Vertex Standard FT-1807M - Manual (Page 7)

7
Circuit Description
Receive Signal Path
Incoming RF signal is from the antenna jack is deliv-
ered to the Main Unit and passed through the low-
pass filter network consisting capacitors C1213,
C1236, C1239, & C1238 and coils L1017, L1018, &
L1019, antenna switching diode D1020 and D1028
(both RLS135), delivery to the RF amplifier Q1045
(3SK296ZQ). The amplified RF signal is passed
through the another varactor-tuned band-pass filter
consisting of capacitors C1198, C1199, C1200, C1290,
C1218, C1249, C1251, C1219, C1248, C1221 & C1268,
coils L1012, L1015, & L1026, and diodes D1017,
D1021 and D1025 (both HVC350B). The amplified
RF signal is passed through the another varactor-
tuned band-pass filter consisting of capacitors C1198,
1199, 1200, & 1218, coils L1012 and L1015, and di-
odes D1017 and D1021 (both HVC350B), then ap-
plied to the 1st mixer Q1037 (3SK296ZQ) along with
the first local signal from the PLL circuit.
The first local signal is generated between 352.75
MHz and 422.75 MHz by the VCO, which consists
of Q1009 (2SC5374) and varactor diode D1002
(HVC355B) according to the receiving frequency.
IF and Audio Circuits
The 47.25 MHz first IF signal is applied to the mono-
lithic crystal filters XF1001 and XF1002 which strip
away unwanted mixer products, and the IF signal is
applied to the first IF amplifier Q1032 (2SC4400).
The amplified first IF signal is then delivered to the
FM IF subsystem IC Q1028 (NJM2591V), which con-
tains the second mixer, limiter amplifier, noise am-
plifier, and FM detector.
The 46.8 MHz second local signal which is made from
quadrupled 11.7 MHz crystal X1001 produces the 450
kHz second IF signal when mixed with first IF sig-
nal within Q1028 (NJM2591V).
The 450 kHz second IF signal is applied to the ce-
ramic filter CF1001 (for Narrow FM) or CF1002 (for
Wide FM) which strip away unwanted mixer prod-
ucts to the ceramic discriminator CD1001 which re-
moves any amplitude variations in the 450 kHz IF
signal before detection of speech.
The detected audio from the Q1028 (NJM2591V)
passes through the de-emphasis circuit consisting of
resistors R1082 & R1113, and capacitors C1120 &
C1122, to the audio mute gate Q1034 (2SJ364).
The audio signal passes through a band-pass filter
consisting of Q1046 and Q1047 (both 2SC4154), and
the audio mute gate Q1039 (2SJ347), to the audio
VR which adjusts the audio sensitivity to compen-
sate for audio level variations. The adjusted audio
signal is delivered to the audio amplifier Q1035
(LA4425A) which provides up to 3 Watts, to the ex-
ternal speaker jack or a 4-Ohm loudspeaker.
Squelch Control
When no carrier received, the noise signal from
Q1028 (NJM2591V) is amplified by Q1051
(2SC4617), and is detected by D1011 and D1013
(both DA221). The resulting DC voltage passes
through the SQL knob to main CPU Q2002
(HD64F2266TF13). While no carrier is received,
main CPU Q2002 (HD64F2266TF13) control Q1048
(CD4094BPWR), thus, audio mute gate Q1034
(2SJ364) and Q1039 (2SJ347) turns “OFF” to dis-
able the audio output from the speaker.
Transmit Signal Path
The speech signal from the microphone is amplified
by Q1049 (LA2902PWR). The amplified speech sig-
nal is subjected to the low-pass filter network Q1049
(LA2902PWR) to deviation controlled by Q1043
(M62364FP).
The adjusted speech signal from Q1043 (M62364FP)
is delivered to VCO Q1009 (2SC5374) which fre-
quency modulates the transmitting VCO made up
of D1004 (HSC277).
The modulated transmit signal passes through buff-
er amplifier Q1010 and Q1023 (both 2SC5374).
The transmit signal applied to the drive amplifier
Q1026 (2SC5226), then finally amplified by power
amplifier module Q1030 (RA55H4047M) up to 50
Watts. The APC circuit controls the Q1030
(RA55H4047M) power amplifier’s gain.
The 50 Watts RF signal passes through low-pass fil-
ter network consisting of Capacitors C1210 and
C1211 and coil L1013, antenna switch D1018 and
D1019 (both XB15A709), and another low-pass fil-
ter network consisting capacitors C1213, C1236, &
C1239, C2038 and coils L1017, L1018, & L1019, and
then deliver to the ANT jack.