Motorola Astro Digital Mobile Radio Xtl5000 Detailled 6881096c74 B Manual

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Front-End3-15
3.4.1.5  Mixer (D3258)
The received signal is down-converted by a double-balanced mixer to an Intermediate Frequency 
(IF) of 109.65 MHz. The mixer is designed to provide low conversion loss and high intermodulation 
performance. The injection buffer provides a 20 dBm LO signal to the mixer. High-side injection is 
used.
3.4.2 UHF Range 1 (380–470 MHz) Band
The receiver circuits primary duties are to detect, filter, amplify, and...

May 25, 20056881096C74-B
3-16Theory of Operation: Receiver Front-End
3.4.2.1  Highpass Filter and First Low-Noise Amplifier
The highpass filter and first low-noise amplifier (LNA) (11 dB gain) can be switched in and out of the 
signal path by diode switches. When switched into the signal path, the sensitivity of the radio is 
improved at the cost of degraded intermodulation performance. This can be necessary in fringe 
areas when strong interference that can lead to intermodulation problems are not...

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Front-End3-17
3.4.3 UHF Range 2 (450–520 MHz) Band
The receiver circuits primary duties are to detect, filter, amplify, and demodulate RF signals in the 
presence of strong interfering noise and unintended signals. The receiver (see Figure 3-15) is 
broken down into the following blocks:
• Front-end, which includes:
- High pass fIlter and first low-noise amplifier (LNA)
- Preselector filter
- Switchable 15 dB attenuator
- Second LNA
- Image Filter
-...

May 25, 20056881096C74-B
3-18Theory of Operation: Receiver Front-End
3.4.3.2  Preselector Filter
The front-end operates in the 450 to 520 MHz band. The front-ends primary function is to optimize 
half IF rejection, image rejection, and selectivity while providing the first conversion. The front-end 
uses a varactor-tuned filter that is tuned by the controller. The tuning signal is a DC control voltage 
between 0 and 9V that come from the PA power control section. Low voltages are for lower 
frequencies...

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Front-End3-19
3.4.4 700–800 MHz Band
The receiver circuits primary duties are to detect, filter, amplify, and demodulate RF signals in the 
presence of strong interfering noise and unintended signals. The receiver (see Figure 3-16) is 
broken down into the following blocks:
• Front-end, which includes:
- Preselector filters
- Low-noise amplifier (LNA)
- First mixer
•IF
• Back-end
Figure 3-16.  Receiver Front-End and Back-End (700–800 MHz)
3.4.4.1...

May 25, 20056881096C74-B
3-20Theory of Operation: Receiver Back-End
3.4.4.3  Mixer (U6251)
The monolithic, passive mixer IC down-converts the received signal to an Intermediate Frequency 
(IF) of 73.35 MHz. The mixer is designed to provide low conversion loss (< 7.0) and high 
intermodulation performance. To improve the performance of the mixer in both bands, a shunt 9.1 pF 
capacitor (C6297) along with a resistive PI network (R6278, R6280, R6281) is designed at the IF 
port (pin 5) of the mixer. The...

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Back-End3-21
Figure 3-17.  ABACUS III (AD9874) IC Functional Block Diagram from Data Sheet (VHF)
3.5.1.2.1  Second Local Oscillator
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the 
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is 
107.4 MHz by default, or 111.9 MHz in special cases as necessary to avoid radio self-quieters. The 
second LO signal mixes with IFIN to...

May 25, 20056881096C74-B
3-22Theory of Operation: Receiver Back-End
3.5.2.1  Intermediate Frequency (IF) Filter
The XTL 5000 radio uses two leadless, surface-mount, two-pole, third-overtone, quartz crystal filters 
(Y5400, Y5401) separated by a 20 dB gain IF amplifier. The filter is centered at 109.65 MHz. This 
narrow-bandpass filter gives the radio part of its adjacent-channel and alternate-channel rejection 
performance. Impedance-matching networks are located at the input and output of each crystal....

6881096C74-BMay 25, 2005
Theory of Operation: Receiver Back-End3-23
3.5.2.2.1  Second Local Oscillator (LO)
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the 
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is 
107.4 MHz by default, or 111.9 MHz in special cases as necessary to avoid radio self-quieters. The 
second LO signal mixes with IFIN to produce a 2.25 MHz final IF. The external VCO consists of 
transistor Q5002,...

May 25, 20056881096C74-B
3-24Theory of Operation: Receiver Back-End
Figure 3-19.  ABACUS III (AD9874) IC Functional Block Diagram from Data Sheet (UHF Range 2)
3.5.3.2.1  Second Local Oscillator (LO)
The ABACUS III IC local oscillator (LO) synthesizer controls the second LO. Signal FREF is the 
16.8 MHz reference from the frequency generation unit (FGU). The second LO frequency is 
107.4 MHz by default, or 111.9 MHz in special cases as necessary to avoid radio self-quieters. The 
second LO signal mixes...