Uhf Range 2 (450-520 Mhz) Band; Highpass Filter And First Low-Noise Amplifier; Figure 3-16. Receiver Front-End And Back-End (Uhf Range 2) - Motorola ASTRO XTL 1500 Detailed Service Manual

3-20
3.10.3 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
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
- First mixer
• Back-end, which includes:
- Intermediate Frequency (IF)
- ABACUS III IC
RF Input
Pre-Amp
Ant. SW.
Switch
Harm. FLT
High Pass
Filter

Figure 3-16. Receiver Front-End and Back-End (UHF Range 2)

3.10.3.1 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 present and the
desired signal is weak.
The preamplifier version of the radio must be purchased to be able to control this option. If it has not
been purchased, the direct path created by the diode switches is the only one available, giving the
radio standard model performance with enhanced intermodulation rejection. Purchasing the
preamplifier option allows the user to select either mode with the CPS.
June 15, 2005
450-520MHz
Pre-Amp
Switch Preselector 15 dB
Att.
A
LNA
Theory of Operation: Receiver Front-End
Low Pass
LNA
Mixer
Filter
24dBm
1st LO
2nd LO
LO Synth.
ABACUS III IC
Backend A/D Converter
Figure 3-16) is
109.65MHz
Crystal IF Amp Crystal
10 dB
Att.
A
Dec.
ADC SSI
Filter
CLK
18MHz
Synth.
CLK
6815854H01-A