BK Radio P150, Service Manual

Page: 109 / 264

Page 4-6 Service Manual - KNG Portables
theory of operation
Tunable 1st
Pre Selector Filter
380-470 MHz (P400)
440-520 MHz (P500)
LO
Low Noise
Pre-Amplifier
Variable
Gain IF Amp
Tx / Rx
Crystal Filter
90MHz
1st IF
90MHz
Gain
Control
Mixer
Antenna
Switch
Harmonic
Filter
1st Preselector
Tune Control
To Tx
DAC
From uP
2nd Preselector
Tune Control
Tuanble
Impedance
Match
IF Match
Tune Control
Tunable 2nd
Pre Selector Filter
380-470 MHz (P400)
440-520 MHz (P500)
AD9864 IC
IF Digitizer
To uP
LNA
ADC
Decimation
Filter
Formatting /
SSI
SPI
To DSP
TCXO
Reference
19.6608 MHz
LO Synthesizer
Mixer
LO VCO
87.984 MHz
Loop Filter
Control Logic
2nd IF Filter
2.016 MHz
Clock VCO
16.128 MHz
Loop Filter
Clock Synthesizer
2nd IF
2.016 MHz
uhf receiver Block diagram
The Tunable Preselector Filters are varactor-tuned bandpass filters that pass the receive frequency and
reject images and out of band signals. The varactor control voltages vary in the range of about 0 V to 4 V.
The tuning control voltage is supplied by a digital-to-analog converter (DAC) which receives control signals
from the radio’s microprocessor.
The Low Noise Preamplifier in VHF models operates linearly and uses a BJT as the gain element. This
block provides the necessary signal drive for the Mixer input.
The Fixed Image Rejection Filter is a bandpass type that serves the dual purpose of filtering spurious
signals such as image frequencies and providing an impedance match between the Low Noise
Preamplifier and the Mixer.
The LT5526 IC is employed as the mixer in UHF models. It uses the receiver’s local oscillator (LO) to
down-convert the RF input signal to the 1st intermediate frequency (IF) of 90 MHz.
In VHF models a discrete, FET-based mixer uses the receiver’s local oscillator (LO) to down-convert the
RF input signal to the 1st intermediate frequency (IF) of 45 MHz.
The Crystal Filter provides frequency selectivity for the IF signal at 45 MHz for VHF models or 90 MHz
for UHF models.
The tunable impedance match in UHF models provides an optimum impedance match between the
Crystal Filter and the Variable Gain IF Amp. The DAC provides the tuning control voltage.
The Variable Gain IF Amp is normally operated at a constant gain. However, this amplifier is shut off
when the radio is in alignment mode. The gain control signal for this dual FET amplifier comes from the