This circuit is for a superheterodyne receiver where the image frequency is suppressed without the use of an input filter. Instead, it uses two NE(SA)612 type mixer ICs that each work 90° out of phase. With a quadrature front-end, the image frequency is rejected and the noise associated with it disappears. In theory, this increases the sensitivity of the receiver by 6 dB.
The phase shift of the local oscillator (LO) is provided by two D-type flip-flops configured as a ring counter. The outputs of the flipflops always change in the same order. The result is a frequency that is half that of the oscillator, but with a 90° phase shift between them. These signals are normally called ‘Q’ (quadrature) and ‘I’ (in phase). Phase shifting of the output is provided by two simple RC networks. For the Q mixer the phase shift is set to –45° using a capacitor; for the I mixer it is set to +45° using trimmer (C14). The total phase difference is therefore 90°. The signals are added very simply, using a preset (P1). In this configuration the input frequency is equal to fo – fif and the image frequency fi = fo + fif, where the latter is suppressed.
With a low IF, such as used in Software Defined Radio, the phase shifting following the mixers has to cover a relatively wide band, because the IF frequency is low compared to the IF bandwidth. This is much easier to achieve using software rather than a complex phase shifting RC network. With this AM receiver the IF bandwidth is small compared to the center IF frequency of 455 kHz and the maximum phase error is almost negligible even when a simple RC network is used.
We’ve used a standard IC for the demodulation: the TDA1072. To drive a loudspeaker we’ve added a simple amplifier stage using a pair of transistors (BC547 and BC557) along with a potentiometer (P2) for the volume control.
When setting up the receiver the lowest frequency of the VCO can be configured such that DC can be received. This can be done by ear because the noise disappears and a 50 Hz hum becomes audible. Setting up the phase shifting can be done with the help of a station that’s on the same frequency as the image frequency.
It could happen that the fixed phase shift at the output of the Q mixer isn’t exactly – 45°, but could be –43°, for example. If you now adjust the trimmer such that the phase shift becomes +47,° the difference becomes 90° again. This is a matter of making small adjustments to the preset and trimmer alternately, while the suppression becomes progressively better until the station is no longer audible due to the image rejection.