Receiver Back End - Motorola ASTRO Digital Saber Service Manual

Receiver Back End

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the first IF frequency. The first IF frequency of VHF and UHF bands are
45.15MHz and 73.35MHz respectively. The 1st LO signal for VHF is
45.15MHz higher than the carrier frequency while that for the UHF is
73.35MHz lower than the carrier frequency. The 1st IF signal output,
at U2 pins 4 and 6, is routed through transformer T2 and impedance
matching components, and applied to a 2-pole crystal filter (FL1),
which is the final stage of the receiver front end. The 2-pole crystal
filter removes unwanted mixer products. Impedance matching
between the output of the transformer (T2) and the input of the filter
(FL1) is accomplished by capacitor C605 and inductor L605 (VHF); or
C611, C614 and L605 (UHF).
The output of crystal filter FL1 is matched to the input of IF buffer
amplifier transistor Q601 by components C610 and L604 (VHF) and
C609, C610, and L600 (UHF). Transistor Q601 is biased by the 5V
regulator (U202). The IF frequency on the collector of Q601 is applied
to a second crystal filter through a matching circuit. The second crystal
filter (FL2) input is matched by C604, C603, and L601 (VHF); or C604,
L601, and L602 (UHF). The filter supplies further attenuation at the IF
sidebands to increase the radios selectivity. The output of FL2 routed
to pin 32 of U401 through a matching circuit which consists of L603,
L606, and C608 (VHF); or L603, C606, and C605 (UHF).
In the ABACUS IC, (U401) the first IF frequency is amplified and then
down converted to 450kHz, the second IF frequency. At this point, the
analog signal is converted into two digital bit streams via a sigma-delta
A/D converter. The bit streams are then digitally filtered and mixed
down to baseband and filtered again. The differential output data
stream is then sent to the ADSIC (U406) on the VOCON board where
it is decoded to produce the recovered audio.
The ABACUS IC (U401) is electronically programmable, and the
amount of filtering which is dependent on the radio channel spacing
and signal type is controlled by the microcomputer. Additional
filtering, which used to be provided externally by a conventional
ceramic filter, is replaced by internal digital filters in the ABACUS IC.
The ABACUS IC contains a feedback AGC circuit to expand the
dynamic range of the sigma-delta converter. The differential output
data contains the quadrature (I and Q) information in 16-bit words,
the AGC information in a 9-bit word, imbedded word sync
information and fill bits dependent on sampling speed. A fractional-n
synthesizer is also incorporated on the ABACUS IC for 2nd LO
generation.
The 2nd LO/VCO is a Colpitts oscillator built around transistor Q401
(VHF) and Q1 (UHF). The VCO has a varactor diode, VR401 (VHF) and
CR5 (UHF), to adjust the VCO frequency. The control signal for the
varactor is derived from a loop filter consisting of C426, C428, and
R413.