Motorola HT 1000 series Theory/Troubleshooting Manual page 28

a. Sub-audible Data (PL/DPL)
Sub-audible data is composed of low-frequency
PL and DPL waveforms for conventional opera-
tion. Although it is referred to as "sub-audible
data," the actual frequency spectrum of these
waveforms may be as high as 250 Hz, which is
audible to the human ear. However, the radio
receiver filters out any audio below 300 Hz,
so these tones are never heard in the actual
system.
Only one type of sub-audible data can be gen-
erated by U705 at any one time. The process is
as follows. Using the SPI, the microcomputer
programs the ASF IC to set up the proper low-
speed data deviation and select the PL or DPL
filters. The microcomputer then generates and
produces a square wave at U705 pin 6, PA5,
which strobes the ASF IC PL/DPL encode input
at U701 pin 40. Module U701 reacts to the
strobe input by generating a staircase approxi-
mation to the PL sine wave or the DPL data
pattern. This internal waveform is low-pass fil-
tered and summed with voice or data. The
resulting waveform appears at U701 pin 13,
VCO ATN, where it is sent to the transceiver
board as previously described for transmit
audio.
b. DTMF Data
DTMF data is a dual-tone waveform used
during phone interconnect operation. There are
seven frequencies; four in the low group
(697-941Hz) and three in the high group
(1209-1477Hz). The high-group tone is generat-
ed by the microcomputer (U705 at pin 22, PH0),
strobing the ASF IC (U701 at pin 29) at six
times the tone frequency for tones lower than
1440Hz, or twice the frequency for tones higher
than 1440Hz. The low-group tone is generated
by the microcomputer (U705 pin 23, PH1)
strobing the ASF IC (U701 pin 28) at six times
the tone frequency. Circuits inside module
U701 sum the low-group and high-group tones
(with the amplitude of the high-group tone being
approximately 2db greater than that of the low-
group tone) and send the summed signal
through a pre-emphasis network. The resultant
signal is routed through a summer and splatter
filter. After filtering, the signal is routed through
modulation attenuators and sent from the ASF
IC to the transceiver board. The signal path is
from U701 pin 13 through the controller/jumper
flex connector (J704/P704 pin 3), and through
the jumper flex/transceiver board connector
(P301/J301 pin 3) to the RF synthesizer (U204).
The input signal is VCO MOD.
20
c. MDC Data
The MDC signal follows exactly the same path
as the DTMF high group tone. MDC data uti-
lizes MSK modulation, in which a logic zero is
represented by one cycle of a 1200Hz sine
wave, and a logic one is represented by 1-1/2
cycles of an 1800Hz sine wave. To generate
the data, the microcomputer first programs the
ASF IC (U701) to the proper filter and gain set-
tings. It then begins strobing module U701
pin 29 (TRK CLK IN) with a square wave (from
U705 pin 22, PH0) at the same baud rate as the
data. The output waveform from U701 is fed to
a post-limiter, to a summer block, and then to a
splatter filter. The resultant signal is routed
through modulation attenuators and sent from
the ASFIC to the transceiver board using the
same signal path as the DTMF data described
in the previous paragraph.
3. Receive Audio Circuits
The major circuits in the receive audio path are the
ASF IC (U701) and the audio PA (U706). The ASF
IC is an SPI programmable device, while the audio
PA has direct control lines.
The radio's RF circuits are constantly producing an
output at the discriminator. In the conventional
standby mode, the radio's receiver is always moni-
toring the squelch line and/or sub-audible data. The
raw discriminator input signal (DISC) from the
transceiver board enters the controller board on
connector jack J704 pin 10. In addition to the raw
discriminator signal, the transceiver board's IF IC
also provides a pre-filtered version of the discrimi-
nator signal, SQ OUT, that is dedicated to the
ASFIC's squelch-detect circuitry. The SQ OUT
signal enters the controller board via connector
jack J704 pin12 and is routed to the ASF IC, U701
pin 14. When the microcomputer is satisfied that it
has received the proper data or signal type for
unsquelching, it sets up the receive audio path and
sends data for the ASF IC (U701) to process.
a. U701 Audio Processing and
Digital Volume Control
The signal enters the ASF IC (U701) pin 16 for
further processing. Inside the IC, the signal first
passes through a low-pass filter to remove any
frequency components above 3000 Hz, and
then a high-pass filter to strip off any sub-audi-
ble data below 300 Hz. Next, the recovered
audio passes through a de-emphasis filter to
reduce the effects of FM noise. Finally, the IC
amplifies the audio and passes it through an 8-
bit programmable attenuator, whose level is set
in accordance with the voltage sensed on the
volume potentiometer, which is connected to