El kit de radio Tecsun 2P3 viene con una hoja de papel con el siguiente esquema y descripción, pero no explica mucho sobre todos los componentes. Para un principiante es un poco difícil saber el sentido de todas las piezas, especialmente las que no tienen nombre. Tomando la descripción manual como punto de partida, sería útil si alguien pudiera extenderla con fines de aprendizaje.
Specifications:
Frequency range: 530kHz~1620kHz
DC supply: Power requirements: DC 3V, two R6 AA batteries
Sensitivity: <1mV/m
Maximum Output: 120mW
Quiescent Current: 7mA
- Mixing circuit: Mixing circuit is broken down into three sections,
input tuned circuit, local oscillator and mixer. Magnetic antenna
inducts radio signal, feeds into a tunning circuit comprised of
variable capacitator VC4 and L1, where the signal frequency
indentical to the tunning circuit resonance frequency is selected
and inducted on L2, this process is called "tunning". L2 feeds the
tuned radio signal into the base of mixer Q1 (S9018). Q1 has two
functions.
- A. Local Oscillator: Q1 working with VC3, C2 and T1, generate an
equiamplitude sine wave at a frequency 455Khz higher than the tuned
radio signal.
- B. Mixer: Q1 mixes L2 inducted radio signal with local generated
oscillator signal, a 455Khz intermediate signal (IF signal = Local
Oscillator signal - tuned radio signal = 455Khz) is selected to pass
the IF transformer T2, in fact, plural frequencies are generated but
T2 is tuned at 455Khz so only 455Khz IF are passed and all other
frequencies are filtered. The 455Khz IF is the fed to the IF
amplifier stage.
- IF Amplifier: In order to obtain enough gain, 2P3 has two IF
amplifiers, comprised of Q2, Q3 and peripheral components. 455Khz IF
signal passes T2, amplified by Q2, passes 455Khz ceramic filter CF1
and feeds into Q3 for second stage IF amplification. Amplified IF
signal passes T3 then feeds into detector stage.
- Detector. When radio signal trasnformed into 455Khz IF signal, only
carrier wave frequency changes, audio signal riding on carrier
remains unchanged, but this IF signal is not audible by human ears,
it must pass a detector to separate audio signal from its carrier,
this detector is carried out by diode D1 (1N60) and peripheral
components.
- Automatic Gain Control: After passing detector D1, IF is filtered by
C7, R11, part of it feeds through C6, R9, C4 to be stepdown and
filtered, providing a negative bias to Q2. When radio signal is
stronger, higher IF output causing Q2 to reduce amplification, it
works reversely when signal is weaker, hence stronger and weaker
stations will have relatively similar volume.
- Audio amplifier: Part of detector output feeds to Audio amplifier IC1
(CD7368) through C8, to be amplified and fed into speaker.
