BK Radio KNG Series, Service Manual, Page: 59 / 248

Page 3-4 Service Manual - KNG Portables
theory of operation
Rx VCO
VCO Buffer
Loop Filter
Programming
Bus
Synthesizer
IC
Tx VCO
Steering Line
Tx Audio
Coarse Tune
TCXO
Reference
10 MHz
Tx / Rx
Tx / Rx
Coarse Tune
Control
Digital
Potentiometer
SPI
LO
Coarse Tune
DAC
From uP
RX VCO
181-219 MHz (P150)
290-380 MHz (P400)
350-430 MHz (P500)
TX VCO
136-174 MHz (P150)
380-470 MHz (P400)
440-520 MHz (P500)
synthesizer Block Diagram
The temperature compensated crystal oscillator (TCXO) serves as a stable frequency reference over the
operating temperature of the radio.
The Synthesizer IC contains programmable counters to divide and compare the VCO frequency from the
feedback loop to the TCXO reference frequency and adjust the VCO control voltage in such a way that
the VCO maintains a stable output frequency.
The Loop Filter smooths the output of the Synthesizer IC in order to reduce undesired modulation of the
VCO frequency.
The Rx VCO is normally in operation, as the PTT is normally de-activated. The Coarse Tune and
Steering inputs serve as the control voltages for this device.
The Tx VCO is in operation whenever the PTT is activated. Its frequency bias point is controlled by
the Coarse Tune and Steering inputs, and its frequency varies with the amplitude of the Tx Audio input
signal. The Coarse Tune control signal comes from the DAC. The Tx Audio signal is, in the case of
analog transmission, an analog audio signal. If the radio is set for digital transmission, the Tx Audio
signal is a baseband digital signal. Because the modulation sensitivity of the VCO varies over its RF
bandwidth, the Digital Potentiometer is used to adjust the level of the Tx Audio signal so that constant
modulation sensitivity is maintained throughout the RF bandwidth.
The VCO buffer provides the necessary signal power to drive the LO input of the 1st IF mixer and the PA
driver of the transmitter.