Enhancing Image Frequency Suppression: Superheterodyne Receiver Circuit
This specific circuit is designed for a superheterodyne receiver, employing a unique method to suppress the image frequency without the reliance on an input filter. Instead of conventional filtering techniques, the circuit integrates two NE(SA)612 type mixer ICs, each operating 90° out of phase. Through this quadrature front-end configuration, the image frequency is effectively rejected, eliminating the associated noise. In theory, this innovative approach results in a 6 dB increase in receiver sensitivity.
Generating Quadrature and In-Phase Signals: Phase Shifting Technique
In this design, the local oscillator’s (LO) phase shift is managed by employing two D-type flip-flops organized as a ring counter. These flip-flops consistently change their outputs in the same sequence, producing signals designated as ‘Q’ (quadrature) and ‘I’ (in phase). The frequency generated is half that of the oscillator, with a 90° phase difference between them. Phase alterations are introduced through straightforward RC networks: for the Q mixer, a –45° shift is set using a capacitor, while for the I mixer, a +45° shift is established through a trimmer (C14). The cumulative phase difference remains 90°. This configuration ensures the input frequency equals fo – fif, suppressing the image frequency fi = fo + fif.
Adaptable Phase Shifting for Low IF in Software Defined Radio
In cases of low Intermediate Frequency (IF) as seen in Software Defined Radio, post-mixer phase shifting must encompass a relatively broad range due to the IF frequency being notably lower than the IF bandwidth. Achieving this task becomes considerably more straightforward using software-based methods compared to intricate RC phase-shifting networks. In this AM receiver, the IF bandwidth, concerning the central IF frequency of 455 kHz, is comparatively small. Therefore, even with a basic RC network, the maximum phase error is practically negligible.
Demodulation and Audio Amplification Stage
For demodulation purposes, a standard IC, the TDA1072, is employed. To drive a loudspeaker, a basic amplifier stage utilizing BC547 and BC557 transistors, coupled with a volume control potentiometer (P2), has been added.
Fine-Tuning the Setup and Image Rejection
During receiver configuration, the VCO’s lowest frequency can be adjusted to receive DC signals. This adjustment can be made audibly, discerning the disappearance of noise and the emergence of a 50 Hz hum. Precision in phase shifting is accomplished by tuning the station to the image frequency. Suppose the fixed phase shift at the Q mixer’s output deviates, for instance, to –43°. In such cases, adjusting the trimmer to +47° restores the 90° difference. This fine-tuning process involves subtle alterations to the preset and trimmer alternately, progressively enhancing suppression until the station becomes inaudible due to impeccable image rejection.