Sec 1: 3-6 Basic Theory of Operation: Analog Mode of Operation
3.2.1.2 VHF Front-End
From the first band select switch, a VHF signal is routed to the first pre-selector filter followed by an
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 output of the second pre-selector filter is applied to the RF port of the Mixer IC via a second
VHF/700,800 band select switch. The Mixer IC is also excited by a Local Oscillator (LO) signal at the
LO port to down-convert the RF signal to a 109.65 MHz intermediate frequency (IF). The down
converted IF signal is passed through a crystal filter and IF amplifier which drives the input of the
Abacus 3 Analog to Digital Converter IC (AD9864).
3.2.1.3 700/800 MHz Front-End
From the first band select switch, a 700 MHz or 800 MHz band signal is routed to a second band
SPST switch which selects the 700 or the 800 band signal and routes it to the appropriate first
pre-selector filter. A third band select switch 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 whose output is applied to a discrete image filter. Both preselector filters are Surface
Acoustic Wave designs 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 via a second VHF/700,800 band select switch. The Mixer IC is also excited
by a Local Oscillator (LO) signal at the LO port to down-convert the RF signal to a 109.65 MHz
intermediate frequency (IF). The down converted IF signal is passed through a crystal filter and IF
amplifier which drives the input of the Abacus 3 Analog to Digital Converter IC (AD9864).
3.2.1.4 UHF1 Front-End
From the first band select switch, a UHF1 signal is routed to the first pre-selector filter followed by an
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
output of the second pre-selector filter is applied to the RF port of the Mixer IC via a second
UHF1/700,800 or UHF1/VHF band select switch. The Mixer IC is also excited by a Local Oscillator
(LO) signal at the LO port to down-convert the RF signal to a 109.65 MHz intermediate frequency
(IF). The down converted IF signal is passed through a crystal filter and IF amplifier which drives the
input of the Abacus 3 Analog to Digital Converter IC (AD9864).
3.2.1.5 UHF2 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 (U1932) with active bias
provided by transistor pair Q1922. 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.
To Next Page
Loading...
