Receiver Back End; Transmitter; Closed Architecture Controller; Microcomputer (U705) - Motorola HT 1000 series Theory/Troubleshooting Manual

The final stage in the receiver front end is a 2-pole
crystal filter, FL12. The crystal filter provides some of
the receiver's adjacent channel selectivity. The receiv-
er's backend IF IC (U3) provides most of the adjacent
channel selectivity, using integrated baseband low-pass
filters. The input to the crystal filter is matched to the 1st
mixer using components L36, L20, C35 and C36. The
output of the crystal filter is matched to pin 2 of the IF IC
using inductor L22, and a capacitive tap C38 and C39.
D. Receiver Back End
The IF frequency is applied to the IF IC (U3), where
it is down converted, amplified, filtered, and demodulat-
ed to produce the recovered audio. This IF IC is
electronically programmable and the amount of filtering
(which is dependent on the radio channel spacing) is
controlled by the microprocessor. Additional filtering,
which used to be provided externally by conventional
ceramic filters, is replaced by internal filters in the IF IC.
The IF IC uses a type of direct conversion process
where the second LO frequency is very close to the IF
frequency. The IF IC controls the second LO VCO and
causes the VCO to track the first IF frequency, produc-
ing a phased lock operation. The IF IC also provides a
recovered signal strength indicator (RSSI) and squelch
output for use in other parts of the radio.
E. Transmitter
The 800MHz and 900MHz RF PAs are 5-stage
amplifiers. Both RF power amplifiers have nominal input
and output impedances of 50 ohms.
An RF input drive level of approximately +3 dBm,
supplied from the VCO buffer IC, U201 is applied to pin
1 of U105. The dc bias for the internal stages of U105 is
applied to pins 2,5,and 6 of the module. Pins 2 and 5
being switched through Q101 and pin 6 being
unswitched B+ to the final amplifier stage. Power control
is achieved through the varying of the dc bias to pins 3
and 4, the third and fourth amplifier stages of the mod-
ule. The amplified RF signal leaves the PA module via
pin 7 and is applied to the directional coupler, U104.
The purpose of U104 is to sample both the forward
power and the reverse power. The reverse power will be
present when there is other than a 50-ohm load at the
antenna port. The sampling will be achieved by coupling
some of the reflected power, forward and/or reverse, to
a coupled leg on the coupler. The sampled RF signals
are applied to diode CR101 for rectification and sum-
ming. The resultant dc signal is applied to the ALC IC
(U101 pin 2) as RFDET to be used as an strength indi-
cator of the RF signal being passed through the
directional coupler, U104.
The transmit ALC IC, U101, is the heart of the
power control loop. The REF V line (U101 pin 7), a dc
signal supplied from the D/A IC (U102), and the RF DET
signal described earlier, are compared internally in the
ALC IC to determine the amount of C BIAS, pin 4, to be
applied to the base of transistor Q110. Transistor Q110
responds to the base drive level by varying the dc con-
trol voltages applied to pin 3 and 4 of the RF PA,
controlling the RF power level of module, U105. The
ALC IC also controls the base switching to transistor
Q101 via pin 12, BIAS.
The D/A IC, U102, controls the dc switching of the
transceiver board. Its outputs, SC1 and SC3, pins 12
and 14 respectively, control transistor Q108, which then
supplies TX 5V and RX 5V to the transceiver board. The
D/A also supplies the dc bias to the detector diode
(CR101) via pin 7, and the REF V signal to the ALC IC,
U101.
V. CLOSED ARCHITECTURE CONTROLLER
Since the controller is the central interface between
the various subsystems of the radio, and because of the
controllers complexity, this section will be divided into
two areas of discussion, the microcomputer and its func-
tions, and the controller board circuit operation.
A. Microcomputer (U705)
The heart of the HT 1000 controller consists of a
new generation Motorola microcomputer, U705. The
microcomputer consists of 640 bytes of EEPROM, 760
bytes of RAM, and 24K of ROM. It operates in single-
chip mode. The microcomputer is powered by a
regulated 5V output from voltage regulator U709.
The microcomputer clock is generated by the ASFIC,
U701, which has a built in programmable clock
synthesizer.
1. Functions
The microcomputer, has two basic functions: inter-
facing to the outside world and controlling the
internal workings of the radio. It interfaces directly to
the side buttons, PTT, rotary switch, toggle switch,
and 13-pin side connector. It is constantly monitor-
ing a numerous amount of inputs, interpreting any
changes that may be occurring, and responding
with commands that control the rest of the radio.
Some functions that it performs include:
• loading the synthesizer with the desired RF
frequency,
• turning the RF PA on or off,
• turning the microphone and speaker on or off,
• enabling and disabling audio and data paths,
and
• generating tones.
Operations and operating conditions within the radio
are interpreted by the microcomputer and fed back
to the operator as audible (alert tone) indications of
the radio's immediate status.
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