Vertex Standard Yaesu VX-2R - Manual (Page 8)

8
Circuit Description
to varactors D1016 and D1017 (both HVC355B) in the
variable frequency band-pass filters. By changing the elec-
trostatic capacitance of the varactors, optimum filter char-
acteristics are provided for each specific operating fre-
quency.
(6) 47.25-MHz First Intermediate Frequency
The 47.25-MHz first intermediate frequency from first
mixers is delivered to the first IF circuit. On the MAIN-
UNIT, the first IF signals on the AM and FM-narrow are
passed through NAR/WIDE switch D1037 (DAP222M),
47.25-MHz monolithic crystal filter (MCF) XF1001, and
narrow IF amplifier Q1017 (2SC4915) to pin 16 of Nar-
row IF IC Q1033 (TA31136FN) after amplitude limiting
by D1042 (DA221M).
Meanwhile, a portion of the output of 11.7-MHz crystal
X1001 is multiplied fourfold by Q1035 and Q1022 (both
2SC4915
) to provide the 46.8-MHz second local signal,
applied to the Narrow IF IC Q1033 (TA31136FN). Within
this IC, the 46.8-MHz second local signal is mixed with
the 47.25-MHz first intermediate frequency signal to pro-
duce the 450-kHz second intermediate frequency.
This second IF signal is filtered by ceramic filter CF1002
and amplified by the limiting amplifier within the Nar-
row IF IC Q1033 (TA31136FN) before quadrature detec-
tion by ceramic discriminator CD1001.
Demodulated audio is fed from pin 9 of the Narrow IF IC
Q1033 (TA31136FN) through the “narrow mute” analog
switch Q1044 and squelch gate Q1053 (both 2SJ364) be-
fore de-emphasis at Q1039 (DTC144EM).
The resulting audio is amplified by AF amplifier Q1079
(NJM2135V), and fed through MIC/EAR jack J1007 to the
internal speaker, SP1001, or an external earphone.
(7) Squelch Control
Signal components in the neighborhood of 15 kHz con-
tained in the discriminator output pass through an active
band-pass filter composed of R1142, R1156, R1160, C1168,
C1177, and the operational amplifier between pins 7 and
8 within Narrow IF IC Q1033 (TA31136FN). They are then
rectified by D1049 and D1050 (both DA221M) to obtain a
DC voltage corresponding to the level of noise. This volt-
age is fed to pin 51 of CPU Q1076 (HD64F2266), which
compares the input voltage with a previously set thresh-
old. When the input voltage drops below the threshold,
normally due to the presence of a carrier, squelch gate
Q1071 (2SC4617) turns on and allows any demodulated
audio to pass. At the same time, Q1001 (UMW1N) switch-
es on, causing the BUSY/TX lamp D1004 (CL-165) to light.
Transmitter Signal Flow
(1) 145-MHz-Band T/R Switching
Closing PTT switch S3001 on the SW-UNIT pulls the base
of Q1007 (DTA114EM) low, causing the collector to go
high. This signal is fed to pin 45 (PTT) of CPU Q1076
(HD64F2266), allowing the CPU to recognize that the PTT
switch has been pushed. When the CPU detects closure of
the PTT switch, pin 70 (TX/RX) goes high. This control
signal switches Q1054 (UMD6N) to produce the TX con-
trol signal that activates Q1050 (2SA1774). At the same
time, PLL division data is applied to PLL IC Q1034
(MB15A01PFV) from the CPU, to disable the receiver
power saver. Also, Q1049 (EMG2) is switched on to dis-
able the receiver circuits, causing the red side of BUSY/
TX lamp D1004 (CL-165) to light.
(2) Modulation
Voice signal input from either the built-in microphone
MC1001 on the MAIN-UNIT or external microphone in-
put from external mic jack J1007 is pre-emphasized by
C1036 and R1031, and processed by microphone amplifi-
er Q1083 (NJM3403AV), the IDC (instantaneous devia-
tion control) circuit Q1083 to prevent over-modulation,
and active low-pass filter Q1083.
During CTCSS operation, the voice signal is mixed with
the TONE ENC subaudible tone signal from pin 43 of the
CPU and delivered to the VCO. During DCS operation,
the voice signal is mixed with the DCS ENC subaudible
tone signal from pin 44 of the CPU and delivered to the
VCO. During DTMF operation, the DTMF tones from pin
44 of the CPU are fed directly to the IDC stage.
(3) 145-MHz-Band Transmission
Modulating audio passes through deviation setting D/A
converter Q1010 (M62364FP), then delivered to the VHF
modulator of the VCO-UNIT mounted on the MAIN-
UNIT. This signal is applied to varactor D4005 (HSC277)
in the tank circuit of VHF VCO Q4004 (EC3H07B), which
oscillates at the desired VHF transmitting frequency. The
modulated VCO signal is buffered by amplifier Q4006
(EC3H07B) and Q1025 (2SC5374) and delivered through
VHF T/R diode switch D1041 (DAN222M) to the MAIN-
UNIT. The modulated low-level VHF transmit signal from
the VCO is passed through diode switch D1047
(DAN222M) to amplifier Q3004 (2SC5374) on the SW-
UNIT. The amplified VHF transmit signal from Q3004
(2SC5374) is amplified by Q3001 (2SC5226) and RF pow-
er amplifier Q3002 (2SK3079) on the SW-UNIT up to 0.1,
0.3, 1.5, or 3 Watts (depending on the power source). The
RF output passes through TX diode switch D1058
(RLS135) then fed via the T/R switch and low-pass filter
(to suppress harmonics and spurious products) to the an-
tenna terminal.