Circuit Description - Kenwood TK-250 Service Manual

TK-250
CIRCUIT DESCRIPTION
1. OVERVIEW
The KENWOOD model TK-250 is a VHF/FM hand-held
transceiver designed to operate in the frequency range of
150 to 174 MHz. The unit consists of a receiver, a transmit-
ter, a phase-Iocked loop (PLL) frequency synthesizer, power
supply circuits a contra I unit, a display unit.
ANT
TX,RX : 150-174MHz K,M, T, E
TX,RX : 136-150MHz K2,M2
MCF
2. CIRCUIT CONFIGURATION BY FREQUENCY
The receiver is a double-convers ion superhetrodyne with
a first intermediate frequency (IF) of 45.05MHz and a second
IF of 455kHz. Incoming signals from the anten na are mixed
with the local signal from the PLL to produce the first IF of
45.05MHz.
This is th en mixed with the 45.505MHz second local os-
cillator output to produce the 455Hz second IF. This is de-
tected to give the demodulated signa!.
The transmit signal frequency is generated by the PLL
VCO, and modulated by the signal from the micraphone. It is
then anplified and sent to the antenna.
CF
455kHz
SP
195.05-219.05 : K,M, T, E
181.05-195.05 : K2,M2
MIC
150-174MHz K,M, T, E
136-150MHz K2,M2
Fig. 1 Frequency configuration
16
3. RECEIVER SYSTEM
The receiver is double conversion superhetrodyne, de-
signed to operate in the frequency range of 150 to
174MHz(K,M) and 136-150MHz(K2,M2).
3.1 Front-end RF amplifier
An incoming signal from the antenna is applied to an RF
amplifier (017) after passing thraugh a transmitlreceive switch
circuit (08 and 09 are of!) . After the signal is amplified, the
signal is filtered by a Band-shift type band-pass filter to elimi-
nate unwanted signals before it is routed to the first mixer.
3.2 First mixer
The signal from the RF amplifier is heterodyned with the
first local oscillator signal fram the PLL frequency synthe-
sizer circuit at the first mixer (014) to become a 45.05MHz
first intermediate frequency (1st IF) signal. The first IF signal
is fed through two monolithic crystal filters (MCFs : XF1) to
further remove spurious signais.
Item
Rating
Nominal center frequency
45.05MHz
Pass band width
±7.5kHz or more at 3dB
Attenuation band width
±22kHz or less at 2.5dB
Ripple
1 .OdB or less
Insertion loss
4dB or less
Guaranteed attenuation
80dB or more within ±910kHz
(Spurious : 40dB or more)
Terminating impedance
800Q/2pF
XF1 : L71-0409-15(ExceptT)
Item
Rating
Nominal center frequency
45.05MHz
Pass band width
±3.75kHz or more at 3dB
Attenuation band width
±14.0kHz or less at 40dB
Ripple
1 .OdB or less
Insertion loss
4dB or less
Guaranteed attenuation
80dB or more within ±1000kHz
(Spurious : 40dB or more)
Terminating impedance
290Q/7pF
XF1 : L71-0461-05( T)