Feature: RF
AM/SW1 radio receiver with 455kHz ceramic filter
By Raj K. Gorkhali, Nepal H
ere is a simple design of an AM/SW1 radio receiver with 455kHz ceramic filter. Components such
as the MW/SW1, SW2
antenna coil and MW/SW1, SW2 oscillator coil can be easily taken from an old MW/ SW1 and SW2 transistor or valve radio receiver. Two-pin 455kHz ceramic filters can be taken from an old TV set or VCR IR remote control. A variable capacitor VC1 can be salvaged from an old radio set, which means this AM/SW1 radio receiver can be built for free. Its main advantage is that it uses a 455kHz ceramic filter, so it’s not necessary to tune IFT coils. Te radio receiver can be tuned to
stations between 530kHz and 1625kHz on the MW band and 2.7MHz-7.7MHz on the SW1 band. By replacing a long-wave antenna coil and oscillator coils you may also be able to tune in on a long-wave band. Te circuit is powered by a 6V battery and has an AF amplifier (using LM386)
36 June 2021
www.electronicsworld.co.uk
that draws power of only 200mW, extending the life of the battery. An LM386 integrated circuit is used to amplify the signal. A diode D1 – OA79 or 1N4148 – will detect the amplitude- modulated signal; see Figures 1 and 2. Like most common radio receivers, this
MW, SW1 radio receiver works on the heterodyne receiver principle.
Circuit description Referring to Figure 2, the ferrite rod in the antenna coil acts as an aerial and picks up radio signals within the 530kHz to1625kHz range. Te assembly, which includes the antenna coil and the section of the gang capacitor, selects the amplitude-modulation RF signal from the desired radio station. Te tuned signal is then coupled to the base of mixer transistor Q1 (BF494) via a tapping at the antenna coil and capacitor C10 (0.01uF). Te oscillator’s winding is tuned
455kHz higher (i.e., 985kHz-2080kHz) than the received signal by the oscillator
section of the gang capacitor. Tis oscillator signal is coupled through capacitor C9 (0.01uF) to the emitter of transistor Q1. Te base bias for Q1 is supplied through resistor R1 (390K). Te modulated RF signal at the base
and the oscillator signal at the emitter are mixed together in Q1 to produce a modulated 455kHz difference frequency, or intermediate frequency (IF). Te antenna and oscillator-tunning capacitor are mechanically ganged so that the antenna input and oscillator circuits are adjusted simultaneously to maintain this difference frequency (IF) constant throughout the tunning. Trimmers across this tunning capacitor
are adjusted to maintain the required tracking relationship by a fine variation of the total capacitance. Te positive feedback for the oscillator circuit is provided by the inductive coupling between the collector winding and the tuned winding of the oscillator’s coil. Te 455kHz IF is coupled to X1
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