5
Receiver filters and amplifies RF signal through SAW filter and LNA
RX part is double super heterodyne type, and consists of LNA, LPF, Mixer, SAW filter and
IF IC part. There happens RX signal from antenna and the signal is low-noise-
amplification (LNA) through SAW filter and LNA, and it comes to 2nd IF 455KHz via 2nd
X-TAL44.545MHz.
SAW filter rejects Image frequency(RX ±2*IF) generated from 1’st Mixer while receiving.
For example, if the input channel is 858Mhz, the output frequency would be 45Mhz from
mixer while local frequency is 813Mhz. However, if SAW filter will not filter the image
frequency of 858±2*IF, S/N would be worse with 45Mhz(813-768=45) of noise. And SAW
filter should filter to prevent local frequency becoming spurious through LNA and reverse
path.
LNA amplifies and sends the faint signal from antenna to mixer, and mixer TR mixes and
generates 1st IF with frequency from antenna.
IF comes from mixer. And IF contains inter-modulation product component. MCF
removes that product component. So clean IF(45MHz) can be inserted to IF IC. 2nd local
frequency entered IF IC is mixed with the IF(45MHz) signal to be 2nd IF of 455KHz
simultaneously.
2nd IF signal of 455KHz passes LPF(Ceramic filer) to remove the noise, and the signal is
de-modulated with discriminator method. Here detected RF signal entered into receiver
and RSSI signal which indicates the strength, and they are passed to micro processor in
analogue value.
5 Transmitter
Transmitter consists of driver amp and power amplifier, and TX power control. It
generates carrier frequency while share RX with frequency synthesizer.
Local frequency is used to direct transmission frequency because there is difference
between transmission and reception. So it only amplifies and propagates through PA
(Power Amp.) which can get high level of gain easily with low power.
B. LOGIC Circuit
1 Summary
This unit consists of CPU part, Decoder part, modem part, and power supply part.
2 CPU part(IC3)
The CPU adopted by this unit has 16/32-bit ARM7TDMI RISC processor (66MHz)
and works on 4.9152MHz. The main functions are as follows;
-Execute MASC protocol
-Control PLL circuit of R/F part and perform Power Saving function
-Perform Data transaction function( receiving and transmitting) through Data
Pump(Modem)part
-Checking and processing of RSSI Level come from R/F part
-Perform Data transaction with DTE through DTE interface part
3 Memory part(IC1, IC3)
Memory part consists of FLASH Memory(2Mb, IC1) and SRAM(2Mb, IC3). Flash
