Posted on Feb 7, 2014

The RF signal is split into two signals that are shifted 90 degrees out of phase (one plus 45 and one minus 45 degrees). Both are mixed to audio frequencies. The two audio signals are again shifted 90 degrees out of phase (again one plus 45 and one minus 45 degrees). When we add the two signals, one side band is in phase and added, the signals fro

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m the other side band are 180 degrees out of phase and substracted. The RF phase shift circuits are simple RC combinations, the trimmers are adjusted for maximum suppression at 3550 kHz. The disadvantage of such a simple phase shift network is that the side band suppression is a little frequency dependent. Both mixers are a CMOS switch. One 74HC4066 IC contains even four CMOS switches of which only two are used here! So a very simple and cheap solution for the mixer! At the input, you will find the very useful RF attenuator. It is the main volume control. The preselector is a bandfilter tuned to 3550 kHz. The RF preamplifier has some gain, a high input impedance for the bandfilter and a low output impedance for the phase shift networks. After this preamplifier we have the RF phase shift filters. It are simple RC networks, both tuned with the trimmers for approximately 45 degrees phase shift. They also compensate for amplitude differences. For optimum settings for phase and amplitude, it is possible that one network is plus 55 degrees, the other minus 35 degrees as long as the difference is 90 degrees. Adjust it by ear, try different trimmer settings while adjusting the other at 3550 kHz while listening to a signal on the suppressed side band. Usually, phase shifting is not done in the RF signal path but in the VFO. In that case, the VFO works at 4 times the reception frequency. The 90 degrees phase shift is obtained by...

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