Guardian G25AMK005, Техническое руководство

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2-10 G U A R D I A N V H F 1 1 0 W M O B I L E
2.5.1.3 PIN Diode Switch
The antenna PIN diode switch is made up of CR7, CR8, CR9, CR10, and other associated components. This switch
is a four-port design. The four ports are antenna 1 (TOP RF), antenna 2 (SIDE RF), receive, and transmit. Receive
and transmit ports can be switched to only one of the two antenna ports. Transmit signals are routed from the
transmit/receive PIN diode switch (to be discussed in the following paragraph) to the antenna port. The receive
signal is routed from the selected antenna port to the transmit/receive PIN diode switch. The antenna PIN diode
switch and receiver circuits share current in the receive mode of operation via the signal labeled RXSINK at Q11,
pin 3. The insertion loss through the antenna pin switch is 0.2 dB typically.
The transmit/receive PIN diode switch is made up of C6, L60, L38, L40, L39, D9, D7, D10, C97, C98, C104, C105,
C106, C107, and other associated components. C127, C114, L42, and C115 are the 1/4 wave simulator circuit. The
1/4 wave simulator is critical to the design of the switch. In the transmit mode of operation CR13 and CR14 are
forward biased. C116 resonates with the internal series inductance of CR14 at 155 MHz and the receive port (RX
INPUT) is RF shorted to ground. With the receive port RF shorted to ground; the parallel combination of C127,
C114, and L42 forms a tank circuit resonating at 155 MHz. Consequently, the receive port appears as an open
circuit to the transmit signal and is routed to the antenna PIN diode switch. In the receive mode of operation, CR13
and CR14 are biased off so C114/L42/C115 appears as a low-pass filter (LPF) to signals at the antenna port of the
switch. The insertion loss through the transmit/receive PIN diode switch is 0.4 dB in the transmit mode and 0.2 dB
in the receive mode typically.
Q10 to Q19 and associated components are switching transistors used to control the antenna and transmit/ receive
PIN diode switches. The current flowing through the entire PIN diode circuit is approximately 45 mA in the
transmit mode of operation. In the receive mode of operation the transmit/receive PIN diode switch is disabled, and
nominal 85 mA flows through the antenna PIN diode switch.
2.5.2 Receiver
The VHF signal enters into the RX INPUT via the PIN diode switch (discussed previously). D1 and D2 are
Schottky protection diodes to protect the front-end circuitry from RF overloads that could occur if the PIN diode
switch failed to work properly or if a transmitter is very close to a receiver. Typical insertion loss is 0.1 dB for the
protection diodes. L25/C61 form a band-stop filter (BSF) at the first IF frequency of 45 MHz. Typical insertion
loss for the BSF is 15 dB at 45 MHz but less than 0.1 dB in the VHF band.
L14, L6, CR3, CR4, L7, CR27, CR28, L8, and L15 make up the very high frequency (VHF) preselector band-pass
filter (BPF). The BPF is inductively coupled for improved high-side attenuation. This filter provides attenuation to
spurious signals such as the first image and the half-IF. The BPF is varactor diode tuned by DAC line RXVTF.
Typical insertion loss (138 to 174 MHz) is 1 dB for the VHF BPF.
The RF amplifier (Q1, T1, etc.) utilizes loss-less feedback to deliver reasonable gain, low-noise figure, and a high
third order intercept point simultaneously. Typical gain (136 to 174 MHz) is 11.5 dB for the RF amplifier.
C14, L1, C9, C15, L2, C10, C16, L3, C11, C17, and L9 form a VHF LPF. This filter provides additional RX
spurious attenuation as well as image noise attenuation. L4, C12, L16, C25, L5, and C13 form a BSF at the first IF
frequency of 45 MHz. The insertion loss is 1.0 to 2.0 dB (136 to 174 MHz) typically for the cascade. The IF BSF
insertion loss is typically 40 dB at 45 MHz, but less than 0.3 dB in the VHF band.
U1 is a double-balanced mixer (DBM). U1 converts the desired RF signal down to the first IF of 45 MHz. High-
side local oscillator (LO) injection is used. Therefore, the LO is 45 MHz higher than the receiver tuned frequency.
The LO drive level is +10 dBm nominal at U1, pin 1. The conversion loss of the mixer (RF to IF) is 5.5 dB
typically.
The LO signal is generated in the synthesizer section (to be discussed later). The LO signal is designated RXLO on
the schematic diagram. The LO signal is routed to a LPF consisting of C31, L21, C87, C30, L20, C75, and C28.
L19 and C28 are also used to impedance match the LO port of the mixer. The insertion loss of the VHF LO LPF is
0.3 dB typically at 174 MHz.
R4, L17, C6, L10, R5, and C23 make up the diplexer network. This network properly terminates the DBM both in
and out of band. The diplexer also provides some additional half-IF spurious rejection. The diplexer insertion loss
is 0.8 dB typically at 45 MHz.