TRANSCEIVER 400 MHz
MTP700 Portable Radio / Detailed Service Manual
4.1 - 11
Pre-selector
The signal arriving at the receiver input is routed through the integrated, tuneable Pre-
selector Filter FL2100 into the Low Noise Amplifier (LNA) U2200 via a matching network
C2200, L2200, R2200 and C2201.
The pre-selector attenuates unwanted incoming RF signals, especially the image fre-
quency (25dB). Typically, the insertion loss is 2dB.
Low Noise Amplifier (LNA)
The LNA U2200 amplifies the RF signal. The overall gain of the LNA including the output
matching circuit, L2201, R2202, C2209, C2210 and L2202 typically is 18dB. Because the
LNA is the first amplifier in the RX line, it is important that it provides high gain and has a
low noise figure.
The LNA has an “enable” pin (pin 3) which is called VPD. When this pin receives a signal
of 2.7V from DC switch Q2200 the amplifier is enabled (via signal GP05_DCLICP from
WPIC). In the transmit slot, Q2200 is switched off. The LNA current is controlled by
R2201. Resistor R2202 ensures the amplifier is stable throughout its operating range.
From the LNA, the signal passes to the fixed-tuned post-selector.
Post-selector
The Post-selector, consisting of discrete components C2300-C2306, L2300-L2302,
attenuates the non-linear products of the LNA and also the image product. Typically, its
insertion loss is 2dB. From the output of the post-selection filter, the received signal is
routed to the mixer RF input via attenuator R2404-R2406 and band pass L2402, C2402.
Mixer
The mixer U2400 down-converts (at high side) the incoming RF signal to an IF signal
(109.65 MHz). The mixer IC consists of a mixer and an LO buffer amplifier.
The RF signal enters the mixer at pin 1, and the IF is produced at pin 6. The LO signal
from the main synthesizer passes via the attenuator R2400-R2402 and the wide band
matching network L2400, C2400, R2403, C2401 into the IC at pin 3. The LO power is
boosted internally before being applied to the mixer. The LO signal power at the LO mixer
input is -4dBm.
The 2.7V DC is supplied by switch Q2400, that is enabled during receive
mode (via signal GP06_DCLICIN from WPIC).
The mixer conversion gain is -6 dB.
An impedance matching network is placed between the mixer and the IF filter FL2500. It
consists of C2405, L2403, C2500, L2500 and C2501.
Содержание IMPRES Tetra MTP700
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