8
DR-142 THEORY OF OPERATION
INTRODUCTION
The DR-142 is an eight-channel synthesized receiver unit capable of rapid data reception at rates up to
9600 bps when using 4-level FSK. The unit maintains under 20 mA current consumption at 13.8 VDC
when in the receive standby mode. The current drain goes up as the voltage goes down.
POWER SUPPLY AND VOLTAGE DISTRIBUTION
The DR-142 is powered by an 8 to 15 VDC external power supply. F601 is a 3A fuse in series with
J301 for short circuit protection. Zener diode D601 prevents over voltage damage by blowing the F601
fuse when over 18 volts is applied. If reverse battery voltage is applied F601 will blow by conducting
current through D601.
The DR-142 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. The buck converter outputs about
40 mA at 5.4 volts. The conversion efficiency of the buck converter is about 90%. Lowering the supply
voltage will cause more current to be drawn from the supply. The buck converter’s output of 5.4 volts
feeds a 5 volt low drop out regulator U402. A voltage clamp consisting of Q601, 2 and 3 limits audio PA
voltages to about +14 volts. This circuitry is enabled by Q604 only when receiving a carrier or when
AUX_MON is enabled.
REFERENCE OSCILLATOR
Reference oscillator Y301 is a temperature controlled, voltage controlled oscillator (TCVCXO)
operating at 14.4 MHz. The Pin 4 output of the TCVCXO provides a reference for the frequency
synthesizer U401 at Pin 8. The reference oscillator also feed pin 7 of the U301 microcontroller through a
buffer amplifier Q310 and a Schmit inverter U303 to obtain a14.4 MHz clock. The Y301 pin 4 output is
also multiplied (tripled) up to 43.2 MHz by Q104 and its associated circuitry to provide a receiver second
local oscillator signal.
SYNTHESIZER
The DR-142 radio is built around a PLL synthesizer that consists of a receive voltage-controlled
oscillator (VCO). U401 contains both a prescaler and synthesizer controller. The prescaler squares and
divides the VCO output present at pin 5 by either 8 or 9, determined by a synthesizer controller logic
signal. The prescaler changes once every reference cycle at the appropriate time to yield the right
frequency. U401 contains a digital phase detector that works as follows. Microcontroller U301 clocks
new data into U401 internal buffer in synchronization with clock pulses. The channel information is
stored in EE memory of U301 and is loaded into RAM when the channel is selected.
The receive synthesizer works 43.65 MHz higher than the actual RF receiver band The loop filter
C403, C404,C405, R407 and R408 transform the pin 2 output signal to a DC voltage for application to
the VCO tuning varactors CR405 and CR406. The synthesizer system is “locked” when the phase and
frequency of both the reference and the divided VCO signal are the same. When the synthesizer is
locked pin 14 goes high. If the loop becomes unlocked pin 14 goes low.
