Kenwood TH-77A Service Manual page 4

TH-77A/E
CIRCUIT DESCRIPTION
(1) Frequency Configuration
The TH-77 has independent PLL circuits and intermediate-fre-
quency amplifiers for the VHF and UHF bands. A VHF signal
and a UHF signal can thus be received at the sz.1e time. The
UHF signal can be received at the same time as the VHF signal
by doubling the local oscillation frequency for the VHF band.
(See Figure 1.)
The received VHF single is converted to the first intermediate
frequency (IF) of 45.05 MHz using the first local oscillator, fre-
quency of 181.05 to 219.05 MHz, and is mixed with the second
local oscillator frequency of 45.505 MHz to produce the second
IF of 455 kHz.
The received UHF signal band is converted to the first IF of
58.525 MHz using the first local oscillator frequency of 371.475
to 391.475 MHz, and is mixed with the second local oscillator
136-174 MHz (E2)
144~148 MHz (K, P, M, X)
144~146 MHz (T, E1)
430-440 MHz (M, T, X, E1, £2)
438~450 MHz (K, P)
|44MHz BAND
136~174 MHz ('st MIX 45.05MHz
136-174 MHz
192.475 MHz
202.475 MHz
384.95 MHz~404.95 MHz
430MHz BAND
371.4 MHz
$
391.4 MHz
VHF PLL
frequency of 58.070 MHz to produce the second IF of 455 kHz.
The local oscillator frequency for the VHF band is doubled,
when the UHF signal is received at the same time. The UHF
signal is converted to the first IF of 45.05 MHz for the VHF band
using a frequency of 384.95 to 404.95 MHz obtained when the
first loca! oscillator frequency of 192.475 to 202.475 MHz for
the VHF band is doubled. The resulting frequency is mixed with
the second local oscillation frequency of 45.505 MHz to pro-
duce the second IF of 455 kHz.
As described above, signal reception for the VHF, UHF, or sub-
UHF band is based on a double-conversion system. In the
transmit signal channel, a directly oscillated voltage-controlled
oscillator (VCO) signal for the VHF and UHF bands is sent to the
reactance modulator, amplified to the required level by a linear
amplifier, and transmitted.
455 KHz
455 KHz
12.8 MHz REF OSC
ae
MIC AMP
(2.8MHz
Fig. 1 Circuit configuration by frequency