Motorola MTP700 Detailed Service Manual page 134

4.2 - 12
Front Filter (First Pre-selector)
The signal arriving from the limiter is routed through the front SAW filter (FL500) into the
Low Noise Amplifier (LNA) (U500) via a matching network C500, L501 and C501.
The front filter is also called the 1st pre-selector and is used to attenuate the incoming
parasitic RF frequencies, especially the image frequency. The Insertion Loss (IL) is typi-
cally 2dB.
Low Noise Amplifier (LNA)
The LNA (U500) amplifies the RF signal. The LNA gain including output matching circuit,
L502, R502 and C514, is typically 15dB. Because the LNA is the first amplifier in the line,
it is important that it will maintain large gain and small Noise Figure.
The LNA has an "enable" pin (pin 3) which is called VPD. When this pin receives a supply
of 2.7 V from DC switch (Q500), the amplifier is enabled (controlled by GP05_DCLICP
from WPIC). The LNA current is controlled by R500. Resistor R502 ensures the amplifier
is stable through out its operating range. From the LNA the signal passes to second pre-
selector, SAW filter (FL501).
Second Pre-selector
The 2nd Pre-selector (FL501) is another SAW filter which attenuates the non-linear prod-
ucts of the LNA and also the image product. Its IL is typically 2dB. From the output of the
2nd SAW filter the received signal is routed to the Mixer RF input via a LC match.
Mixer
The Mixer (U502) function is to down convert the incoming RF signal into IF signal
(109.65 MHz). The mixer IC consists of a mixer and LO injection buffer.
The RF signal enters the Mixer at pin 4, and the IF is produced at pin 6. The LO (from the
main VCO) signal passes via low pass filter FL503 and R514 into the IC at pin 3. The LO
power is boosted internally by the mixer IC's injection amplifier before being applied to
the mixer. The LO buffer output choke, L513, is used to resonate out the chip internal
capacitance, allowing a high injection signal into the mixer. The mixer IF current is partly
controlled by R511 and R510. R511 controls the mixer gain while C528 provides RF
bypass. The LO power at the LO mixer input is -2dBm to -4dBm. The 2.7V DC is supplied
by switch Q501 (controlled by GP06_DCLICIN from WPIC).
The mixer conversion gain is about 3dB.
An impedance matching net is placed between the mixer and the IF filter (FL502). It con-
sists of L514, C534, C535, C537, L509, R516 and C538.
IF Filter
The IF signal is routed via the IF filter (FL502), which has IL of 2.5dB and attenuates par-
asitic products of the mixer. It is connected via an impedance matching network that con-
sists of C539, C540 and L510 to the Balun.
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