RF Mixers (Extracted heavily from the ARRL Handbook) “When you can measure what you are speaking about, and express it in numbers, you begin to know something about it;” Lord Kelvin 1842-1907 He also said: “Radio has no Future” Mixers are used for: • Frequency conversion in heterodynes o Up-conversion o Down-conversion • Modulation (AM, SSB, PM and FM) • De-modulation • Phase detection • Image rejection Superheterodyne Mixer Double Conversion Superhet. Two types of “mixers” “Audio” (and “RF”) • Audio mixers combine one sound track with another, adding voltages of the two tracks. Frequencies of the two tracks are unaffected. Audio Mixer (Addition) Circuit “RF” Mixers • RF mixers combine two RF signals to produce two different frequencies. o Usually, the lower (shifting) frequency, called “LO” (for local oscillator). It is mixed with a higher (constant) frequency, called “RF”, to produce two new (shifting) “IF” (for intermediate) frequencies, “RF + LO” and “RF – LO”. o Usually, the higher frequencies (“RF”, “LO” and “RF + LO” are filtered out. Leaving only “RF – LO”. RF Mixer – Why Does a Diode Do This?