Technical Manual
Table Of Contents
- Front Cover
- Foreword
- Table of Contents
- Safety
- ASTRO Digital XTS 3000 Model Chart (“A” Models)
- Portable Radio Model Numbering System
- ASTRO Digital XTS 3000 Model Chart (”B” Models)
- ASTRO Digital XTS 3000 R (Ruggedized) Model Chart
- ASTRO Digital XTS 3000 R (Ruggedized) Yellow Model Chart
- Model History Tables
- Glossary
- Introduction
- General Overview of an ASTRO Digital XTS 3000 Radio
- Radio Power
- VHF/UHF Transceiver Board Detailed Theory of Operation
- 800MHz Transceiver Board Detailed Theory of Operation
4-4
The RF signal is applied to the mixer at U2 pins 1 and 15. An injection
signal (1st LO) of about -10dBm, supplied by the FGU, is applied to U2,
pin 8. Mixing of the RF and the 1st LO results in an output signal that
is the first IF frequency. The first IF frequencies of the 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 two-pole crystal
filter (FL1), which is the final stage of the receiver front end. The two-
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 C605 and L605 (VHF); or C611,
C614, and L605 (UHF).
Receiver Back End 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 by a sigma-delta
A/D converter. The bit streams are then digitally filtered, mixed down
to baseband, and filtered again. The differential output data stream is
then sent to the ADSIC (U406) on the vocoder 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) or Q1 (UHF). The VCO has a varactor diode, VR401 (VHF) or
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.