What changes were made to the internal (non software) bandpass filters in the FLEX-5000?
First, the transmitter and both receivers have their own independent filter banks. Second, all filters are low pass instead of band pass or “roofing.” Direct conversion receivers, like the SDR-1000 and FLEX-5000, do not require bandpass or roofing filters as found in superhet receivers. The quadrature sampling detector (QSD) will not respond to signals below its passband but is susceptible to odd harmonics above its local oscillator (LO) frequency. A low pass filter blocks signals above its cutoff frequency and not below. This type of filter has much lower loss and is less sensitive to component tolerance than a band pass filter. Also the QSD acts as a 400 kHz wide “tracking” filter centered at the LO frequency. However, no receive filter can remove real harmonics from a transmitter located at a harmonically related frequency below the received frequency. In other words, if you are transmitting on 7.0 MHz, there will be a real signal emitted at 21.0 MHz that can be heard in any receive
