Overview

The VX-900is a UHF FM hand-held transceiver de-

signed to operate in the frequency range of 400 to

512 MHz.

Circuit Configuration by Frequency

The receiver is a double-conversion superhetero-

dyne with a first intermediate frequency (IF) of 44.25

MHz and a second IF of 450 kHz. Incoming signals

from the antenna are mixed with the local signal

from PLL to produce the first IF of 44.25 MHz.

This is then mixed with the 43.8 MHz second local

oscillator (using the 14.6 MHz reference crystal) out-

put to produce the 450 kHz second IF. This is subse-

quently detected to produce the demodulated signal.

The transmit signal frequency is directly generated

by the PLL VCO, and modulated by the signal from

the microphone. It is then amplified and sent to the

antenna.

Receive Signal Path

Front-end RF Amplifier

Incoming RF from the antenna jack is delivered to

the RF Unit and passes through a low-pass filter and high-pass filter consisting of coils L1002, L1003,

L1005, L1007, L1001, L1004, L1006, L1008, L1009 and L1011, capacitors C1002, C1005, C1009, C1011, C1019, C1028, C1003, C1012, C1015, C1018, C1022, C1026, C1030, C1035, C1036, C1039 and C1042, and

antenna switching diode D1016 (HVU131).

Signals within the frequency range of the transceiver are then amplified by Q1012 (BIC702C) and enter a

varactor-tunedband-pass filter consisting of coils L1014, L1015, and L1017, capacitors C1081, C1083,

C1094, C1097, C1105, C1106, C1115, C1117, C1121, C1123, C1134, C1139, and C1145, resisters R1079, R1106 and R1124, and diodes D1027, D1028, D1030, D1031, D1032, and D1033 (all HVC350) before de-

livery of the RF signal to the first mixer.

First Mixer

Buffered output from the VCO is amplified by Q1029 (2SC5226-4/5) to provide a pure first local signal be-

tween 355.75 and 467 MHz for injection to the first mixer D1037 (GN2011-Q). The 44.25 MHz first mixer

product then passes through monolithic crystal fil-

Circuit Description

ters XF1001 and XF1002 (MDF0028, ±5.5 kHz BW)

to strip away all undesired mixer products.

IF Amplifier

The first IF signal is amplified by Q1044 (2SC5226- 4/5).

The amplified first IF signal is applied to FM IF subsystem IC Q1049 (TA31136FN), which contains the

second mixer, second local oscillator, limiter ampli-

fier, noise amplifier, and S-meter amplifier.

A second local signal is generated by Q1051 (2SC4617) using the 14.6 MHz crystal X1002 as a

reference, producing a 43.8 MHz signal; this which

yields a 450 kHz second IF when the reference signal is mixed with the first IF signal within Q1046.

The second IF then passes through the ceramic filter CF1001 (PBFC450R15D: wide channels), CF1002 (PBFC450R9DR: narrow channels) to strip

away all but the desired signal, and is applied to the

limiter amplifier in Q1049, which removes ampli-

tude variations in the 450kHz IF, before detection of

the speech by the ceramic discriminator CD1001

(CDBC450CX24).

Audio Amplifier

Detected audio from Q1049 is applied to Q1027(AK2345) and the audio low-pass filter, and then through the volume control (Q1021:M62364FP) to the audio amplifier Q1007 (TDA2822D;external speaker) or Q1011 (TDA2822D;internal speaker), pro-

viding up to 0.5 Watt to the optional headphone jack

or 16-ohm loudspeaker.

Attention: Audio output is BTL output. Both sides

of the audio output are above ground, and this line

must not be connected to a speaker line which uses

a grounded shield.

Squelch Control

The squelch circuitry consists of a noise amplifier and band-pass filter within Q1049, and noise detector D1058/D1059 (DA221).

When no carrier received, noise at the output of the detector stage in Q1049 is amplified and band-pass

filtered by the noise amplifier section of Q1049 and

the network between pins 7 and 8, and then recti-

fied by D1058.

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