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Theory of Operation: Transceiver Board 3-9

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