7-20
Theory of Operation
The outputs of the Audio PA (EXT-SPKR+, INT/EXT SPKR-) are routed to the
external speaker through the accessory connector. The INT/EXT SPKR- is
directly connected to the INT-SPKR(-).
The EXT-SPKR+ can be connected to the internal s) in two ways: 1)
internal connection via R456 (0 Ohm); 2) Externally, via a jumper between pin 16
and pin 13 of the accessory connector.
The audio power amplifier has one inverted and one non-inverted output that
produces the differential audio output SPK+ / SPK- (U0401-4/6). The inputs for
each of these amplifiers are pins 1 and 9 respectively; these inputs are both tied to
the received audio. The audio PA’s DC biases are not activated until the audio PA
is enabled at pin 8.
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 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 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).
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.
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 being equal value set up the op-amp as a unity gain device, i.e. a buffer.
Note:
Any volume adjustment of the signal on this path must be
done by the accessory.
Discriminator Audio
(Unfiltered)
Note:
Discriminator audio DET AUDIO from the IF IC U5201, in
addition to being routed to the ASFIC, is also routed to the
option connector J0103-5. (See Secure Rx description
blocks for further information.)
