Copyright @ 2010 Amateur Radio Association of Lanka. Reproduction of material from ARAL web page without written permission is strictly prohibited
Modified 2M receiver.
Prepared by Achira - 4S6DDA
This experiment was done when I was a SWL & also studying for RAE in year 2000. This circuit was based on a FM radio originally made for receiving FM broadcast stations. I modified its RF coil & local OSC coil & removed BPF at the front end & replaced a 50 ohms matching part to suitable 50 ohms antennas. Also AM section was disabled in the receiver.
As this design is a single conversation receiver & trying to receive narrow band FM signals, it might be difficult to lock the tuning between 144 MHz & 146MHz. But once locked it receives 2M signals nicely.
A homebrewed ground plane antenna was used during the experiment to receive a local repeater on 145.625 MHz. Later connected a homebrewed 8 element beam with a folded dipole antenna. Both worked with this receiver without any problems.
KA 22426
This IC is all-in-one chip made to receive FM broadcast stations. This Integrated chip has a FM front end RF AMP, Local OSC , 10.7 MHz IF section, FM quadrature detector & an AF power amplifier. Additionally it has tuning meter, FM AFC & Also a variable capacitance diode for fine tunings arrangement.
# Capacitors # Resistors
1 C1 220 µF R1 10k
2 C2 0.1 µF R2 100 ?
3 C3 470 µF R3 100K
4 C4 10 µF R4 220 ?
5 C5 0.47 µF
6 C6 0.022 µF Ceramic filter 10.7 MHz
7 C7 10 µF Crystal 10.7 MHz
8 C8 4.7 µF
9 C9 0.01 µF Tuning capacitor 15PF - 30PF
10 C10 10 µF Speaker 4 ? / 0.25W
11 C11 4.7 µF
12 C12 1PF IC
13 C13 0.01 µF KA22426
14 C14 10 µF
15 C15 22 PF
16 C16 22PF
17 C17 22PF
18 C18 18PF
After I got the radio amateur license, this modified radio was gifted to a friend of mine who is an another SWL & therefore I don't have it now to publish photos with this article. Also I didn't prepare a PCB pattern, I only modified a working FM radio.
My next experiment with KA22426.
First a PCB pattern will be designed. As this chip has a variable capacitance diode , an AFC block. So a manual variable capacitance diode tuning system will be added using the internal very cap diode. Also will add a tunings meter system to indicate signal strength an S meter level. All modifications will be published here when the experiment will be completed.