0.2.4 Baseband Audio-Processing Circuits; Baseband Audio-Processing Circuits; Digital Control And Power Conditioning - Motorola R-20010 Maintenance Manual

1 0.2.4 BASEBAND AUDIO-PROCESSING
CIRCUITS
10.2.4 .1 General
Under processor control, the audio-p rocessing cir-
cuits provide filtering, pola rity inversion, gain scaling,
squelch, and detector selection. With the special func-
tion codes, the operator has override control of certain
filt e r selections.
A block diagram of the baseband audio-processing
circuits is s hown at t he end of the secti on in Figure
I 0- 1
e.
1 0.2.4.2 Modulation Selection/Squelch Gate
Analog gate U I 01. under direct processor control,
selects modulation, routing the AM, FM, or product-
detector out put to t he squelch gate, which is also part
of UIOI. T o provide t he squelch-gate drive, the log-
amp output is compa red to a reference level derived
from the front panel's Squelch control. T he log-amp
output provides a sta ble, logarithmic, linear-squelch
cont rol.
1 0.2.4.3 Speaker Buffer Amplifier
The squelch-gate output is routed directly to the
spea ker audio amplifier on the Scope/DVM Control
board (A 7). This output goes through the front panel's
Volume cont rol and the Receiver buffer amplifier
(U314 ), bypassing t he Receiver audio filters. To main-
tai n output vo lume, the buffe r am plifier provides a
20-dB gain boost in narrowband FM. Ul06 serves as a
processor-controlled speaker-enable. A series-output
capacit or serves as a ! -pole, 300-Hz, high-pass filter,
while a low-pass filter on the A7 board selects either a
3-dB, 1-kHz, !-pole roll-off or a 75-j..ls equaliza tion at
t he s peaker a udi o amplifier.
1 0.2.4.4 Audio Low-Pass Filters
A . : 1 -pole. selectable bandwidth, low-pass filter pro-
vides :lOO-Hz.
:~- kHz
or 20-kHz fi lteri ng. The filter is
s plit into two 2- pole sections to 1) initially filter wide-
band noise and RF resid ual at the detector outputs and
~~supp ress
wirleband noise introd uced by the various
high-pass filters, the polarity-inversion a mplifier, and
the ci rcuit -board pickup at the output.
1 0.2.4.5 Modulation-Sense Amplifier
Hecause the System Analyzer can select either high-
or low-sidt- mixing, t he FM-detector output must be
polarit.\·-selectable to maintain correct digital-data
10-4
se nse as in DPL etc. Under processor control, Ul03
performs t his function by providing a gain of ± 1.
1 0.2.4 .6 Audio High-Pass Filters
A 4-pole, selectable-bandwid th, hi gh-pass filte r
prm·ides
:~00- H z
and 3-kH z high -pass filte ring. The
fi lter can be bypassed for low-frequency signals. Ul05A
and B. which provide two poles each, are cascaded to
form the 4-pole active fi lter.
1 0.2.4. 7 Audio Output-Buffer Amplifiers
To prevent limiting in t he audio filters, t he audio
signals are filtered at low level and t hen amplified 12
dH by U108. U108 p rovides audio to t he internal
detectors and scope. QlOl , Q l 02, and U207B provide
a
buffered . unc:ali brated a udio output to the front-
panel port. U207 B provides a processor-selectable gain
of I X/ I OX for narrowband FM.
10.2.5 DIGITAL CONTROL AND POWER
CONDITIONING
1 0.2.5.1 General
The Receiver is programmed by a serial data stream
to min im ize t he system's interconnect complexity.
Detector acc uracy requ ires a stable, accurate power
suppl_v_ A precision reference provides the Receiver
with a temperatu re-stable refe rence from which all
nit ic:al voltages a re ge nerated.
10.2.5.2 Digital Control
Digital control is provided by a pair of cascad ed,
la tchi ng. 8-bit , serial-in, parallel-out, CMOS shift reg-
i;;ters. To obt ain the processor-programming data, t he
Ht!ceiver strips oH· the last 16 bits of the Synthesizer's
control-data stream and routes t he shift-register out-
put to the Synt hesizer. Tables 10- 1 t hrough 10-4 detail
fu1Wt ion::. of the d igital control bits.
1 0.2.5 .3 Power Conditioning
lli
J i)
provide::. the Receive r's power-supply refer-
t'nre or
~.:)V.
U ll l and associated components gener-
nte - :2.F,V. + !JV Ref., and ±9V for the various
Hec:eiver ci rcuits. No current limiting is provided on-
hoard: the refo re, the Receiver's voltage regulators
de pend u pon the current limit of the system's powe r
supply. This is particu larly important if a test fixt ure
is used .