MMark,
Below about 160 MHz, the SM-series is in its direct conversion band. But in order to squeeze this into our normal I/Q signal processing, we do an upfront digital tune for some combinations of center frequency and bandwidth. This will ultimately move the pass band, and will digitally filter out some frequencies. If you email our support team, we can go into more specifics for your use case, but this might be what you’re observing.
