Direct Conversion Receiver using sub-harmonic mixer
Used this sub-harmonic mixer, popularised by Vladimir Polyakov (RA3AAE), to make a 40M direct conversion receiver. Sub-harmonic mixer Example: Local oscillator is set to 3572.5 kHz to receive 7145 kHz in this sub-harmonic mixer. LO leakage to antenna causes most problems in direct conversion receivers. Anticipated little problem because LO and Rx frequencies are different in a sub-harmonic mixer. Tried a single tuned front end without RF amplifier, DDS VFO as LO, a simple 12 Volt power supply and audio amplifier. Audio amplifier was followed by a LM386 speaker amplifier. Result was an unusable radio with strong hum. 2nd harmonic of LO was reaching the antenna or the power supply had ripple ? Assumed both and applied Grandma-approved home remedies. RFC with ferrite core, bypassed with 100nF ceramic disc capacitor, in every conceivable point 10000uF capacitor after rectifiers of power supply 18 Volt from first regulator, goes to radio box through RF filter 18 volt is inpu