Kirisun DM588, Service Manual

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DM588 Service Manual
using a microstrip to reduce loss of output power caused by impedance mismatching. Then,
transmitting signals go through the TX/RX switch (composed of the D14, D15, D16 and D17) and
enter the low-pass filter (LPF).

Low-Pass Filter (LPF) Circuit
The low-pass filter for inhibiting harmonic wave is a high-order Chebyshev filter composed of a
lumped parameter inductance and a capacitance. Under certain in-band fluctuation performance, the
filter enhances inhibition of out-of-band harmonic wave and spurious signals. Transmitting signals
after wave filtering is radiated through the antenna.

Automatic Power Control Circuit (APC) (Included)
For transmitting signals, a power-oriented coupler composed of a microstrip, an inductance and a
capacitance detects forward and reverse power, and detects and samples waves through detector
diodes (the D30 and D201). The output voltage is compared first with the voltage (the IC13-A) and
then with the preset standard voltage (the IC13-B) to get a voltage, which is used to adjusts the grid
voltage of the pre-drive amplifier Q18 and the last-stage amplifier Q29 to detect standing wave and
control power. Meanwhile, the RF power amplification circuit is protected through controlling the APC
voltage.
4.3.Rx Circuit
The Rx circuit is composed of an RF circuit, an MF circuit and MF signal processing, which are
described as below.

RF Circuit
Signals from the antenna are filtered through a low-pass filter circuit, transmitted through the RX/TX
switch (the D14, D15, D16 and D17) to a tunable band-pass filter for filtering, amplified by the
low-noise amplifier Q12, filtered again by a tunable band-pass filter, and then mixed with the first
local oscillator signals in a mixer to get the first MF signals at a frequency of 49.95MHz.

MF Circuit and MF Signal Processing
The first MF signals are filtered by the crystal filter Z2, amplified by the amplifiers Q13 and Q14, and
then mixed again and processed by the MF decoding chip Q17 to get noise signals, signal strength
and audio signals, which are processed by the DSP in the master chip LT1901.
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