Analog Mode Of Operation; Receiving; Transmitting; Figure 2. Receiver Block Diagram - Motorola XTS 2500 Basic Service Manual

Basic Theory of Operation
Analog Mode of
Operation

Receiving

Transmitting

6
When the radio is receiving (see Figure 2), the signal comes from the antenna connector to
the radio board, passes through the RX/TX antenna switch and into the receiver front end.
A 15dB step attenuator is included in the front end to provide additional protection against
strong signals. The attenuator is controlled by a DSP based algorithm that continuously
monitors signal strength. When the ON threshold is exceeded (approximately -95dBm), the
attenuator is activated via a DAC in the PCIC by the host. The attenuator remains activated
until the signal drops below the OFF threshold (approximately -115dBm including the
15dB attenuation). Hysterisis and timer functions are included in the algorithm to enhance
performance. The algorithm controlling the attenuator is enabled via the CPS for each
personality. When the algorithm is disabled, the attenuator is essentially a short circuit from
input to output.The signal is then filtered, amplified, and mixed with the first local-
oscillator signal generated by the voltage-controlled oscillator (VCO).
RF Input
Antenna
Harmonic
Switch
Filter
XTAL
Filter
The resulting intermediate frequency (IF) signal is fed to the IF circuitry, where it is again
filtered and amplified. This amplified signal passes to the digital back-end IC, where it is
mixed with the second local oscillator to create the second IF at 450 kHz. It is then
converted to a digital bit stream and mixed a third time to produce a baseband signal. This
signal is passed to the VOCON circuitry through a current-driven differential output.
In the VOCON circuitry, the digital-signal processor (DSP) support IC digitally filters and
discriminates the signal, and passes it to the DSP. The DSP decodes the information in the
signal and identifies the appropriate destination for it. For a voice signal, the DSP routes
the digital voice data to the CODEC for conversion to an analog signal. The CODEC then
presents the signal to the audio power amplifier, which drives the speaker. For signalling
information, the DSP decodes the message and pass it to the microcontrol unit.
When the radio is transmitting (see Figure 3), microphone audio is passed through gain
stages to the CODEC where the signal is digitized. The CODEC passes digital data to the
DSP where pre-emphasis and low-pass (splatter) filtering are done. The DSP passes this
signal to a digital-to-analog (D/A) converter where it is reconverted into an analog signal
and scaled for application to the voltage-controlled oscillator as a modulation signal.
RX Front End
Tuneable
Attenuator RF
Preslector
Amp
ABACUS III - RX Back End
CKO
LO

Figure 2. Receiver Block Diagram

Tuneable
1st LO
Preselector
Filter
1st
Mixer
3
RX_SSI_ DATA
ADC
to VOCON Board
MAEPF-27278-O