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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.

Kirisun DM588 User Manual

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