![]() ![]() This is a very common and effective way of filtering audio-frequency signals.) See the capacitors in the op-amp feedback loops? Those turn the amplifiers into additional low-pass filters. (Though looking at the schematic, those aren't the only filters - there's also some active filtering in the following amplifier stages. ![]() ![]() It's a quadrature direct conversion receiver - instead of converting the signal to IF, it converts it to a pair of I and Q signals at around audio frequencies, and then uses a pair of lowpass filters to filter out all the other frequencies, which can easily be implemented as standard LC filters. Ceramic filters are very similar but cheaper, less narrow, and less stable. Crystal filters are particularly well suited here because they're extremely narrow, stable bandpass filters - far narrower and more stable than can be achieved with LC bandpass filters. A traditional superheterodyne receiver, which is what you'd find in most older radios, uses a mixer to convert the desired signal to a fixed IF of say 10.7MHz and then uses a very narrow bandpass filter at that frequency to select only that signal. As Phil Genera says, the KX3 uses a different type of receiver that requires a different kind of filter. ![]()
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