the signal on this path
must be done by the accessory.
6.6

RX Secure Audio (optional)

Discriminator audio, which is now encrypted audio, enters the Secure board at connector J0103-5.
On the Secure board, the encrypted signal is converted back to normal audio format, and then fed
back through (J0103-2) to RX IN of the ASFIC (U0201-H6). From then on it follows a path identical
to conventional receive audio, where it is filtered (0.3 - 3kHz) and de-emphasized. The signal UNAT
RX OUT from the ASFIC (U0201-H5), also routed to option connector J0103-4, is not used for the
Secure board but for other option boards.
5C.3-16 Theory of Operation