Can the 12 kHz FM filter be used on AM?
Unlike most DSP-based radios, the K3 converts from the main IF (8.215 MHz) to the 15 kHz DSP IF without an intervening 455 kHz IF. This helps give the K3 a cleaner sound and better dynamic range since there are fewer conversions. However, this also means the IF crystal filter must have exceptional skirt selectivity to prevent the 15 kHz “image” from getting through. In FM mode, capture effect helps suppress the effect of “image” leakage. AM and (E)SSB modes have far more dynamic range and the 15 kHz IF image must be suppressed by some 10s of dB more than is required for excellent FM operation. It is for this reason that the FM filter is recommended only for FM use. It may also be used for AM receive, but is not recommended for AM transmit. The 6 kHz filter has the necessary 15 kHz IF image rejection to be suitable for use with either AM or ESSB.
Unlike most DSP-based radios, the K3 converts from the main IF (8.215 MHz) to the 15 kHz DSP IF without an intervening 455 kHz IF. This helps give the K3 a cleaner sound and better dynamic range since there are fewer conversions. However, this also means the IF crystal filter must have exceptional skirt selectivity to prevent the 15 kHz “image” from getting through. In FM mode, capture effect helps suppress the effect of “image” leakage. AM and (E)SSB modes have far more dynamic range and the 15 kHz IF image must be suppressed by some 10s of dB more than is required for excellent FM operation. It is for this reason that the FM filter is recommended only for FM use. It may also be used for AM receive, but is not allowed for AM transmit. The 6 kHz filter has the necessary 15 kHz IF image rejection to be suitable for use with either AM or ESSB transmit.
