Advertisement
1.
The front-end (RF amplifier)
From the antenna signal into the transmit/receive switch diode circuits（D501，D503and D504）to
amplify.This signal passes through BPF (L508) and then enters mixing.These BPFs are adjusted by the
variable capacitors (D505, D506 and D512).The voltage of the input variable capacitor is adjusted by the
voltage output of the microprocessor (U301).
2.
First mixer
The signal of the front end is mixed with the first local oscillator signal generated by the PLL
circuit at Q1 to generate a first intermediate frequency signal at a frequency of 49.95 MHz,and the
resulting signal is filtered by XF501
3. If amplifier circuit
The first intermediate frequency signal eliminates the signal of adjacent channel by crystal filter
(XF501) and amplifies the first frequency signal by the first intermediate frequency amplifier (Q507) and
enters the intermediate frequency system chip (U501).The IF system chip provides a second mixer, a second
local oscillator signal, a limiting amplifier, a quadrature detector, and an RSSI (Received Signal Strength
Indicator).The second mixer mixes the first intermediate frequency signal with a second local oscillation
signal output of 50.8 MHz and generates a second intermediate frequency signal of 400 MHz.
The second intermediate frequency signal continues to cancel the signal of the adjacent channel through
the ceramic filter (CF501).The filtered second intermediate frequency signal is amplified by a limiting
amplifier and demodulated by a quadrature detector with a ceramic discriminator (FD501). The demodulated
signal enters the audio power circuit.
4. wide / narrow switching circuit
Set the width and width of each channel by turning on the wide and narrow card switch.
When receiving WIDE (low level) data, Q511 connected.
When receiving NARROW (high) data, Q511 is disconnected.
Q511 is turned on / off with wide / narrow data, the U501 detector output level changes
frequently, with a constant output level that maintains a wide or narrow signal process.
As picture 2 receiving department
Advertisement
