Ivan,
The TG44A produces 10 Hz to 4.4 GHz, -30 dBm to -10 dBm. This is why 0 dBm and -40 dBm do not read accurately–they are beyond the amplitude range of the device.
The TG44A, as a tracking generator (not a signal generator), has a very simple job: generate a signal that tracks the SA44B’s input frequency, at a specific frequency and a repeatable amplitude. Since the SA44B can filter out harmonics, we do not filter them on the TG44A. This reduces cost, size, weight, and power, without really affecting performance as a tracking generator.
Below 28 MHz, the frequency is generated with a DDS, thus the sinusoidal output. Above 28 MHz, it is produced by a clock or divided LO, and will be closer to square than sinusoid. A external low pass filter can easily remove harmonics to produce a sinusoid if needed.
