The BEL, 7220 is an integrated AM/FM IC designed for economical and high quality receivers. The BEL
7220 IC can be used in battery or mains operated AM/FM receivers working from a nominal voltage of 1.6 to 6 volts. The functional block diagram is shown in Fig.l. With the exception of FM front end and audio output stage, the following functions are integrated.
(a) Pre-amplifier and double balanced mixer with AGC
(b) One pin local oscillator
(c) IF amplifier with internal AGC
(d) FM/IF amplifier and limiter
(f) FM quadrature detector
(g) Audio pre-amplifier
The audio power amplifier stage makes use of IC BEL 1895. This is an audio power amplifier IC designed specially for radio application delivering 1W power into 4 ohms at 6V. The output exhibits low distortion and good noise characteristics. The schematic of AM/FM receiver is given in Fig.2.
FM section. The FM radio section consists of FM tuner, FM-IF amplifier, limiter and FM detector. All these sections except the FM tuner are integrated within the IC BEL 7220. The AM/FM selection is achieved by the selector switch. The FM selection is effected by applying DC voltage at pin 13 which switches the internal reference. The FM tuner covers the frequency from 87 to 108 MHz. The three-transistor FM front end schematic is shown in Fig. 3. It consists of a RF amplifier and a self-oscillating stage followed by a buffer amplifier. The front end transistors have been selected to give low noise figure and the design is stable under all conditions.
The tuned circuit at the IF input of IC provides high selectivity to. the receiver. Ceramic filter can also be used in place of tuned coil, and in this case, pin 16 is,biased from pin 14 by a low value resistor. The 10.7 MHz IF signal from the input coil is amplified and limited by the four stage differential IF amplifier. The gain of the IF stage is 81dB typical.
The FM IF output at pin 12 is applied to the discriminator network. The signal at pin 13 will be in quadrature (90 degree shift) with the input signal at centre frequency (10.7 MHz). The linearity can be improved by using a double tuned network, implying lower distortion comparatively. However, this leads to a reduction in the level of the recovered audio. A compromise between the two is achieved in the circuit. The audio output is available at pin 9. The capacitor at pin 9 filters the IF signal and also acts as de-em phasis network.
AM section. The AM section is designed to receive medium and short wave frequency ranges as given below: MW 520 kHz to 1620 kHz SW 4.5 MHz to 16.5 MHz.
The RF amplifier stage of the IC BEL 7220 receives the input from the secondary winding of the ferrite rod antenna coil, and SW antenna coil for the MW and SW operation respectively. The band selection is done by the band-switch. The RF input of IC BEL 7220 is biased by pin 4 through antenna coil. Gain of the RF amplifier is about 30dB.
The local oscillator of IC BEL 7220 is a cross coupled differential amplifier which oscillates at the frequency determined by the resonant circuit formed by the oscillator coil with the gang condensor. Transformer coupling is used to improve the Q factor and to obtain better frequency stability of the oscillator circuit.
The mixer employed in IC BEL 7220 is a double balanced multiplier type. The IF output available at pin 3 is directly connected to the IF filter coil. The coil has been designed to have good selectivity. The tuned output is transformer coupled to IF input at pin 5.
For higher selectivity either an additional coil or ceramic filter can be used. The IF output available at pin 7 is detected. The capacitor at pin 6 acts as the peak envelope detector. This capacitor not only helps to recover the audio signal but also reduces the IF radiation and other spurious detector products. The audio output is available at pin 9.
The capacitor at pin 8 derives a mean DC signal to drive the AGC circuit. The IC has AGC range of about 90dB. The AGC action is obtained by the control of the transconductance of the RF amplifier, mixer and the IF amplifier stages. With weak signals, the AGC controls the IF stage gain maintaining the S/N (signal-to-noise ratio) maximum. With strong signal the gain of IF stages reduces first and then the RF stages. The detected output at pin 9 is coupled to audio amplifier stage through a capacitor.
The most popular BEL 1895 IC is used as the audio power amplifier.