Ritron RIT17-445 用户手册

 

 

 

The DTX-445 is powered by a 9 to 17 VDC external power supply.  F601 is a 3A reset able fuse in series with  

J301 for short circuit or reverse connection protection. If reverse voltage is applied, F601 will temporarily “blow” open 
when excess current flows through D601. Disconnect the supply to reset the fuse. Reconnect power correctly and 
proceed. An input voltage below 17 VDC must be maintained. 

The DTX-445 is designed to consume low current by using a switching DC/DC converter called a buck converter. 

Receiver standby current is less than 20 mA at 13.8 volts. In the receive mode current drain goes up as the voltage 
goes down. The buck converter outputs about 40 mA at 6.2 volts. The conversion efficiency of the buck converter is 
about 85%. Lowering the supply voltage will cause more current to be drawn from the supply. The buck converter’s 
output of 6.2 volts feeds a 3.3 volt low drop out regulator U602.  The 3.3 volt regulator supplies the logic and audio 
processing ICs. A voltage regulator consisting of Q209, Q210 and Q214 limits the RF final PA and audio PA voltages to 
+14 volts. This circuitry is enabled by Q211 only when transmitting. An 8 volt regulator consisting of Q204 and Q205 
supply power to the VCO buffer stages Q402, Q404 and Q203 during transmission. When supply voltages greater than 
14 volts are used the internal regulator can get hot with prolonged transmissions. 
 

 

Reference oscillator Y301 is a temperature compensated, voltage controlled crystal oscillator (TCVCXO) operating 

at 14.4 MHz.  The output of the TCVCXO provides a reference for the frequency synthesizer U401 at Pin 8.  The 
reference oscillator also feeds pin 7 of the U301 microcontroller through a buffer amplifier Q303.  The Y301 output is 
also multiplied (tripled) up to 43.2 MHz by Q104 and its associated circuitry to provide the receiver second local 
oscillator signal.   
 

 

The DTX-445 radio is built around a single PLL synthesized voltage-controlled oscillator (VCO). When the receiver 

or transmitter mode is switched, a new synthesizer operating frequency is selected.  Microcontroller U301 clocks new 
data into the U401 internal buffer in synchronization with clock pulses. The channel information is stored in the EE 
memory of U301 and is loaded into RAM when the channel is selected. The single VCO operates from 406 to 470 MHz. 
In receive mode, the synthesizer operates 43.65 MHz lower than the receive frequency. When transmitting, the voice or 
data is applied to the modulation varactor diode CR401. The loop filter C407, C408, C413, R402 and R405 transform 
the pin 2 output signal to a DC voltage for application to the VCO tuning varactor CR402.  The synthesizer system is 
“locked” when the phase and frequency of both the reference and the divided VCO signal are the same. The VCO 
control voltage can be measured with a DVM at test point 1 (TP1) on the bottom of the board below the synthesizer 
shield. 
 

 
Receiver Front End 

The signal from the antenna passes through the transmitter lowpass filter and then goes to the SAW bandpass 

filter FL101. Q101 amplifies the signal about 11 dB before going through another SAW bandpass filter FL102 and the 
mixer matching stage. The receiver is turned on by Q212 and Q213 supplying RX_3.3v when RXEN of U301 goes high. 

The amplified received input signal is applied to the input of mixer U103.  The 1st local oscillator signal from the 

synthesizer is applied to the local oscillator input of the mixer. Y101 and Y103, a 43.65 MHz four-pole crystal filter forms 
the first IF filtering.  Q103 and associated components amplify the 43.65 MHz IF signal and apply it to the input of the 
2nd mixer at Pin 16 of U101. 
 
FM Receiver Subsystem 

A multi-function integrated circuit, U101 and associated components form the FM-receiver subsystem. The 

subsystem performs the functions of a 2nd mixer, IF amplifier and FM detector.  The second local oscillator at 43.2 MHz 
is applied to the 2nd local oscillator input at Pin 1 of U101.  The 43.65 MHz signal at Pin 16 and the 2nd local oscillator 
are mixed, with the resulting 450 kHz output signal appearing at Pin 3.  This signal is filtered by a 450 kHz 6-pole 
ceramic filter YF101 and applied to the input of the limiting IF amplifier at Pin 5.  IC101 pin 6 de-couples the IF amplifier.  
An internal quadrature detector, whose center frequency is determined by the 450 kHz quadrature resonator Y102 
detects the FM IF signal.  One input of the quadrature detector is connected internally to the IF signal from pin 11 while 
the other input is the phase-shifted signal from Y102 at Pin 10. Demodulated audio appears at Pin 9, where a lowpass 

 

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