Motorola APX 7000 Detailed Service Manual page 37

Theory of Operation: Transceiver Board
3.1.2.1 VHF Front-End
From the first band select switch, U1122, 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 a second
band select switch, U501. 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.
3.1.2.2 700/800 Front-End
From the first band select switch, U1122, a 700 MHz or 800 MHz band signal is routed to a second
band SPST switch, U402, which selects the 700 or the 800 band signal and routes it to the
appropriate first pre-selector filter, FL401. A third band select switch, U404, selects the output of the
appropriate filter and applies it to an LNA followed by a similar pre-selector filter/ band-select switch
circuit. The signal is then routed to second LNA, U407, whose output is applied to a discrete image
filter. Both preselector filters are Surface Acoustic Wave designs (EPCOS B4232) used to band limit
the received energy and suppress known spurious responses such as Image and the ½ IF spur. The
output of the discrete image filter is applied to the RF port of the Mixer IC, U506, via a second band
select switch, U501. The Mixer IC is driven by an LO signal generated by the Trident synthesizer IC,
U702, and applied to the LO port to down-convert the RF signal to a 109.65 MHz intermediate
frequency (IF).
The IF signal is passed through a crystal filter, FL501, and an IF amplifier, Q503, which drive the
input of the Analog to Digital Converter IC, U601.
3.1.2.3 UHF1 Front-End
From the first band select switch, U1122, 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 second
band select switch, U501. 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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