Motorola APX 5000 Service Manual page 35

Theory of Operation: Transceiver Board
3.1.2.1 VHF Front-End
From the attenuator, U1149, a VHF signal is routed to the first pre-selector filter followed by a Low
Noise Amplifier (LNA) and a second pre-selector filter. Both filters are discrete and fixed designs and
are used to band limit the incoming energy and suppress known spurious responses such as Image
and the ½ IF spur. The LNA active device is an NPN transistor (U302) with active bias provided by
transistor pair Q302.
The output of the second pre-selector filter is applied to the RF port of the Mixer IC via balun
transformer, T503. The Mixer IC, U506, is driven by a Local Oscillator (LO) signal generated by the
Trident synthesizer IC, U702, at the LO port to down-convert the RF signal to a 109.65 MHz
intermediate frequency (IF). It is a passive, high linearity design with balanced inputs at the RF and
IF ports and internal LO buffer. The down converted IF signal is passed through a 3-pole crystal filter,
FL501, and an IF amplifier, Q503, which drive the input of the Analog to Digital Converter IC, U601
3.1.2.2 UHF1 Front-End
From the attenuator, U1149, a UHF1 signal is routed to the first pre-selector filter followed by a Low
Noise Amplifier (LNA) and a second pre-selector filter. Both filters are discrete and tunable designs
and are used to band limit the incoming energy and suppress known spurious responses such as
Image spur. The LNA active device is an NPN transistor (U2032) with active bias provided by
transistor pair Q2022. The output of the second pre-selector filter is applied to a discrete Low Pass
Filter (LPF). The output of the LPF is applied to the RF port of the Mixer IC via a balun transformer,
T503. The Mixer IC, U506, is driven by a Local Oscillator (LO) signal generated by the Trident
synthesizer IC, U702, at the LO port to down-convert the RF signal to a 109.65 MHz intermediate
frequency (IF). It is a passive, high linearity design with balanced inputs at the RF and IF ports and
internal LO buffer. The down converted IF signal is passed through a 3-pole crystal filter, FL501, and
an IF amplifier, Q503, which drives the input of the Analog to Digital Converter IC, U601.
3.1.2.3 UHF2 Front-End
From the attenuator, U1149, a UHF2 signal is routed to the first pre-selector filter followed by a Low
Noise Amplifier (LNA) and a second pre-selector filter. Both filters are discrete and tunable designs
and are used to band limit the incoming energy and suppress known spurious responses such as
Image spur. The LNA active device is an NPN transistor (U2032) with active bias provided by
transistor pair Q2022. The output of the second pre-selector filter is applied to a discrete Low Pass
Filter (LPF). The output of the LPF is applied to the RF port of the Mixer IC via a balun transformer,
T503. The Mixer IC, U506, is driven by a Local Oscillator (LO) signal generated by the Trident
synthesizer IC, U702, at the LO port to down-convert the RF signal to a 109.65 MHz intermediate
frequency (IF). It is a passive, high linearity design with balanced inputs at the RF and IF ports and
internal LO buffer. The down converted IF signal is passed through a 3-pole crystal filter, FL501, and
an IF amplifier, Q503, which drives the input of the Analog to Digital Converter IC, U601.
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