Motorola HT 1000 series Theory/Troubleshooting Manual page 34

Although it is referred to as "sub-audible data,"
the actual frequency spectrum of these wave-
forms may be as high as 250Hz, which is
audible to the human ear. However, the radio
receiver filters out any audio below 300Hz,
so these tones are never heard in the actual
system.
Only one type of sub-audible data can be gen-
erated by U701 at any one time. The process is
as follows: using the SPI, the microprocessor
programs the ASF IC (U701) to set up the prop-
er low-speed data deviation and select the PL
or DPL filters. The microprocessor then gener-
ates a square wave from U705-PA6 which
strobes the ASF IC PL/DPL encode input U701-
C3 at twelve times the desired data rate. (For
example, for a PL frequency of 103Hz, the fre-
quency of the square wave at U701-C3 would
be 1236Hz.) This drives a tone generator inside
U701, which generates a staircase approxima-
tion to a PL sine wave or DPL data pattern. This
internal waveform is then low-pass filtered and
summed with voice or data. The resulting
summed waveform then appears on U701-H8
(VCO MOD), where it is sent to the transceiver
board as previously described for transmit
audio.
b. High-Speed Data
High-speed data refers to the 3600 baud data
waveforms (ISWs and OSWs) used in a trunk-
ing system for high-speed communication
between the radio and the central controller. To
generate an ISW, the microprocessor first pro-
grams the ASF IC (U701) to the proper filter
and gain settings. It then begins strobing U701-
G1 (Trunking Clock In) with a square wave
(from U705-PA5) at the same baud rate as the
data. The output waveform from U701's 5-3-2
State Encoder is then fed to the post-limiter
summer block and then the splatter filter. From
that point it is routed through the mod attenua-
tors and then out of the ASF IC to the
transceiver board via the VCO MOD connector
jack, J704 pin 3.
c. DTMF Data
DTMF data is a dual-tone waveform used dur-
ing phone interconnect operation. There are
seven frequencies, with four in the low group
(697-941Hz) and three in the high-group (1209-
1477Hz). The high-group tone is generated by
U705-PA5 strobing U701-G1 at six times the
tone frequency for tones less than 1440Hz, or
twice the frequency for tones greater than
1440Hz. The low-group tone is generated by
U705-PA4 strobing U701-G2 (DTMF CLOCK)
at six times the tone frequency. Inside U701 the
26
low-group and high-group tones are summed
(with the amplitude of the high group tone being
approximately 2dB greater than that of the low-
group tone) and then pre-emphasized before
being routed to the summer and splatter filter.
The DTMF waveform then follows the same
path as was described for high-speed data.
d. 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 by 1.5 cycles of an
1800Hz sine wave. To generate the data, the
microprocessor first programs the ASF IC
(U701) to the proper filter and gain settings. It
then begins strobing U701-G1 (Trunking Clock
In) with a square wave (from U705-PA5) at the
same baud rate as the data. The output wave-
form from U701 is fed to the post-limiter
summer block and then the splatter filter. From
that point it is routed through the mod attenua-
tors and then out of the ASF IC to the
transceiver board via the VCO MOD line, con-
nector jack J704 pin 3.
3. Receive Audio Circuits
There are three major circuits in the receive audio
path. These are the ASF IC (U701), the HearClear
IC (U601), and the audio PA (U702). The ASF IC is
an SPI-programmable device, while the other two
ICs have direct control lines.
The radio's RF circuits are constantly producing an
output at the discriminator. Whenever the radio is in
trunked standby mode, it is processing data from
the control channel. While in conventional standby
mode, it is always monitoring the squelch line
and/or or sub-audible data. The raw discriminator
from the transceiver board enters the controller
board at connector jack J704 pin 10. In addition to
the raw discriminator signal (DISC), the transceiver
board's IF IC also provides a pre-filtered version of
the discriminator signal that is dedicated to the ASF
IC squelch-detect circuitry. This signal, which is
labelled SQ IN, enters the controller board at con-
nector jack J704 pin 12, and is routed to the ASF IC
on U701-H7. When the microprocessor is satisfied
that it has received the proper data or signal type for
unsquelching, it sets up the receive audio path and
sends data to U701 to do the same within.
a. HearClear (Noise Muting)
For the 900MHz Hear Clear controllers, the raw
discriminator (which contains both audio and
sub-audible data) is routed to U601-E4, the
input to the flutter fighter circuit inside U601.
The purpose of this section is to eliminate any