Receiver Back End - Motorola ASTRO Digital Saber Service Manual

Receiver Back End

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of U205 via bypass C213. The mixer's LO port is matched to the radio's
PLL by a capacitive tap, C207 and C206. A balun transformer (T202) is
used to couple the RF signal into the mixer. The primary of T202 is
matched to the preceding stage by capacitor C223, with C227
providing a dc block to ground. The secondary of T202 provides a
differential output, with a 180° phase differential being achieved by
setting the secondary center tap to virtual ground using bypass
capacitors C210, C211 and C212. The secondary of transformer T202
is connected to pins 1 and 15 of the mixer IC, which drives the source
leg of dual FETs used to toggle the paralleled differential amplifier
configuration within the Gilbert Cell.
The final stage in the receiver front end is a 2-pole crystal filter (FL1).
The crystal filter provides some of the receiver's adjacent channel
selectivity. The input to the crystal filter is matched to the 1st mixer
using components L605, C611 and C614. The output of the crystal
filter is matched to the input of IF buffer amplifier transistor Q601 by
components L600, C609, and C610.
The IF frequency on the collector of Q601 is applied to a second crystal
filter (FL2) through a matching circuit consisting of L601, L602, C604,
and C612. The filter supplies further attenuation at the IF sidebands to
increase the radio's selectivity. The output of FL2 is routed to pin 32 of
U401 through a matching circuit which consists of L603, C605, and
C606 and dc block capacitor C613.
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 second LO/VCO is a Colpitts oscillator built around transistor Q1.
The VCO has a varactor diode (VR401), which is used to the adjust the
VCO frequency. The control signal for the varactor is derived from a
loop filter consisting of C426, C428, and R413.