Hi Henry, > Could you explain why in a little more detail? > > (I thought that AM noise was not a problem in balanced mixers?)
When you feed an LO signal into a schottky mixer the power level is high. A low level mixer would need +7 dBm or so. The noise floor is at perahps -164 dBm/Hz (NF=10dB) The mixer is balanced so it would suppress AM modulation by perhaps 30 or 40 dB so AM noise would have to be -130 to -140 dBc/Hz to not create problems. That is easy at a frequency separation of 10 kHz, but at a separation of 100 Hz it is not so easy. Supply voltages have to be filtered through RC filters with big electrolytic capacitors of good quality to avoid producing AM modulation. http://www.sm5bsz.com/linuxdsp/hware/optiq.htm Also phase noise will cause problems. A schottky mixer is also an FM detector although the sensitivity for FM modulation is a bit lower than the sensitivity for AM modulation. Close range phase noise is often much higher than AM noise...... 73 Leif / SM5BSZ
