Handset Section; Block Diagram - Panasonic KX-FPC135 Service Manual

KX-FPC135 / KX-FPC141

9 HANDSET SECTION

9.1.

Block diagram

The signal flow of the KX-FPC135/KX-FPC141 handset is explained in Fig.A. The diagram´s frequency is shown as the first
channel´s value.
Receiving Route
1. The RF (Radio Frequency) receiving signal modulated into 900MHz band entered from the antenna is separated into a
transmitting signal in the duplexer constructed from the band pass filter, etc., and enters the receiving route.
2. After it is revised into the appropriate amplitude in the RF anplifier, it drops to the first lower carrier frequency (1st IF
(Intermediate Frequency) Receiving Signal) of 10.7MHz in the first mixer circuit. In this circuit, the signal supplied from the
RXVCO is used. Here, that frequency becomes 891.4000MHz based on the formula (the RF receiving signal frequency
(902.1000MHz) -1st (10.7MHz) = 882.3125MHz).
3. Next, it falls to the next carrier frequency of 450KHz (2nd IF receiving Signal). Here, the signal frequency supplied becomes
10.25MHz based on the formula (the 1st IF receiving signal frequency (10.7MHz) - 2nd IF receiving signal frequency (450KHz)
= 10.25MHz). It is supplied from the reference oscillator through the buffer circuit because the frequency is the same.
4. The 2nd IF receiving signal is demodulated to a receiving voice signal in the detector circuit. It passes through the filter and
amplifier and is sent to the speaker. The signal for the link before communication is also demodulated in the detector circuit,
and input to the CPU through the RX data filter circuit and decoded. Also, the carrier detection circuit detects whether the carrier
signal from the base unit has stopped or not. (Refer to 14.3 Carrier detection circuit for details.)
If the RF receiving signal directly demodulates to a voice frequency band, the signal frequency supplied from RXVCO will
become very close to the receiving signal frequency (the RF receiving signal frequency (902.100MHz) - 2nd IF receiving
frequency (450KHz) = 901.6500MHz). To prevent obstacles in the receiving signal for a weak wave condition, the frequency will
drop in 2 steps. This method is called the Double Super Heterodyne Method and is a technique often used in the FM receiver.
5. The expander circuit decreases the noise from fhe AF (Audio Frequency) receiving signal demodulated by the detector. An
expander circuit and compander circuit in the communication path decrease the noise. It is similar to the DOLBY system in an
audio product. Here, the explanation of the detailed priniciple is omitted. After that, an AF signal is output from the speaker
through the side tone control circuit and amplifier circuit. The side tone control circuit makes proper side tone using the
transmission signal from the microphone, and returns it to the speaker.
Transmitting Route
The transmitting signal entered from the microphone goes through the amplifier, compander and filter, is modulated to each carrier
frequency in the TXVCO circuit according to the selected channel, goes through the RF amplifier and duplexer and is output by the
antenna.
Also, the handset operates with a Ni-Cd battery and when there is no communication, power is not supplied and prevents the
battery from running out. Furthermore, the surrounding circuits in the dotted lines in Fig.A. make up the heart of the unit which is
called the RF unit. Usually, it is shielded by a metallic plate. This protects it from exterior obstacles and radiated waves from the
internal components.
This base unit block diagram exchanges the speaker and microphone from the handset block diagram with a telephone line. The
telephone line interface circuit is added and the construction is almost the same.
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