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Theory of Operation: Receiver Front-End
- Low-noise IF amplifier
- ABACUS III digital back-end IC
RF Input
Front-end
Ant. SW.
Harm. FLT
Att.
3.10.1.1 15 dB Step Attenuator (U3250)
The 15 dB step attenuator provides protection from strong on-channel signals and interferences.
3.10.1.2 PIN Diode Switches
The VHF front end operates in two modes: Standard mode and Preamp mode. The PIN diode
switches select between the two modes. In standard mode the intermodulation performance is
improved by bypassing the first HPF and LNA. Preamp mode is characterized by higher sensitivity.
3.10.1.3 Preselector Filters
The front-end operates in the 136-174 MHz band. The front-end filters primary function is to provide
protection against out-of-band spurious responses such as image, IF, half-IF, etc. while providing flat,
low-loss response in the receive band. The front-end uses discrete LC filter technology. The first filter
is a high-pass filter that protects the first LNA from strong out-of-band signals at frequencies which
are below the receive band and is used in preamp mode only. The second and third filters are band-
pass filters that provide additional out-of-band spurious rejection.
3.10.1.4 Low-Noise Amplifiers (Q3255, Q3252)
The function of the Low-Noise Amplifiers (LNA) is to amplify the received signal with minimal noise
contribution. The first LNA (Q3255) has 10 dB of gain and is activated in Preamp mode only. A
clamping diode pair (D3256) located after the High Pass and before the first LNA protects the
receiver from strong RF signals by limiting the signal amplitude going into the amplifier. The second
LNA (Q3252) has 15 dB of gain. Both LNAs are biased with 9.3V.
6815854H01-A
Pre Mode
LNA 1
HPF
Std Mode
Back-end
2 Pole
Crystal
ABACUS
Back-end
IC
Figure 3-14. Receiver Block Diagram (VHF)
BPF 1
LNA 2
LNA
2 Pole
Crystal
3-17
BPF 2
Mixer
LO
June 15, 2005
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